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REPL

NAME

REPL - a more easily configurable REPL

SYNOPSIS 


use REPL;

# Start a Read, Evaluate, Print Loop
repl;

DESCRIPTION

The REPL module is a re-imagining of the REPL (Read, Evaluate, Print Loop) functionality as offered by Rakudo before the 2024.11 release. It provides both a programmable interface, as well as a ready made "repl" CLI.

COMMAND LINE INTERFACE 


# Simplest invocation
$ repl

# Pre-loading module Foo from local distribution
$ repl -I. -MFoo

# Invocation with some custom settings
$ repl --the-prompt='[:index:] :HHMM:' --symbols=🦋,🔥

[0] 20:51 🦋 if 42 {
[0] 20:51 🔥 say "foo"
[0] 20:51 🔥 }
foo
[0] 20:52 🦋

The REPL command-line interface can be invoked with named arguments that have the same name as the named arguments to the REPL.new call. They are:

  • --editor - the editor to use (default: Any)

  • --header - whether to show the full header (default: yes)

  • --multi-line-ok - can expression span more than one line (default: yes)

  • --output-method - the output method to be used (default: gist)

  • --symbols - symbols for prompt state (default: ">,*")

  • --the-prompt - string for prompt (default: "[:index:] :symbol: ")

It is also possible to specify -I and -M arguments for pre-loading any module while starting the REPL.

Any other command-line arguments will be ignored.

SUBROUTINES

repl 


repl;

The repl subroutine creates a new REPL object for that context and returns that. When called in sink context, it will activate the interactive Read, Evaluate, Print Loop. This is the most common usage when debugging your code. 

#line 666 foo.raku
if 42 -> $answer {
    repl "Looking for clues", :$answer;
}

Looking for clues
answer: 42
[0] >

Note that when the repl subroutine is used in a debugging mode, it will not show any header, other than the one that is specified with the positional argument (in this case "Looking for clues").

If no explicit heafer is specified, it will show the location of the callframe that the code is currently in (in this case that would have been: line 666 in the file "foo.raku").

Finally it will show the value of any named arguments to help you debug your code (in this case "answer: 32").

It is also possible to save the REPL object at one place in the code, and actually run the REPL at a later time in another scope.

If you just press ENTER when using the REPL as a debugging aid in this manner, then it will be interpreted the same as "=quit" for convenience (especially if the repl sub is called in a loop). 


my $repl = do {
    my $a = 42;
    repl
}

# ...

# activate REPL later, allowing access to $a even though that
# variable is no longer in scope
$repl.run;

standard-completions

The standard-completions subroutine provides the standard completion logic if no :additional-completions argument has been specified with either REPL.new or the repl subroutine.

It is provided to allow it to be added with the <:additional-completions> argument, and to allow testing of the standard completion logic.

uniname-words

The uniname-words subroutine provides the same functionality as the uniname-words subroutine provided by the uniname-words distribution if that distribution is installed.

Otherwise it will always return Nil.

RUNNING A REPL

The REPL role is what usually gets punned into a class. 


my $repl = REPL.new;

$repl.run;

Same as above, but with all named arguments spelled out: 


my $repl = REPL.new:
  :editor(Any),    # or "Readline", "LineEditor", "Linenoise"
  :output-method<gist>,  # or "Str", "raku"
  :out = $*OUT,
  :err = $*ERR,
  :val = $*OUT,
  :header,
  :multi-line-ok,
  :the-prompt("[:index:] :symbol: "),
  :symbols(">", "*"),
  :is-win($*DISTRO.is-win),
  :@completions,
  :@additional-completions,
  :compiler<Raku>,
;

The REPL role embodies the information needed to run a Read, Evaluate, Print Loop. It allows for these named arguments:

:editor

Optional. String indicating which editor logic to be used.

If the INSIDE_EMACS environment variable is set with a true value, then the Fallback editor will be used.

If it can be determined from the environment that the process is running inside the "rlwrap" wrapper, then the Fallback editor will be used.

If the RAKUDO_LINE_EDITOR environment variable is set, then its contents will be assumed as an indication of preference and will first be tried. If that fails, an error message will be shown.

Whatever is then the value, that value will be used to create a Prompt object.

:output-method

String. The name of the method to be called to display the value of an expression. This defaults to the value of the RAKU_REPL_OUTPUT_METHOD environment variable, or to "gist" if that has not been specified.

Used value available with the .output-method method.

:header

Boolean. Indicate whether to show the REPL header upon entry. Defaults to True.

Used value available with the .header method.

:out

Optional. The :out named argument specifies the value of $*OUT whenever a command is executed. If not specified, or specified with an undefined value, will assume the value of $*OUT at command execution time.

:err

Optional. The :err named argument specifies the value of $*ERR whenever a command is executed. If not specified, or specified with an undefined value, will assume the value of $*ERR at command execution time.

:multi-line-ok

Optional, Boolean. Indicate whether it is ok to interprete multiple lines of input as a single statement to evaluate. Defaults to True unless the RAKUDO_DISABLE_MULTILINE environment variable has been specified with a true value.

Used value available with the .multi-line-ok method.

:the-prompt

Optional. Specifies what will be shown to the user before the user can enter characters. Defaults to what has been specified with the RAKUDO_REPL_PROMPT environment variable, or "[:index:] :symbol: ".

Supports all of the expansions offered by the Prompt::Expand distribution.

Note that if there is a prompt (implicitely) specified, the string ":symbol: " will be added if there is no ":symbol:" specified, to make sure the user actually sees a prompting symbol, and to make specifying a user prompt a bit easier.

The expanded prompt is also available with the .the-prompt method.

:symbols

Optional. Specifies the symbols that should be used for the ":symbol:" placeholder in the different REPL states. Defaults to what has been specified as a comma-separated list with the RAKUDO_REPL_SYMBOLS environment variable. Defaults to ">", "*" if that is not specified.

Currently the following states are recognized:

  • 0 - accepting expression to be evaluated

  • 1 - previous expression not complete, accepting continuation

:is-win

Boolean. Indicate whether certain OS dependent checks should assume Windows semantics. Defaults to $*DISTRO.is-win.

Used value available with the .is-win method.

:completions

A List of strings to be used tab-completions. If none are specified, then a default Raku set of completions will be used.

Used value available with the .completions method.

:additional-completions 


# completion that uppercases whole line if ended with a !
sub shout($line, $pos) {
    ($line.chop.uc,) if $pos == $line.chars && $line.ends-with("!")
}

my $repl = REPL.new(:additional-completions(&shout));

A List of Callables to be called to produce tab-completions. If none are specified, the standard-completions will be assumed.

Each Callable is expected to accept two positional arguments: the line that has been entered so far, and the position of the cursor. It is expected to return a (potentially) empty List with the new state of the line (so including everything before and after the completion).

:prompt

The Prompt object to be used. If specified, overrides anything related to the <C:editor> named argument. If not specified, will use whatever was (implicitely) specified with :editor.

:compiler

String. The HLL compiler to be used. This defaults to "Raku", which is the only compiler supported at this time.

Used value available with the .compiler method.

method run

Actually run the REPL.

OTHER METHODS

err

The object to be used for error output. Defaults to $*ERR.

history

Expected to take no arguments, and return an object that represents the (possibly persistent) history of the REPL's interactive sessions.

By default it will first look for a RAKUDO_HIST environment variable and return an IO::Path object for that. If that environment variable is not specified, will check the $*HOME and $*TMPDIR dynamic variables for the existence of a .raku subdirectory in that. If found, will return an IO::Path for the "rakudo-history" file in that subdirectory and try to create that if it didn't exist yet (and produce an error message if that failed).

out

The object to be used for standard output. Defaults to $*OUT.

prompt

The Prompt object to be used when obtaining input from the user. Also handles the read, readline, load-history, add-history, save-history and editor-name methods.

supports-completions

Returns a Bool indicating whether the selected editor supports completions.

teardown

Expected to take no arguments. Will be called whenever the user indicates that they want to exit the REPL. Will call the save-history method by default.

val

The object to be used for outputting values that weren't shown already. Defaults to $*OUT.

USER COMMANDS

The following REPL commands are currently supported: if a command is not recognized, then Raku code will be assumed and executed if possible. Note that all REPL commands start with "=" to prevent confusion with possibly legal Raku code.

completions

Shows an introduction to completions. Can be shortened to "=com".

context

Allows creation of and switching between two or more contexts: "new" creates a new context and switches to it, "switch" switches to an already existing context, and "list" shows the available contexts. Can be shortened to "=con".

edit

Edit the file given, or the last file that was saved with =write. Can be shortened to "=ed".

exit

Leaves the REPL. Can be shortened all the way to "=ex".

help

Shows a list of available commands. Can be shortened all the way to "=h".

info

Shows the name of the editor logic being used. Can be shortened to to "=inf".

introduction

Shows a general introduction. Can be shortened to "=int".

output [method]

Shows the current output method. If a second argument is specified, it indicates the name of the output method to be used from now on. Typical values are "raku", "Str", "gist". Can be shortened all the way to "=o".

quit

Leaves the REPL. Can be shortened all the way to "=q", and is thus the shortest way to leave the REPL with a REPL command.

read [path]

Read the code from the file with the indicated path and compiles and executes it. Remembers the path name so that subsequent =write commands need not have it specified.

reset

Reset the status of the REPL as if it was freshly entered.

stack

Show the caller stack from where the REPL has been called. Only makes sense if the REPL is being called from within a program, rather than from the command line. Can be shortened to "=s".

write [path]

Write all lines entered that did not produce any output to the indicated path. Remembers the path name from =read and =write so that subsequent =write commands need not have it specified.

TAB COMPLETIONS

If the supports-completions method returns True, the standard tab-completion logic will provide:

  • all relevant items from the CORE:: namespace

  • any relevant items from the direct context (such as a REPL command)

  • \123 wil change integer value to superscript: ¹²³, subscript: ₁₂₃

  • foo! will tab-complete to FOO, foo, Foo

Additionally, if the uniname-words module is installed:

ENVIRONMENT VARIABLES

These environment variables will override default settings.

RAKUDO_REPL_PROMPT

The prompt shown to the user. May contain escape sequences as supported by the Prompt.expand method.

RAKUDO_REPL_SYMBOLS

A comma separated list of symbols representing the states of the REPL. Defaults to >,*>.

RAKUDO_HIST

Path where the history file is / should be stored.

RAKU_REPL_OUTPUT_METHOD

The name of the method with which to show results to the user.

RAKUDO_DISABLE_MULTILINE

Whether multi-line evaluations should be disabled or not.

AUTHOR

Elizabeth Mattijsen <[email protected]>

Source can be located at: https://codeberg.org/lizmat/REPL . Comments and Pull Requests are welcome.

If you like this module, or what I'm doing more generally, committing to a small sponsorship would mean a great deal to me!

COPYRIGHT AND LICENSE

Copyright 2024, 2025, 2026 Elizabeth Mattijsen

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.

# vim: expandtab shiftwidth=4

REPL v0.0.25

A more easily configurable REPL

Authors

  • Elizabeth Mattijsen

License

Artistic-2.0

Dependencies

Test Dependencies

Provides

  • REPL

Documentation

The Camelia image is copyright 2009 by Larry Wall. "Raku" is trademark of the Yet Another Society. All rights reserved.