(c) Software Lab. Alexander Burger
This document demonstrates some aspects of the PicoLisp system in detail and example. For a general description of the PicoLisp kernel please look at the PicoLisp Reference.
This is not a Lisp tutorial, as it assumes some basic knowledge of programming, Lisp, and even PicoLisp. Please read these sections before coming back here: Introduction and The PicoLisp Machine. This tutorial concentrates on the specificities of PicoLisp, and its differences with other Lisp dialects.
If not stated otherwise, all examples assume that PicoLisp was started from a global installation (see Installation) from the shell prompt as
$ pil +
:
It loads the PicoLisp base system and the debugging environment, and waits
for you to enter input lines at the interpreter prompt (:). You can
terminate the interpreter and return to the shell at any time, by either hitting
the Ctrl-D key, or by executing the function (bye).
Please note that special handling is done during character input. This one
is incompatible with rlwrap for example but is more powerful.
vi-like command-line editing (typos fixes and history with ESC,
h, j, k and l but not
arrows)," appear).If you prefer to use Emacs, please use the picolisp-mode bundled in "@lib/el".
If you feel that you absolutely have to use an IDE, rlwrap or
another input front-end, please remove the entry "@lib/led.l" from "lib.l" and
"dbg.l". Note that in this case, however, you will not have the TAB symbol
completion feature available during command line editing.
If you are new to PicoLisp, you might want to read the following sections in the given order, as some of them assume knowledge about previous ones. Otherwise just jump anywhere you are interested in.
PicoLisp permanently reads input from the current input channel (i.e. the console in interactive mode), evaluates it, and prints the result to the current output channel. This is called a "read-eval-print-loop" (REPL).
It is very helpful - though not absolutely necessary - when you know how to
use the vi editor.
To alleviate the task of manual line input, a command line editor is provided
which is similar to (though much simpler than) the readline feature
of the bash shell. Only a subset of the vi mode is
supported, which is restricted to single-key commands (the "real"
vi supports multi-key commands and the modification of most
commands with count prefixes). It is loaded at startup in debug mode, you find
its source in "lib/led.l".
You can enter lines in the normal way, correcting mistypes with the BACKSPACE key, and terminating them with the ENTER key. This is the Insert Mode.
If you hit ESC, you get into Command Mode. Now you can navigate
horizontally in the current input line, or vertically in the history of
previously entered lines, with key commands borrowed from the vi
editor (only h, j, k and l
and not arrows). Note, however, that there is always only a single line visible.
Let's say you did some calculation
: (* (+ 2 3) (- 7 2))
-> 25
:
If you want to repeat a modified version of this command, using
8 instead of 7, you don't have to re-type the
whole command, but type
k to get one line "up"
f and 7 to "find" the character 7
r and 8 to "replace" with 8
Then you hit ENTER to execute the modified line. Instead of jumping to the
7 with the "find" command, you may also type l (move
"right") repeatedly till you reach the correct position.
The key commands in the Command Mode are listed below. Some commands change the mode back to Insert Mode as indicated in parentheses. Deleting or changing a "word" take either the current atom (number or symbol), or a whole expression when the cursor is at a left parenthesis.
k - Go up one line
j - Go down one line
l - Go right one character
h - Go left one character
w - Go right one word
b - Go back (left) one word
0 - Go to the beginning of the line
$ - Go to the end of the line
i - Enter Insert Mode at the cursor position
a - Append (Insert Mode) after the cursor position
A - Append (Insert Mode) at the end of the line
I - Insert (Insert Mode) at the beginning of the line
x - Delete the character at the cursor position
X - Delete the character left of the cursor position
r - Replace the character at the cursor position with the next key
s - Substitute the character at the cursor position (Insert Mode)
S - Substitute the whole line (Insert Mode)
d - Delete the word at the cursor position (Insert Mode)
D - Delete the rest of the line
c - Change the word at the cursor position (Insert Mode)
C - Change the rest of the line (Insert Mode)
f - Find next key in the rest of the current line
p - Paste data deleted with x, X, d or D after the cursor position
P - Paste data deleted with x, X, d or D before the cursor position
/ - Accept an input pattern and search the history for it
n - Search for next occurrence of pattern (as entered with /)
N - Search for previous occurrence of pattern
% - Go to matching parenthesis
~ - Convert character to opposite (lower or upper) case and move right
u - Undo the last change (one level only)
U - Undo all changes of the current line
g - Display current contents of cut buffer (not in vi)
Notes:
d command corresponds to the dw command of the
vi editor, and c corresponds to cw.
@" characters as wildcards.
The following two key-combinations work both in Insert and Command Mode:
Ctrl-D will immediately terminate the current process.
Ctrl-X discards all input, abandons further processing, and
returns to the interpreter's top level (equivalent to invoking quit). This is also useful when the program
stopped at a breakpoint (see single-stepping Debugging), or
after program execution was interrupted with Ctrl-C.
Besides these two keys, in Insert Mode only the following keys have a special meaning:
Ctrl-H) and DEL erase the character to the left
Ctrl-V inserts the next key literally
Please take some time to experiment and to get used to command line editing. It will make life much easier in the future :-)
PicoLisp provides some functionality for inspecting pieces of data and code within the running system.
print,
size
...
But you will appreciate some more powerful tools like:
match, a predicate which
compares S-expressions with bindable wildcards when matching,The most commonly used tool is probably the show function. It takes a symbolic argument,
and shows the symbol's name (if any), followed by its value, and then the
contents of the property list on the following lines (assignment of such things
to a symbol can be done with set,
setq, and put).
: (setq A '(This is the value)) # Set the value of 'A'
-> (This is the value)
: (put 'A 'key1 'val1) # Store property 'key1'
-> val1
: (put 'A 'key2 'val2) # and 'key2'
-> val2
: (show 'A) # Now 'show' the symbol 'A'
A (This is the value)
key2 val2
key1 val1
-> A
show accepts an arbitrary number of arguments which are
processed according to the rules of get, resulting in a symbol which is showed then.
: (put 'B 'a 'A) # Put 'A' under the 'a'-property of 'B'
-> A
: (setq Lst '(A B C)) # Create a list with 'B' as second argument
-> (A B C)
: (show Lst 2 'a) # Show the property 'a of the 2nd element of 'Lst'
A (This is the value) # (which is 'A' again)
key2 val2
key1 val1
-> A
Similar to show is edit. It takes an arbitrary number of symbolic
arguments, writes them to a temporary file in a format similar to
show, and starts the vim editor with that file.
: (edit 'A 'B)
The vim window will look like
A (This is the value)
key1 val1
key2 val2
(********)
B NIL
a A # (This is the value)
(********)
Now you can modify values or properties. You should not touch the
parenthesized asterisks, as they serve as delimiters. If you position the cursor
on the first character of a symbol name and type 'K' ("Keyword
lookup"), the editor will be restarted with that symbol added to the editor
window. 'Q' (for "Quit") will bring you back to the previous view.
edit is also very useful to browse in a database. You can follow
the links between objects with 'K', and even - e.g. for low-level
repairs - modify the data (but only if you are really sure about what you are
doing, and don't forget to commit
when you are done).
The pretty-print function pp
takes a symbol that has a function defined (or two symbols that specify message
and class for a method definition), and displays that definition in a formatted
and indented way.
: (pp 'pretty)
(de pretty (X N . @)
(setq N (abs (space (or N 0))))
(while (args) (printsp (next)))
(if (or (atom X) (>= 12 (size X)))
(print X)
(while (== 'quote (car X))
(prin "'")
(pop 'X) )
(let Z X
(prin "(")
(cond
((memq (print (pop 'X)) *PP)
(cond
((memq (car Z) *PP1)
(if (and (pair (car X)) (pair (cdar X)))
(when (>= 12 (size (car X)))
(space)
(print (pop 'X)) )
(space)
(print (pop 'X))
(when
(or
(atom (car X))
(>= 12 (size (car X))) )
(space)
(print (pop 'X)) ) ) )
((memq (car Z) *PP2)
(inc 'N 3)
(loop
(prinl)
(pretty (cadr X) N (car X))
(NIL (setq X (cddr X)) (space)) ) )
((or (atom (car X)) (>= 12 (size (car X))))
(space)
(print (pop 'X)) ) ) )
((and (memq (car Z) *PP3) (>= 12 (size (head 2 X))))
(space)
(print (pop 'X) (pop 'X)) ) )
(when X
(loop
(T (== Z X) (prin " ."))
(T (atom X) (prin " . ") (print X))
(prinl)
(pretty (pop 'X) (+ 3 N))
(NIL X) )
(space) )
(prin ")") ) ) )
-> pretty
The style is the same as we use in source files:
size is greater than 12), pretty-print the CAR
on the current line, and each element of the CDR recursively on its own line.
more is a simple tool that displays
the elements of a list one by one. It stops after each element and waits for
input. If you just hit ENTER, more continues with the next element,
otherwise (usually I type a dot (.) followed by ENTER) it
terminates.
: (more (1 2 3 4 5 6))
1 # Hit ENTER
2. # Hit '.' and ENTER
-> T # stopped
Optionally more takes a function as a second argument and
applies that function to each element (instead of the default print). Here, often show or
pp (see below) is used.
: (more '(A B)) # Step through 'A' and 'B'
A
B
-> NIL
: (more '(A B) show) # Step through 'A' and 'B' with 'show'
A (This is the value) # showing 'A'
key2 val2
key1 val1
# Hit ENTER
B NIL # showing 'B'
a A
-> NIL
The what function returns a list of
all internal symbols in the system which match a given pattern (with
'@' wildcard characters).
: (what "prin@")
-> (prin print prinl print> printsp println)
The function who returns "who
contains that", i.e. a list of symbols that contain a given argument
somewhere in their value or property list.
: (who 'print)
-> (query pretty pp msg more "edit" view show (print> . +Date) rules select
(print> . +relation))A dotted pair indicates either a method definition or a property entry. So
(print> . +relation) denotes the print> method of
the +relation class.
who can be conveniently combined with more and
pp:
: (more (who 'print) pp)
(de query ("Q" "Dbg") # Pretty-print these functions one by one
(use "R"
(loop
(NIL (prove "Q" "Dbg"))
(T (=T (setq "R" @)) T)
(for X "R"
(space)
(print (car X))
(print '=)
(print (cdr X))
(flush) )
(T (line)) ) ) )
(de pretty (X N . @)
...
The argument to who may also be a pattern list (see match):
: (who '(print @ (val @)))
-> (show)
: (more (who '(% @ 7)) pp)
(de day (Dat Lst)
(get
(or Lst *DayFmt)
(inc (% (inc Dat) 7)) ) )
(de _week (Dat)
(/ (- Dat (% (inc Dat) 7)) 7) )
The function can returns a list
which indicates which classes can accept a given message. Again, this
list is suitable for iteration with pp:
: (can 'del>) # Which classes accept 'del>' ?
-> ((del> . +List) (del> . +Entity) (del> . +relation))
: (more (can 'del>) pp) # Inspect the methods with 'pp'
(dm (del> . +List) (Obj Old Val)
(and ((<> Old Val) (delete Val Old)) )
(dm (del> . +Entity) (Var Val)
(when
(and
Val
(has> (meta This Var) Val (get This Var)) )
(let Old (get This Var)
(rel>
(meta This Var)
This
Old
(put This Var (del> (meta This Var) This Old @)) )
(when (asoq Var (meta This 'Aux))
(relAux This Var Old (cdr @)) )
(upd> This Var Old) ) ) )
(dm (del> . +relation) (Obj Old Val)
(and ((<> Old Val) Val) )
dep shows the dependencies in a
class hierarchy. That is, for a given class it displays the tree of its
(super)class(es) above it, and the tree of its subclasses below it.
To view the complete hierarchy of input fields, we start with the root class
+relation:
: (dep '+relation)
+relation
+Bag
+Any
+Blob
+Link
+Joint
+Bool
+Symbol
+String
+Number
+Time
+Date
-> +relation
If we are interested in +Link:
: (dep '+Link)
+relation
+Link
+Joint
-> +Link
This says that +Link is a subclass of +relation, and has a single subclass
(+Joint).
Most of the time during programming is spent defining functions (or methods).
In the following we will concentrate on functions, but most will be true for
methods as well except for using dm
instead of de.
The notorious "Hello world" function must be defined:
: (de hello ()
(prinl "Hello world") )
-> hello
The () in the first line indicates a function without arguments.
The body of the function is in the second line, consisting of a single
statement. The last line is the return value of de, which here is
the defined symbol. From now on we will omit the return values of examples when
they are unimportant.
Now you can call this function this way:
: (hello)
Hello world
A function with an argument might be defined this way:
: (de hello (X)
(prinl "Hello " X) )
# hello redefined
-> hello
PicoLisp informs you that you have just redefined the function. This might be a useful warning in case you forgot that a bound symbol with that name already existed.
: (hello "world")
Hello world
: (hello "Alex")
Hello Alex
Normally, PicoLisp evaluates the arguments before it passes them to a function:
: (hello (+ 1 2 3))
Hello 6
: (setq A 1 B 2) # Set 'A' to 1 and 'B' to 2
-> 2
: (de foo (X Y) # 'foo' returns the list of its arguments
(list X Y) )
-> foo
: (foo A B) # Now call 'foo' with 'A' and 'B'
-> (1 2) # -> We get a list of 1 and 2, the values of 'A' and 'B'
In some cases you don't want that. For some functions (setq for example) it is better if the function
gets all arguments unevaluated, and can decide for itself what to do with them.
For such cases you do not define the function with a list of parameters, but give it a single atomic parameter instead. PicoLisp will then bind all (unevaluated) arguments as a list to that parameter.
: (de foo X
(list (car X) (cadr X)) ) # 'foo' lists the first two arguments
: (foo A B) # Now call it again
-> (A B) # -> We don't get '(1 2)', but '(A B)'
: (de foo X
(list (car X) (eval (cadr X))) ) # Now evaluate only the second argument
: (foo A B)
-> (A 2) # -> We get '(A 2)'
As a logical consequence, you can combine these principles. To define a function with 2 evaluated and an arbitrary number of unevaluated arguments:
: (de foo (X Y . Z) # Evaluate only the first two args
(list X Y Z) )
: (foo A B C D E)
-> (1 2 (C D E)) # -> Get the value of 'A' and 'B' and the remaining list
More common, in fact, is the case where you want to pass an arbitrary number
of evaluated arguments to a function. For that, PicoLisp recognizes the
symbol @ as a single atomic parameter and remembers all evaluated
arguments in an internal frame. This frame can then be accessed sequentially
with the args, next, arg and rest functions.
: (de foo @
(list (next) (next)) ) # Get the first two arguments
: (foo A B)
-> (1 2)
Again, this can be combined:
: (de foo (X Y . @)
(list X Y (next) (next)) ) # 'X' and 'Y' are fixed arguments
: (foo A B (+ 3 4) (* 3 4))
-> (1 2 7 12) # All arguments are evaluated
These examples are not very useful, because the advantage of a variable number of arguments is not used. A function that prints all its evaluated numeric arguments, each on a line followed by its squared value:
: (de foo @
(while (args) # Check if there are some args left
(println (next) (* (arg) (arg))) ) ) # Call the last arg (next) returned
: (foo (+ 2 3) (- 7 1) 1234 (* 9 9))
5 25
6 36
1234 1522756
81 6561
-> 6561
This next example shows the behaviour of args and
rest:
: (de foo @
(while (args)
(next)
(println (arg) (args) (rest)) ) )
: (foo 1 2 3)
1 T (2 3)
2 T (3)
3 NIL NIL
Finally, it is possible to pass all these evaluated arguments to another
function, using pass:
: (de foo @
(pass println 9 8 7) # First print all arguments preceded by 9, 8, 7
(pass + 9 8 7) ) # Then add all these values
: (foo (+ 2 3) (- 7 1) 1234 (* 9 9))
9 8 7 5 6 1234 81 # Printing ...
-> 1350 # Return the result
quote will do what you want (see
also this FAQ entry).
: ((quote (X) (* X X)) 9)
-> 81
: (setq f '((X) (* X X))) # This is equivalent to (de f (X) (* X X))
-> ((X) (* X X))
: f
-> ((X) (* X X))
: (f 3)
-> 9
There are two major ways to debug functions (and methods) at runtime: Tracing and single-stepping.
In this section we will use the REPL to explore the debugging facilities, but in the Scripting section, you will learn how to launch PicoLisp scripts with some selected functions debugged:
$ pil app/file1.l -"trace 'foo" -main -"debug 'bar" app/file2.l +
Tracing means letting functions of interest print their name and arguments when they are entered, and their name again and the return value when they are exited.
For demonstration, let's define the unavoidable factorial function (or just
load the file "@doc/fun.l"):
(de fact (N)
(if (=0 N)
1
(* N (fact (dec N))) ) )
With trace we can put it in trace
mode:
: (trace 'fact)
-> fact
Calling fact now will display its execution trace.
: (fact 3)
fact : 3
fact : 2
fact : 1
fact : 0
fact = 1
fact = 1
fact = 2
fact = 6
-> 6
As can be seen here, each level of function call will indent by an additional
space. Upon function entry, the name is separated from the arguments with a
colon (:), and upon function exit with an equals sign
(=) from the return value.
trace works by modifying the function body, so generally it
works only for functions defined as lists (lambda expressions, see Evaluation). Tracing a C-function is possible, however,
when it is a function that evaluates all its arguments.
So let's trace the functions =0 and
*:
: (trace '=0)
-> =0
: (trace '*)
-> *
If we call fact again, we see the additional output:
: (fact 3)
fact : 3
=0 : 3
=0 = NIL
fact : 2
=0 : 2
=0 = NIL
fact : 1
=0 : 1
=0 = NIL
fact : 0
=0 : 0
=0 = 0
fact = 1
* : 1 1
* = 1
fact = 1
* : 2 1
* = 2
fact = 2
* : 3 2
* = 6
fact = 6
-> 6
To reset a function to its untraced state, call untrace:
: (untrace 'fact)
-> fact
: (untrace '=0)
-> =0
: (untrace '*)
-> *
or simply use mapc:
: (mapc untrace '(fact =0 *))
-> *
Single-stepping means to execute a function step by step, giving the
programmer an opportunity to look more closely at what is happening. The
function debug inserts a breakpoint
into each top-level expression of a function. When the function is called, it
stops at each breakpoint, displays the expression it is about to execute next
(this expression is also stored into the global variable ^) and enters a read-eval-loop. The programmer can
then
(d) to recursively debug the
next expression (looping through subexpressions of this expression)
(e) to evaluate the next
expression, to see what will happen without actually advancing on
Thus, in the simplest case, single-stepping consists of just hitting ENTER repeatedly to step through the function.
To try it out, let's look at the stamp system function. You may need to have a
look at
=T (T test),date and time (grab system date and time)
default (conditional
assignments)
pack (kind of concatenation), and
dat$ and tim$ (date and time formats)
: (pp 'stamp)
(de stamp (Dat Tim)
(and (=T Dat) (setq Dat (date T)))
(default Dat (date) Tim (time T))
(pack (dat$ Dat "-") " " (tim$ Tim T)) )
-> stamp
: (debug 'stamp) # Debug it
-> T
: (stamp) # Call it again
(and (=T Dat) (setq Dat (date T))) # stopped at first expression
! # ENTER
(default Dat (date) Tim (time T)) # second expression
! # ENTER
(pack (dat$ Dat "-") " " (tim$ ... # third expression
! Tim # inspect 'Tim' variable
-> 41908
! (time Tim) # convert it
-> (11 38 28)
! # ENTER
-> "2004-10-29 11:38:28" # done, as there are only 3 expressions
Now we execute it again, but this time we want to look at what's happening inside the second expression.
: (stamp) # Call it again
(and (=T Dat) (setq Dat (date T)))
! # ENTER
(default Dat (date) Tim (time T))
! # ENTER
(pack (dat$ Dat "-") " " (tim$ ... # here we want to look closer
! (d) # debug this expression
-> T
! # ENTER
(dat$ Dat "-") # stopped at first subexpression
! (e) # evaluate it
-> "2004-10-29"
! # ENTER
(tim$ Tim T) # stopped at second subexpression
! (e) # evaluate it
-> "11:40:44"
! # ENTER
-> "2004-10-29 11:40:44" # done
The breakpoints still remain in the function body. We can see them when we pretty-print it:
: (pp 'stamp)
(de stamp (Dat Tim)
(! and (=T Dat) (setq Dat (date T)))
(! default Dat (date) Tim (time T))
(! pack
(! dat$ Dat "-")
" "
(! tim$ Tim T) ) )
-> stamp
To reset the function to its normal state, call unbug:
: (unbug 'stamp)
Often, you will not want to single-step a whole function. Just use
edit (see above) to insert a single breakpoint (the exclamation
mark followed by a space) as CAR of an expression, and run your program.
Execution will then stop there as described above; you can inspect the
environment and continue execution with ENTER when you are done.
Input and output in PicoLisp is functional, in the sense that there are not variables assigned to file descriptors, which need then to be passed to I/O functions for reading, writing and closing. Instead, these functions operate on implicit input and output channels, which are created and maintained as dynamic environments.
Standard input and standard output are the default channels. Try reading a single expression:
: (read)
(a b c) # Console input
-> (a b c)
To read from a file, we redirect the input with in. Note that comments and whitespace are
automatically skipped by read:
: (in "@doc/fun.l" (read))
-> (de fact (N) (if (=0 N) 1 (* N (fact (dec N)))))
The skip function can also be used
directly. To get the first non-white character in the file with char:
: (in "@doc/fun.l" (skip "#") (char))
-> "("
from searches through the input
stream for given patterns. Typically, this is not done with Lisp source files
(there are better ways), but for a simple example let's extract all items
immediately following fact in the file,
: (in "@doc/fun.l" (while (from "fact ") (println (read))))
(N)
(dec N)
or the word following "(de " with till:
: (in "@doc/fun.l" (from "(de ") (till " " T)))
-> "fact"
With line, a line of characters is
read, either into a single transient symbol
(the type used by PicoLisp for strings),
: (in "@doc/tut.html" (line T))
-> "<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://..."
or into a list of symbols (characters):
: (in "@doc/tut.html" (line))
-> ("<" "!" "D" "O" "C" "T" "Y" "P" "E" " " "H" "T" "M" "L" ...
line is typically used to read tabular data from a file.
Additional arguments can split the line into fixed-width fields, as described in
the reference manual. If, however, the
data are of variable width, delimited by some special character, the split function can be used to extract the
fields. A typical way to import the contents of such a file is:
(load "@lib/import.l")
(in '("bin/utf2" "importFile.txt") # Pipe: Convert to UTF-8
(until (eof) # Process whole file
(let L (split (line) "^I") # TAB-delimited data
... use 'getStr', 'getNum' etc ... # process them
Some more examples with echo:
(in "a" # Copy the first 40 Bytes
(out "b" # from file "a" to file "b"
(echo 40) ) )
(in "@doc/tut.html" # Show the HTTP-header
(line)
(echo "<body>") )
(out "file.mac" # Convert to Macintosh
(in "file.txt" # from Unix or DOS format:
(while (char)
(prin
(case @
("^M" NIL) # ignore CR
("^J" "^M") # convert CR to LF
(T @) ) ) ) ) ) # otherwise no change
(out "c" # Merge the contents of "a"
(in "b" # and "b" into "c"
(in "a"
(while (read) # Read an item from "a",
(println @ (in -1 (read))) ) ) ) ) # print it with an item from "b"
There are two possibilities to get the PicoLisp interpreter into doing useful work: via command line arguments, or as a stand-alone script.
The command line can specify either files for execution, or arbitrary Lisp
expressions for direct evaluation (see Invocation):
if an argument starts with a hyphen, it is evaluated, otherwise it is loaded as a file. A typical invocation might
look like:
$ pil app/file1.l -main app/file2.l +
It loads the debugging environment, an application source file, calls the main function, and then loads another application source. In a typical development and debugging session, this line is often modified using the shell's history mechanisms, e.g. by inserting debugging statements:
$ pil app/file1.l -"trace 'foo" -main -"debug 'bar" app/file2.l +
Another convenience during debugging and testing is to put things into the command line (shell history) which would otherwise have to be done each time in the application's user interface:
$ pil app/file1.l -main app/file2.l -go -'login "name" "password"' +
The final production release of an application usually includes a shell script, which initializes the environment, does some bookkeeping and cleanup, and calls the application with a proper command line. It is no problem if the command line is long and complicated.
For small utility programs, however, this is overkill. Enter full PicoLisp scripts.
#!", the operating
system kernel will pass this file to an interpreter program whose pathname is
given in the first line (optionally followed by a single argument). This is fast
and efficient, because the overhead of a subshell is avoided.
Let's assume you installed PicoLisp in the directory "/home/foo/picolisp/", and put links to the executable and the installation directory as:
$ ln -s /home/foo/picolisp /usr/lib/picolisp
$ ln -s /usr/lib/picolisp/bin/picolisp /usr/bin/picolisp
#!/usr/bin/picolisp /usr/lib/picolisp/lib.l
(prinl "Hello world!")
(bye)
If you write this into a text file, and use chmod to set it to
"executable", it can be executed like any other command. Note that (because
# is the comment character in PicoLisp) the first line will not be
interpreted, and you can still use that file as a normal command line argument
to PicoLisp (useful during debugging).
The fact that a hyphen causes evaluation of command line arguments can be
used to simulate something like command line options. The following script
defines two functions a and f, and then calls
(load T) to process the rest of the
command line (which otherwise would be ignored because of the (bye) statement):
#!/usr/bin/picolisp /usr/lib/picolisp/lib.l
(de a ()
(println '-a '-> (opt)) )
(de f ()
(println '-f '-> (opt)) )
(load T)
(bye)
opt retrieves the next command line
option)
Calling this script (let's say we named it "testOpts") gives:
$ ./testOpts -f abc
-f -> "abc"
$ ./testOpts -a xxx -f yyy
-a -> "xxx"
-f -> "yyy"
We have to be aware of the fact, however, that the aggregation of arguments like
$ ./testOpts -axxx -fyyy
or
$ ./testOpts -af yyy
cannot be achieved with this simple and general mechanism of command line processing.
Utilities are typically used outside the context of the PicoLisp environment. All examples above assumed that the current working directory is the PicoLisp installation directory, which is usually all right for applications developed in that environment. Command line file arguments like "app/file1.l" will be properly found.
To allow utilities to run in arbitrary places on the host file system, the
concept of home directory substitution was introduced. The interpreter
remembers internally at start-up the pathname of its first argument (usually
"lib.l"), and substitutes any leading "@" character in subsequent
file names with that pathname. Thus, to run the above example in some other
place, simply write:
$ /home/foo/picolisp/dbg @app/file1.l -main @app/file2.l
that is, supply a full path name to the initial command (here 'p'), or put it
into your PATH variable, and prefix each file which has to be
loaded from the PicoLisp home directory with a @ character.
"Normal" files (not prefixed by @) will be opened or created
relative to the current working directory as usual.
Stand-alone scripts will often want to load additional modules from the PicoLisp environment, beyond the "lib.l" we provided in the first line of the hello-world script. Typically, at least a call to
(load "@lib/misc.l")
(note the home directory substitution) will be included near the beginning of the script.
As a more complete example, here is a script which extracts the date, name and size of the latest official PicoLisp release version from the download web site, and prints it to standard output:
#!/usr/bin/picolisp /usr/lib/picolisp/lib.l
(load "@lib/misc.l" "@lib/http.l")
(use (@Date @Name @Size)
(when
(match
'(@Date " " "-" " " @Name " " "(" @Size ")")
(client "software-lab.de" 80 "down.html"
(from "Release Archive")
(from ".tgz">")
(till "") ) )
(prinl @Name)
(prinl @Date " -- " @Size) ) )
(bye)
We recommend that you have a terminal window open, and try the examples by
yourself. You may either type them in, directly to the PicoLisp interpreter, or
edit a separate source file (e.g. "@doc/fun.l") in a second
terminal window and load it into PicoLisp with
: (load "@doc/fun.l")
each time you have modified and saved it.
Once a function is loaded from a source file, you can call 'vim' directly on that function with
: (vi 'fact)
The function 'vi' opens the appropriate source file, and jumps to the right line where 'fact' is defined. When you modify it, you can simply call 'ld' to (re)load that source file
: (ld)
The PicoLisp object model is very simple, yet flexible and powerful. Objects as well as classes are both implemented as symbols. In fact, there is no formal difference between objects and classes; classes are more a conceptual design consideration in the head of the programmer than a physical reality.
Having said this, we declare that normally:
So the main difference between classes and objects is that the former ones
usually are internal symbols. By convention, their names start with a
'+'. Sometimes it makes sense, however, to create named objects (as
global singletons, for example), or even anonymous classes.
Both classes and objects have a list in their value, consisting of method definitions (often empty for objects) and (super)class(es). And both classes and objects have local data in their property lists (often empty for classes). This implies, that any given object (as an instance of a class) may have private (object-local) methods defined.
It is rather difficult to contrive a simple OOP example. We constructed a
hierarchy of geometric shapes, with a base class +Shape and two
subclasses +Rectangle and +Circle.
The source code is included as "@doc/shape.l" in the PicoLisp distribution, so you
don't have to type it in. Just load
the file, or start it from the shell as:
$ pil @doc/shape.l +
Let's look at it piece by piece. Here's the base class:
(class +Shape)
# x y
(dm T (X Y)
(=: x X)
(=: y Y) )
(dm move> (DX DY)
(inc (:: x) DX)
(inc (:: y) DY) )
The first line '(class +Shape)' defines the symbol
+Shape as a class without superclasses. The following method
definitions will go to that class.
The comment '# x y' in the second line is just a convention, to
indicate what instance variables (properties) that class uses. As PicoLisp is a
dynamic language, a class can be extended at runtime with any number of
properties, and there is nothing like a fixed object size or structure. This
comment is a hint of what the programmer thinks to be essential and typical for
that class. In the case of +Shape, x and
y are the coordinates of the shape's origin.
Then we have two method definitions, using the keyword dm for "define method". The first method is
special, in that its name is T. Each time a new object is created,
and a method with that name is found in its class hierarchy, that method will be
executed. Though this looks like a "constructor" in other programming languages,
it should probably better be called "initializer". The T method of
+Shape takes two arguments X and Y, and
stores them in the object's property list.
The second method move> changes the object's origin by adding
the offset values DX and DY to the object's origin.
Now to the first derived class:
(class +Rectangle +Shape)
# dx dy
(dm T (X Y DX DY)
(super X Y)
(=: dx DX)
(=: dy DY) )
(dm area> ()
(* (: dx) (: dy)) )
(dm perimeter> ()
(* 2 (+ (: dx) (: dy))) )
(dm draw> ()
(drawRect (: x) (: y) (: dx) (: dy)) )
+Rectangle is defined as a subclass of +Shape.
The comment '# dx dy' indicates that +Rectangle has a
width and a height in addition to the origin coordinates inherited from
+Shape.
The T method passes the origin coordinates X and
Y to the T method of the superclass
(+Shape), then stores the width and height parameters into
dx and dy.
Next we define the methods area> and
perimeter> which do some obvious calculations, and a method
draw> which is supposed to draw the shape on the screen by
calling some hypothetical function drawRect.
Finally, we define a +Circle class in an analog way, postulating
the hypothetical function drawCircle:
(class +Circle +Shape)
# r
(dm T (X Y R)
(super X Y)
(=: r R) )
(dm area> ()
(*/ (: r) (: r) 31415927 10000000) )
(dm perimeter> ()
(*/ 2 (: r) 31415927 10000000) )
(dm draw> ()
(drawCircle (: x) (: y) (: r)) )
Now we can experiment with geometrical shapes. We create a rectangle at point
(0,0) with a width of 30 and a height of 20, and keep it in the variable
R:
: (setq R (new '(+Rectangle) 0 0 30 20)) # New rectangle
-> $134432824 # returned anonymous symbol
: (show R)
$134432824 (+Rectangle) # Show the rectangle
dy 20
dx 30
y 0
x 0
We see that the symbol $134432824 has a list of classes
'(+Rectangle)' in its value, and the coordinates, width and height
in its property list.
Sending messages to that object
: (area> R) # Calculate area
-> 600
: (perimeter> R) # and perimeter
-> 100
will return the values for area and perimeter, respectively.
Then we move the object's origin:
: (move> R 10 5) # Move 10 right and 5 down
-> 5
: (show R)
$134432824 (+Rectangle)
y 5 # Origin changed (0,0) -> (10,5)
x 10
dy 20
dx 30
Though a method move> wasn't defined for the
+Rectangle class, it is inherited from the +Shape
superclass.
Similarly, we create and use a circle object:
: (setq C (new '(+Circle) 10 10 30)) # New circle
-> $134432607 # returned anonymous symbol
: (show C)
$134432607 (+Circle) # Show the circle
r 30
y 10
x 10
-> $134432607
: (area> C) # Calculate area
-> 2827
: (perimeter> C) # and perimeter
-> 188
: (move> C 10 5) # Move 10 right and 5 down
-> 15
: (show C)
$134432607 (+Circle) # Origin changed (10,10) -> (20,15)
y 15
x 20
r 30
It is also easy to send messages to objects in a list:
: (mapcar 'area> (list R C)) # Get list of areas
-> (600 2827)
: (mapc
'((Shape) (move> Shape 10 10)) # Move all 10 right and down
(list R C) )
-> 25
: (show R)
$134431493 (+Rectangle)
y 15
x 20
dy 20
dx 30
-> $134431493
: (show C)
$134431523 (+Circle)
y 25
x 30
r 30
Assume that we want to extend our shape system. From time to time, we need
shapes that behave exactly like the ones above, but are tied to a fixed
position. That is, they do not change their position even if they receive a
move> message.
One solution would be to modify the move> method in the
+Shape class to a no-operation. But this would require to duplicate
the whole shape hierarchy (e.g. by defining +FixedShape,
+FixedRectangle and +FixedCircle classes).
The PicoLisp Way is the use of Prefix Classes through multiple inheritance. It uses the fact that searching for method definitions is a depth-first, left-to-right search of the class tree. We define a prefix class:
: (class +Fixed)
(dm move> (DX DY)) # A do-nothing method
We can now create a fixed rectangle, and try to move it:
: (setq R (new '(+Fixed +Rectangle) 0 0 30 20)) # '+Fixed' prefix class
-> $134432881
: (move> R 10 5) # Send 'move>' message
-> NIL
: (show R)
$134432881 (+Fixed +Rectangle)
dy 20
dx 30
y 0 # Did not move!
x 0
We see, prefix classes can surgically change the inheritance tree for selected objects or classes.
Alternatively, if fixed rectangles are needed often, it might make sense to
define a new class +FixRect:
: (class +FixRect +Fixed +Rectangle)
-> +FixRect
and then use it directly:
: (setq R (new '(+FixRect) 0 0 30 20))
-> $13455710
PicoLisp has persistent objects built-in as a first class data type. With "first class" we mean not just the ability of being passed around, or returned from functions (that's a matter of course), but that they are a primary data type with their own interpreter tag bits. They are, in fact, a special type of symbolic atoms (called "External Symbols"), that happen to be read from pool file(s) when accessed, and written back automatically when modified.
In all other aspects they are normal symbols. They have a value, a property list and a name.
The name cannot be directly controlled by the programmer, as it is assigned
when the symbol is created. It is an encoded index of the symbol's location in
its database file. In its visual representation (output by the print functions and input by the read functions) it is surrounded by braces.
To make use of external symbols, you need to open a database first:
: (pool "test.db")
If a file with that name did not exist, it got created now. Also created at
the same moment was {1}, the very first symbol in the file. This
symbol is of great importance, and is handled especially by PicoLisp. Therefore
a global constant *DB exists, which
points to that symbol {1}, which should be used exclusively to
access the symbol {1}, and which should never be modified by the
programmer.
: *DB # The value of '*DB'
-> {1} # is '{1}'
: (show *DB)
{1} NIL # Value of '{1}' is NIL, property list empty
Now let's put something into the value and property list of {1}.
: (set *DB "Hello world") # Set value of '{1}' to a transient symbol (string)
-> "Hello world"
: (put *DB 'a 1) # Property 'a' to 1
-> 1
: (put *DB 'b 2) # Property 'b' to 2
-> 2
: (show *DB) # Now show the symbol '{1}'
{1} "Hello world"
b 2
a 1
Note that instead of '(set *DB "Hello world")', we might
also have written '(setq {1} "Hello world")', and instead of
'(put *DB 'a 1)' we might have written '(put '{1} 'a
1)'. This would have the same effect, but as a rule external symbols
should never be be accessed literally in application programs, because the
garbage collector might not be able to free these symbols and all symbols
connected to them (and that might well be the whole database). It is all right,
however, to access external symbols literally during interactive debugging.
Now we can create our first own external symbol. This can be done with
new when a T argument is
supplied:
: (new T)
-> {2} # Got a new symbol
We store it in the database root {1}:
: (put *DB 'newSym '{2}) # Literal '{2}' (ok during debugging)
-> {2}
: (show *DB)
{1} "Hello world"
newSym {2} # '{2}' is now stored in '{1}'
b 2
a 1
Put some property value into '{2}'
: (put *DB 'newSym 'x 777) # Put 777 as 'x'-property of '{2}'
-> 777
: (show *DB 'newSym) # Show '{2}' (indirectly)
{2} NIL
x 777
-> {2}
: (show '{2}) # Show '{2}' (directly)
{2} NIL
x 777
All modifications to - and creations of - external symbols done so far are
not written to the database yet. We could call rollback (or simply exit PicoLisp) to undo
all the changes. But as we want to keep them:
: (commit) # Commit all changes
-> T
: (bye) # Exit picolisp
$ # back to the shell
So, the next time when ..
$ pil + # .. we start PicoLisp
: (pool "test.db") # and open the database file,
-> T
: (show *DB) # our two symbols are there again
{1} "Hello world"
newSym {2}
b 2
a 1
-> {1}
: (show *DB 'newSym)
{2} NIL
x 777
-> {2}
To a database, there is more than just persistence. PicoLisp includes an entity/relation class framework (see also Database) which allows a close mapping of the application data structure to the database.
We provided a simple yet complete database and GUI demo application in
@doc/family.tgz and @doc/family64.tgz. Please unpack the first one if
you use a 32-bit system, and the second one on a 64-bit system. Both contain the
sources in @doc/family.l, and an initial
database in the "family/" subdirectory.
To use it, please unpack it first in your current working directory, then start it up in the following way:
$ pil family.l -main +
:
This loads the source file, initializes the database by calling the
main function, and prompts for user input.
The data model is small and simple. We define a class +Person
and two subclasses +Man and +Woman.
(class +Person +Entity)
+Person is a subclass of the +Entity system class. Usually all objects in
a database are of a direct or indirect subclass of +Entity. We can then define the relations to
other data with the rel function.
(rel nm (+Need +Sn +Idx +String)) # Name
This defines the name property (nm) of a person. The first
argument to rel is always a list of relation classes (subclasses of
+relation), optionally followed
by further arguments, causing relation daemon objects be created and stored in
the class definition. These daemon objects control the entity's behavior later
at runtime.
Relation daemons are a kind of metadata, controlling the interactions between entities, and maintaining database integrity. Like other classes, relation classes can be extended and refined, and in combination with proper prefix classes a fine-grained description of the application's structure can be produced.
Besides primitive relation classes, like +Number,
+String or +Date, there are
+Link (unidirectional link),
+Joint (bidirectional link) or +Hook (object-local
index trees)
+Bag)
+List prefix class
+Blob class for "binary large objects"
+Key (unique
index), +Ref (non-unique index) or +Idx (full text
index)
+Sn (modified soundex algorithm [knuth73]
for tolerant searches)
+Need prefix class, for existence checks
+Dep prefix class controlling dependencies between other
relations
In the case of the person's name (nm) above, the relation object
is of type (+Need +Sn +Idx +String). Thus, the name of each person
in this demo database is a mandatory attribute (+Need), searchable
with the soundex algorithm (+Sn) and a full index
(+Idx) of type +String.
(rel pa (+Joint) kids (+Man)) # Father
(rel ma (+Joint) kids (+Woman)) # Mother
(rel mate (+Joint) mate (+Person)) # Partner
The attributes for father (pa), Mother
(ma) and partner (mate) are all defined as
+Joints. A +Joint is probably the most powerful
relation mechanism in PicoLisp; it establishes a bidirectional link between two
objects.
The above declarations say that the father (pa) attribute
points to an object of type +Man, and is joined with that object's
kids attribute (which is a list of joints back to all his
children).
The consistency of +Joints is maintained automatically by the
relation daemons. These become active whenever a value is stored to a person's
pa, ma, mate or kids
property.
For example, interesting things happen when a person's mate is
changed to a new value. Then the mate property of the old mate's
object is cleared (she has no mate after that). Now when the person pointed to
by the new value already has a mate, then that mate's mate property
gets cleared, and the happy new two mates now get their joints both set
correctly.
The programmer doesn't have to care about all that. He just declares these
relations as +Joints.
The last four attributes of person objects are just static data:
(rel job (+Ref +String)) # Occupation
(rel dat (+Ref +Date)) # Date of birth
(rel fin (+Ref +Date)) # Date of death
(rel txt (+String)) # Info
They are all searchable via a non-unique index (+Ref). Date
values in PicoLisp are just numbers, representing the day number (starting first
of March of the year zero).
A method url> is defined:
(dm url> ()
(list "!person" '*ID This) )
It is needed later in the GUI, to cause a click on a link to switch to that object.
The classes +Man and +Woman are subclasses of
+Person:
(class +Man +Person)
(rel kids (+List +Joint) pa (+Person)) # Children
(class +Woman +Person)
(rel kids (+List +Joint) ma (+Person)) # Children
They inherit everything from +Person, except for the
kids attribute. This attribute joins with the pa or
ma attribute of the child, depending on the parent's gender.
That's the whole data model for our demo database application.
It is followed by a call to dbs
("database sizes"). This call is optional. If it is not present, the whole
database will reside in a single file, with a block size of 256 bytes. If it is
given, it should specify a list of items, each having a number in its CAR, and a
list in its CDR. The CARs taken together will be passed later to pool, causing an individual database file with that
size to be created. The CDRs tell what entity classes (if an item is a symbol)
or index trees (if an item is a list with a class in its CAR and a list of
relations in its CDR) should be placed into that file.
A handful of access functions is provided, that know about database relationships and thus allows higher-level access modes to the external symbols in a database.
For one thing, the B-Trees created and maintained by the index daemons can be used directly. Though this is rarely done in a typical application, they form the base mechanisms of other access modes and should be understood first.
The function tree returns the tree
structure for a given relation. To iterate over the whole tree, the functions
iter and scan can be used:
(iter (tree 'dat '+Person) '((P) (println (datStr (get P 'dat)) (get P 'nm))))
"1770-08-03" "Friedrich Wilhelm III"
"1776-03-10" "Luise Augusta of Mecklenburg-Strelitz"
"1797-03-2m perimeter> ()
(*/ 2 (: r) 31415927 10000000) )
(dm draw> ()
(drawCircle (: x) (: y) (: r)) )
Now we can experiment with geometrical shapes. We create a rectangle at point
(0,0) with a width of 30 and a height of 20, and keep it in the variable
R:
: (setq R (new '(+Rectangle) 0 0 30 20)) # New rectangle
-> $134432824 # returned anonymous symbol
: (show R)
$134432824 (+Rectangle) # Show the rectangle
dy 20
dx 30
y 0
x 0
We see that the symbol $134432824 has a list of classes
'(+Rectangle)' in its value, and the coordinates, width and height
in its property list.
Sending messages to that object
: (area> R) # Calculate area
-> 600
: (perimeter> R) # and perimeter
-> 100
will return the values for area and perimeter, respectively.
Then we move the object's origin:
: (move> R 10 5) # Move 10 right and 5 down
-> 5
: (show R)
$134432824 (+Rectangle)
y 5 # Origin changed (0,0) -> (10,5)
x 10
dy 20
dx 30
Though a method move> wasn't defined for the
+Rectangle class, it is inherited from the +Shape
superclass.
Similarly, we create and use a circle object:
: (setq C (new '(+Circle) 10 10 30)) # New circle
-> $134432607 # returned anonymous symbol
: (show C)
$134432607 (+Circle) # Show the circle
r 30
y 10
x 10
-> $134432607
: (area> C) # Calculate area
-> 2827
: (perimeter> C) # and perimeter
-> 188
: (move> C 10 5) # Move 10 right and 5 down
-> 15
: (show C)
$134432607 (+Circle) # Origin changed (10,10) -> (20,15)
y 15
x 20
r 30
It is also easy to send messages to objects in a list:
: (mapcar 'area> (list R C)) # Get list of areas
-> (600 2827)
: (mapc
'((Shape) (move> Shape 10 10)) # Move all 10 right and down
(list R C) )
-> 25
: (show R)
$134431493 (+Rectangle)
y 15
x 20
dy 20
dx 30
-> $134431493
: (show C)
$134431523 (+Circle)
y 25
x 30
r 30
Assume that we want to extend our shape system. From time to time, we need
shapes that behave exactly like the ones above, but are tied to a fixed
position. That is, they do not change their position even if they receive a
move> message.
One solution would be to modify the move> method in the
+Shape class to a no-operation. But this would require to duplicate
the whole shape hierarchy (e.g. by defining +FixedShape,
+FixedRectangle and +FixedCircle classes).
The PicoLisp Way is the use of Prefix Classes through multiple inheritance. It uses the fact that searching for method definitions is a depth-first, left-to-right search of the class tree. We define a prefix class:
: (class +Fixed)
(dm move> (DX DY)) # A do-nothing method
We can now create a fixed rectangle, and try to move it:
: (setq R (new '(+Fixed +Rectangle) 0 0 30 20)) # '+Fixed' prefix class
-> $134432881
: (move> R 10 5) # Send 'move>' message
-> NIL
: (show R)
$134432881 (+Fixed +Rectangle)
dy 20
dx 30
y 0 # Did not move!
x 0
We see, prefix classes can surgically change the inheritance tree for selected objects or classes.
Alternatively, if fixed rectangles are needed often, it might make sense to
define a new class +FixRect:
: (class +FixRect +Fixed +Rectangle)
-> +FixRect
and then use it directly:
: (setq R (new '(+FixRect) 0 0 30 20))
-> $13455710
PicoLisp has persistent objects built-in as a first class data type. With "first class" we mean not just the ability of being passed around, or returned from functions (that's a matter of course), but that they are a primary data type with their own interpreter tag bits. They are, in fact, a special type of symbolic atoms (called "External Symbols"), that happen to be read from pool file(s) when accessed, and written back automatically when modified.
In all other aspects they are normal symbols. They have a value, a property list and a name.
The name cannot be directly controlled by the programmer, as it is assigned
when the symbol is created. It is an encoded index of the symbol's location in
its database file. In its visual representation (output by the print functions and input by the read functions) it is surrounded by braces.
To make use of external symbols, you need to open a database first:
: (pool "test.db")
If a file with that name did not exist, it got created now. Also created at
the same moment was {1}, the very first symbol in the file. This
symbol is of great importance, and is handled especially by PicoLisp. Therefore
a global constant *DB exists, which
points to that symbol {1}, which should be used exclusively to
access the symbol {1}, and which should never be modified by the
programmer.
: *DB # The value of '*DB'
-> {1} # is '{1}'
: (show *DB)
{1} NIL # Value of '{1}' is NIL, property list empty
Now let's put something into the value and property list of {1}.
: (set *DB "Hello world") # Set value of '{1}' to a transient symbol (string)
-> "Hello world"
: (put *DB 'a 1) # Property 'a' to 1
-> 1
: (put *DB 'b 2) # Property 'b' to 2
-> 2
: (show *DB) # Now show the symbol '{1}'
{1} "Hello world"
b 2
a 1
Note that instead of '(set *DB "Hello world")', we might
also have written '(setq {1} "Hello world")', and instead of
'(put *DB 'a 1)' we might have written '(put '{1} 'a
1)'. This would have the same effect, but as a rule external symbols
should never be be accessed literally in application programs, because the
garbage collector might not be able to free these symbols and all symbols
connected to them (and that might well be the whole database). It is all right,
however, to access external symbols literally during interactive debugging.
Now we can create our first own external symbol. This can be done with
new when a T argument is
supplied:
: (new T)
-> {2} # Got a new symbol
We store it in the database root {1}:
: (put *DB 'newSym '{2}) # Literal '{2}' (ok during debugging)
-> {2}
: (show *DB)
{1} "Hello world"
newSym {2} # '{2}' is now stored in '{1}'
b 2
a 1
Put some property value into '{2}'
: (put *DB 'newSym 'x 777) # Put 777 as 'x'-property of '{2}'
-> 777
: (show *DB 'newSym) # Show '{2}' (indirectly)
{2} NIL
x 777
-> {2}
: (show '{2}) # Show '{2}' (directly)
{2} NIL
x 777
All modifications to - and creations of - external symbols done so far are
not written to the database yet. We could call rollback (or simply exit PicoLisp) to undo
all the changes. But as we want to keep them:
: (commit) # Commit all changes
-> T
: (bye) # Exit picolisp
$ # back to the shell
So, the next time when ..
$ pil + # .. we start PicoLisp
: (pool "test.db") # and open the database file,
-> T
: (show *DB) # our two symbols are there again
{1} "Hello world"
newSym {2}
b 2
a 1
-> {1}
: (show *DB 'newSym)
{2} NIL
x 777
-> {2}
To a database, there is more than just persistence. PicoLisp includes an entity/relation class framework (see also Database) which allows a close mapping of the application data structure to the database.
We provided a simple yet complete database and GUI demo application in
@doc/family.tgz and @doc/family64.tgz. Please unpack the first one if
you use a 32-bit system, and the second one on a 64-bit system. Both contain the
sources in @doc/family.l, and an initial
database in the "family/" subdirectory.
To use it, please unpack it first in your current working directory, then start it up in the following way:
$ pil family.l -main +
:
This loads the source file, initializes the database by calling the
main function, and prompts for user input.
The data model is small and simple. We define a class +Person
and two subclasses +Man and +Woman.
(class +Person +Entity)
+Person is a subclass of the +Entity system class. Usually all objects in
a database are of a direct or indirect subclass of +Entity. We can then define the relations to
other data with the rel function.
(rel nm (+Need +Sn +Idx +String)) # Name
This defines the name property (nm) of a person. The first
argument to rel is always a list of relation classes (subclasses of
+relation), optionally followed
by further arguments, causing relation daemon objects be created and stored in
the class definition. These daemon objects control the entity's behavior later
at runtime.
Relation daemons are a kind of metadata, controlling the interactions between entities, and maintaining database integrity. Like other classes, relation classes can be extended and refined, and in combination with proper prefix classes a fine-grained description of the application's structure can be produced.
Besides primitive relation classes, like +Number,
+String or +Date, there are
+Link (unidirectional link),
+Joint (bidirectional link) or +Hook (object-local
index trees)
+Bag)
+List prefix class
+Blob class for "binary large objects"
+Key (unique
index), +Ref (non-unique index) or +Idx (full text
index)
+Sn (modified soundex algorithm [knuth73]
for tolerant searches)
+Need prefix class, for existence checks
+Dep prefix class controlling dependencies between other
relations
In the case of the person's name (nm) above, the relation object
is of type (+Need +Sn +Idx +String). Thus, the name of each person
in this demo database is a mandatory attribute (+Need), searchable
with the soundex algorithm (+Sn) and a full index
(+Idx) of type +String.
(rel pa (+Joint) kids (+Man)) # Father
(rel ma (+Joint) kids (+Woman)) # Mother
(rel mate (+Joint) mate (+Person)) # Partner
The attributes for father (pa), Mother
(ma) and partner (mate) are all defined as
+Joints. A +Joint is probably the most powerful
relation mechanism in PicoLisp; it establishes a bidirectional link between two
objects.
The above declarations say that the father (pa) attribute
points to an object of type +Man, and is joined with that object's
kids attribute (which is a list of joints back to all his
children).
The consistency of +Joints is maintained automatically by the
relation daemons. These become active whenever a value is stored to a person's
pa, ma, mate or kids
property.
For example, interesting things happen when a person's mate is
changed to a new value. Then the mate property of the old mate's
object is cleared (she has no mate after that). Now when the person pointed to
by the new value already has a mate, then that mate's mate property
gets cleared, and the happy new two mates now get their joints both set
correctly.
The programmer doesn't have to care about all that. He just declares these
relations as +Joints.
The last four attributes of person objects are just static data:
(rel job (+Ref +String)) # Occupation
(rel dat (+Ref +Date)) # Date of birth
(rel fin (+Ref +Date)) # Date of death
(rel txt (+String)) # Info
They are all searchable via a non-unique index (+Ref). Date
values in PicoLisp are just numbers, representing the day number (starting first
of March of the year zero).
A method url> is defined:
(dm url> ()
(list "!person" '*ID This) )
It is needed later in the GUI, to cause a click on a link to switch to that object.
The classes +Man and +Woman are subclasses of
+Person:
(class +Man +Person)
(rel kids (+List +Joint) pa (+Person)) # Children
(class +Woman +Person)
(rel kids (+List +Joint) ma (+Person)) # Children
They inherit everything from +Person, except for the
kids attribute. This attribute joins with the pa or
ma attribute of the child, depending on the parent's gender.
That's the whole data model for our demo database application.
It is followed by a call to dbs
("database sizes"). This call is optional. If it is not present, the whole
database will reside in a single file, with a block size of 256 bytes. If it is
given, it should specify a list of items, each having a number in its CAR, and a
list in its CDR. The CARs taken together will be passed later to pool, causing an individual database file with that
size to be created. The CDRs tell what entity classes (if an item is a symbol)
or index trees (if an item is a list with a class in its CAR and a list of
relations in its CDR) should be placed into that file.
A handful of access functions is provided, that know about database relationships and thus allows higher-level access modes to the external symbols in a database.
For one thing, the B-Trees created and maintained by the index daemons can be used directly. Though this is rarely done in a typical application, they form the base mechanisms of other access modes and should be understood first.
The function tree returns the tree
structure for a given relation. To iterate over the whole tree, the functions
iter and scan can be used:
(iter (tree 'dat '+Person) '((P) (println (datStr (get P 'dat)) (get P 'nm))))
"1770-08-03" "Friedrich Wilhelm III"
"1776-03-10" "Luise Augusta of Mecklenburg-Strelitz"
"1797-03-2m perimeter> ()
(*/ 2 (: r) 31415927 10000000) )
(dm draw> ()
(drawCircle (: x) (: y) (: r)) )
Now we can experiment with geometrical shapes. We create a rectangle at point
(0,0) with a width of 30 and a height of 20, and keep it in the variable
R:
: (setq R (new '(+Rectangle) 0 0 30 20)) # New rectangle
-> $134432824 # returned anonymous symbol
: (show R)
$134432824 (+Rectangle) # Show the rectangle
dy 20
dx 30
y 0
x 0
We see that the symbol $134432824 has a list of classes
'(+Rectangle)' in its value, and the coordinates, width and height
in its property list.
Sending messages to that object
: (area> R) # Calculate area
-> 600
: (perimeter> R) # and perimeter
-> 100
will return the values for area and perimeter, respectively.
Then we move the object's origin:
: (move> R 10 5) # Move 10 right and 5 down
-> 5
: (show R)
$134432824 (+Rectangle)
y 5 # Origin changed (0,0) -> (10,5)
x 10
dy 20
dx 30
Though a method move> wasn't defined for the
+Rectangle class, it is inherited from the +Shape
superclass.
Similarly, we create and use a circle object:
: (setq C (new '(+Circle) 10 10 30)) # New circle
-> $134432607 # returned anonymous symbol
: (show C)
$134432607 (+Circle) # Show the circle
r 30
y 10
x 10
-> $134432607
: (area> C) # Calculate area
-> 2827
: (perimeter> C) # and perimeter
-> 188
: (move> C 10 5) # Move 10 right and 5 down
-> 15
: (show C)
$134432607 (+Circle) # Origin changed (10,10) -> (20,15)
y 15
x 20
r 30
It is also easy to send messages to objects in a list:
: (mapcar 'area> (list R C)) # Get list of areas
-> (600 2827)
: (mapc
'((Shape) (move> Shape 10 10)) # Move all 10 right and down
(list R C) )
-> 25
: (show R)
$134431493 (+Rectangle)
y 15
x 20
dy 20
dx 30
-> $134431493
: (show C)
$134431523 (+Circle)
y 25
x 30
r 30
Assume that we want to extend our shape system. From time to time, we need
shapes that behave exactly like the ones above, but are tied to a fixed
position. That is, they do not change their position even if they receive a
move> message.
One solution would be to modify the move> method in the
+Shape class to a no-operation. But this would require to duplicate
the whole shape hierarchy (e.g. by defining +FixedShape,
+FixedRectangle and +FixedCircle classes).
The PicoLisp Way is the use of Prefix Classes through multiple inheritance. It uses the fact that searching for method definitions is a depth-first, left-to-right search of the class tree. We define a prefix class:
: (class +Fixed)
(dm move> (DX DY)) # A do-nothing method
We can now create a fixed rectangle, and try to move it:
: (setq R (new '(+Fixed +Rectangle) 0 0 30 20)) # '+Fixed' prefix class
-> $134432881
: (move> R 10 5) # Send 'move>' message
-> NIL
: (show R)
$134432881 (+Fixed +Rectangle)
dy 20
dx 30
y 0 # Did not move!
x 0
We see, prefix classes can surgically change the inheritance tree for selected objects or classes.
Alternatively, if fixed rectangles are needed often, it might make sense to
define a new class +FixRect:
: (class +FixRect +Fixed +Rectangle)
-> +FixRect
and then use it directly:
: (setq R (new '(+FixRect) 0 0 30 20))
-> $13455710
PicoLisp has persistent objects built-in as a first class data type. With "first class" we mean not just the ability of being passed around, or returned from functions (that's a matter of course), but that the