Using the interpreter (historical revision 16521)

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Using the interpreter

Using the interpreter

CHICKEN provides an interpreter named csi for evaluating Scheme programs and expressions interactively.

Interpreter command line format

csi {FILENAME|OPTION}

where FILENAME specifies a file with Scheme source-code. If the extension of the source file is .scm, it may be omitted. The runtime options described in Compiler command line format are also available for the interpreter. If the environment variable CSI_OPTIONS is set to a list of options, then these options are additionally passed to every direct or indirect invocation of csi. Please note that runtime options (like -:...) can not be passed using this method. The options recognized by the interpreter are:

--: Ignore everything on the command-line following this marker. Runtime options (-:...) are still recognized.
-i -case-insensitive: Enables the reader to read symbols case insensitive. The default is to read case sensitive (in violation of R5RS). This option registers the case-insensitive feature identifier.
-b -batch: Quit the interpreter after processing all command line options.
-e -eval EXPRESSIONS: Evaluate EXPRESSIONS. This option implies -batch and -quiet, so no startup message will be printed and the interpreter exits after processing all -eval options and/or loading files given on the command-line.
-p -print EXPRESSIONS: Evaluate EXPRESSIONS and print the results of each expression using print. Implies -batch and -quiet.
-P -pretty-print EXPRESSIONS: Evaluate EXPRESSIONS and print the results of each expression using pretty-print. Implies -batch and -quiet.
-D -feature SYMBOL: Registers SYMBOL to be a valid feature identifier for cond-expand and feature?.
-h -help: Write a summary of the available command line options to standard output and exit.
-I -include-path PATHNAME: Specifies an alternative search-path for files included via the include special form. This option may be given multiple times. If the environment variable CHICKEN_INCLUDE_PATH is set, it should contain a list of alternative include pathnames separated by ;.
-k -keyword-style STYLE: Enables alternative keyword syntax, where STYLE may be either prefix (as in Common Lisp) or suffix (as in DSSSL). Any other value is ignored.
-n -no-init: Do not load initialization-file. If this option is not given and the file ./.csirc or $HOME/.csirc exists, then it is loaded before the read-eval-print loop commences.
-no-parentheses-synonyms STYLE: Disables list delimiter synonyms, [..] and {...} for (...).
-no-symbol-escape: Disables support for escaped symbols, the |...| form.
-w -no-warnings: Disables any warnings that might be issued by the reader or evaluated code.
-q -quiet: Do not print a startup message. Also disables generation of call-trace information for interpreted code.
-r5rs-syntax: Disables the Chicken extensions to R5RS syntax. Does not disable non-standard read syntax.
-s -script PATHNAME: This is equivalent to -batch -quiet -no-init PATHNAME. Arguments following PATHNAME are available by using command-line-arguments and are not processed as interpreter options. Extra options in the environment variable CSI_OPTIONS are ignored.
-sx PATHNAME: The same as -s PATHNAME but prints each expression to (current-error-port) before it is evaluated.
-ss PATHNAME: The same as -s PATHNAME but invokes the procedure main with the value of (command-line-arguments) as its single argument. If the main procedure returns an integer result, then the interpreter is terminated, returning the integer as the status code back to the invoking process. Any other result terminates the interpreter with a zero exit status.
-setup-mode: When locating extension, search the current directory first. By default, extensions are located first in the extension repository, where chicken-install stores compiled extensions and their associated metadata.
-R -require-extension NAME: Equivalent to evaluating (require-extension NAME).
-v -version: Write the banner with version information to standard output and exit.

Writing Scheme scripts

Since UNIX shells use the #! notation for starting scripts, anything following the characters #! is ignored, with the exception of the special symbols #!optional, #!key, #!rest and #!eof.

The easiest way is to use the -script option like this:

% cat foo
#! /usr/local/bin/csi -script
(print (eval (with-input-from-string
                (car (command-line-arguments))
                 read)))

% chmod +x foo
% foo "(+ 3 4)"
7

The parameter command-line-arguments is set to a list of the parameters that were passed to the Scheme script. Scripts can be compiled to standalone executables (don't forget to declare used library units).

CHICKEN supports writing shell scripts in Scheme for other platforms as well, using a slightly different approach. The first example would look like this on Windows:

C:>type foo.bat
@;csibatch %0 %1 %2 %3 %4 %5 %6 %7 %8 %9
(print (eval (with-input-from-string
                (car (command-line-arguments))
                read)))

C:>foo "(+ 3 4)"
7

Like UNIX scripts, batch files can be compiled. Windows batch scripts do not accept more than 8 arguments.

Since it is sometimes useful to run a script in the interpreter without actually executing it (for example to test specific parts of it), the option -ss can be used as an alternative to -script. -ss PATHNAME is equivalent to -script PATHNAME but invokes (main (command-line-arguments)) after loading all top-level forms of the script file. The result of main is returned as the exit status to the shell. Any non-numeric result exits with status zero:

% cat hi.scm
(define (main args)
  (print "Hi, " (car args))
  0)
% csi -ss hi.scm you
Hi, you
% csi -q
#;1> ,l hi.scm
#;2> (main (list "ye all"))
Hi, ye all
0
#;3>

Toplevel commands

The toplevel loop understands a number of special commands:

,?: Show summary of available toplevel commands.
,l FILENAME ...: Load files with given FILENAMEs
,ln FILENAME ...: Load files and print result(s) of each top-level expression.
,p EXP: Pretty-print evaluated expression EXP.
,d EXP: Describe result of evaluated expression EXP.
,du EXP: Dump contents of the result of evaluated expression EXP.
,dur EXP N: Dump N bytes of the result of evaluated expression EXP.
,exn: Describes the last exception that occurred and adds it to the result history (it can be accessed using the # notation).
,q: Quit the interpreter.
,r: Show system information.
,s TEXT ...: Execute shell-command.
,t EXP: Evaluate form and print elapsed time.
,x EXP: Pretty-print macroexpanded expression EXP (the expression is not evaluated).
,tr SYMBOL ...: Enables tracing of the toplevel procedures with the given names.

#;1> (fac 10)                       ==> 3628800
#;2> ,tr fac
#;3> (fac 3)
|(fac 3)
| (fac 2)
|  (fac 1)
|   (fac 0)
|   fac -> 1 
|  fac -> 1 
| fac -> 2 
|fac -> 6                          ==> 6
#;4> ,utr fac
#;5> (fac 3)                        ==> 6

,utr SYMBOL ...: Disables tracing of the given toplevel procedures.
,br SYMBOL ...: Sets a breakpoint at the procedures named SYMBOL .... Breakpoint can also be trigged using the breakpoint procedure.
,ubr SYMBOL ...: Removes breakpoints.
,c: Continues execution from the last invoked breakpoint.
,breakall: Enable breakpoints for all threads (this is the default).
,breakonly THREAD: Enable breakpoints only for the thread returned by the expression THREAD.
,info: Lists traced procedures and breakpoints.
,step EXPR: Evaluates EXPR in single-stepping mode. On each procedure call you will be presented with a menu that allows stepping to the next call, leaving single-stepping mode or triggering a breakpoint. Note that you will see some internal calls, and unsafe or heavily optimized compiled code might not be stepped at all. Single-stepping mode is also possible by invoking the singlestep procedure.

You can define your own toplevel commands using the toplevel-command procedure:

toplevel-command

[procedure] (toplevel-command SYMBOL PROC [HELPSTRING])

Defines or redefines a toplevel interpreter command which can be invoked by entering ,SYMBOL. PROC will be invoked when the command is entered and may read any required argument via read (or read-line). If the optional argument HELPSTRING is given, it will be listed by the ,? command.

History access

The interpreter toplevel accepts the special object #[INDEX] which returns the result of entry number INDEX in the history list. If the expression for that entry resulted in multiple values, the first result (or an unspecified value for no values) is returned. If no INDEX is given (and if a whitespace or closing paranthesis character follows the #, then the result of the last expression is returned. Note that the value returned is implicitly quoted.

set-describer!

[procedure] (set-describer! TAG PROC)

Sets a custom description handler that invokes PROC when the ,d command is invoked with a record-type object that has the type TAG (a symbol). PROC is called with two arguments: the object to be described and an output-port. It should write a possibly useful textual description of the object to the passed output-port. For example:

#;1> (define-record-type point (make-point x y) point?
       (x point-x)
       (y point-y))
#;2> (set-describer! 'point 
       (lambda (pt o)
         (print "a point with x=" (point-x pt) " and y=" (point-y pt))))
#;3> ,d (make-point 1 2)
a point with x=1 and y=2

Auto-completion and edition

On platforms that support it, it is possible to get auto-completion of symbols, history (over different csi sessions) and a more feature-full editor for the expressions you type using the http://www.call-with-current-continuation.org/eggs/readline.html egg by Tony Garnock Jones. It is very useful for interactive use of csi.

To enable it install the egg and put this in your ~/.csirc file:

(use readline regex)
(current-input-port (make-gnu-readline-port))
(gnu-history-install-file-manager 
  (string-append (or (getenv "HOME") ".") "/.csi.history"))

More details are available in the egg's documentation.

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