Language::Expr::Manual::Syntax - Description of the Language::Expr language
This document describes version 0.23 of module Language::Expr::Manual::Syntax (in distribution Language-Expr), released on 2014-05-01.
Language::Expr language is very simple. It has just enough features to support mathematical/logical/string operations, arrays, hashes, variables, and function calls.
Language::Expr is (intentionally) not Turing-complete (lacks assignment and loops).
Language::Expr is a lot like Perl. Differences from Perl, if any, are given after each section.
It should be trivial to implement an interpreter or code generator from the parser. In fact, Language::Expr is meant to be easily convertible to Perl, PHP, and Javascript (among others like Python and Ruby).
undef
true false
Differences from Perl: Perl doesn't support native booleans, but there are modules like boolean which practically make Perl behaves like it does.
1 -2.3 inf nan 0x1f # hexadecimal, = 31 in decimal 0o17 # octal, = 15 in decimal 0b100 # binary, = 4 in decimal
Differences from Perl: octal literals in Expr are written using the less error-prone 0o123 syntax, which is also adopted by Python.
Single-quoted strings, e.g. 'single quoted'. Supported escape sequences:
\' literal single quote \\ literal backslash
Double-quoted strings, e.g. "double quoted". Supported escape sequences:
\' literal single quote \" literal double quote \\ literal backslash \$ prevent variable interpolation \t tab \n newline \r linefeed \f formfeed \b backspace \a bell \e escape \0 or \03 or \033 octal char \x7 or \x7B hex char \x{263a} wide hex char
Double-quoted strings will also interpolate variables, e.g.:
"I have $num apples" "This is Foo::Bar version ${perl:/Foo/Bar/VERSION}"
Differences from Perl: Perl supports a few other escape sequences, but they are not commonly found in other scripting languages (e.g.: named Unicode character or the \l, \L, et al), so they are not included.
[] [1, 2, "str"]
Differences from Perl: dangling comma at the end is not allowed in Expr.
{} {a => 1, "b c" => 2+3}
Differences from Perl: in Expr you must always use "=>" to separate key and value, not comma. Dangling comma at the end is not allowed.
Many operators are taken from Perl, along with their precedence levels and associativity, but here are the differences:
No assigment operators.
Because assignment is deliberately not supported.
No smart-match "~~" operator or the other more esoteric or Perl-specific operators.
These include Perl's "..", "...", "->", etc.
Currently no "and", "or", "not" operators.
Use &&, ||, ! instead. Perl supports an extra, low-precedence set of logical operators mostly to separate statements, which we do not have anyway.
"xor" becomes "^^" (to be consistent with "^" as bitwise-xor).
Hash value is accessed using [] instead of {}, and barewords (e.g. $hash[key] instead of $hash["key"]) are not allowed.
Which, BTW, is also the way it is done in Python, Ruby, and PHP.
Comparison operators can be chained (except <=> and cmp).
Example:
1 < 2 < 3 # true 2 == (1+1) != 2 # false
Below is list of supported operators, ordered from lowest precedence, along with their associativity.
left => left || // ^^ right ?: left && left | ^ left & left == != <=> cmp eq ne < > <= >= ge gt le lt nonassoc <=> cmp left << >> left + - . left * / % x right ! ~ unary+ unary- right ** left subscript (hash[s], array[i]) left term (variable, str/num literals, (paren), func())
left =>
left || // ^^
right ?:
left &&
left | ^
left &
left == != <=> cmp eq ne < > <= >= ge gt le lt
nonassoc <=> cmp
left << >>
left + - .
left * / % x
right ! ~ unary+ unary-
right **
left subscript (hash[s], array[i])
left term (variable, str/num literals, (paren), func())
There are two syntax for variables:
$alphanum123 (including $_) $package::separated::var
and:
${anything goes except closing curly brace}
Differences from Perl: In Expr there is just $scalar, no @array or %hash or others. There are no special variables with funny names ($., $$, etc), but if they are enclosed with curly braces they are allowed (e.g. ${.}, ${name/contains/slashes}). In fact, the curly braces syntax allows the compiler/interpreter a greater freedom of defining the namespace scheme aside from the Perl-like double-colon syntax, e.g.:
# Unix-path-like ${../foo} ${/foo/bar/baz} # volume:path (or URL-like) ${schema:/foo/bar/baz} ${data:../../baz}
Examples:
rand() length("foo")
Differences from Perl: parentheses are required in Expr.
The language define just a few functions:
This is similar to Perl's map() (but notice the required parentheses), it will form a new array composed from the result of EXPR. EXPR will be evaluated for each element of ARRAY (stored in $_). The original value of $_ will be restored after EXPR completes.
This is similar to Perl's grep() (but notice the required parentheses), it will form a new array composed from the elements of ARRAY when EXPR evaluates to true (like in Perl, empty string '', the number 0, boolean false, undef are considered false). EXPR will be evaluated for each element of ARRAY (stored in $_). The original value of $_ will be restored after EXPR completes.
false
This is similar to Perl's sort() (but notice the required parentheses), it will return the ARRAY sorted using comparison in EXPR. EXPR will be evaluated for each element of ARRAY ($a and $b will be set with two values to be compared). The original value of $a and $b will be restored after EXPR completes.
Please visit the project's homepage at https://metacpan.org/release/Language-Expr.
Source repository is at https://github.com/sharyanto/perl-Language-Expr.
Please report any bugs or feature requests on the bugtracker website https://rt.cpan.org/Public/Dist/Display.html?Name=Language-Expr
When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.
Steven Haryanto <stevenharyanto@gmail.com>
This software is copyright (c) 2014 by Steven Haryanto.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.
To install Language::Expr, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Language::Expr
CPAN shell
perl -MCPAN -e shell install Language::Expr
For more information on module installation, please visit the detailed CPAN module installation guide.