fat comma
{{Short description|Syntactic construction in computer programming}}
{{Use dmy dates|date=December 2023}}
The fat comma (also termed hash rocket in Ruby and a fat arrow in JavaScript) is a syntactic construction that appears in a position in a function call (or definition) where a comma would usually appear. The original usage refers to the ")letters:(" construction in ALGOL 60. Newer usage refers to the "=>" operator present in some programming languages. It is primarily associated with PHP, Ruby and Perl programming languages, which use it to declare hashes. Using a fat comma to bind key-value pairs in a hash, instead of using a comma, is considered an example of good idiomatic Perl.{{Cite book|last=Conway|first=Damian|authorlink=Damian Conway|editor=Allison Randal and Tatiana Appandi|title=Perl Best Practices|year=2005|publisher=O'Reilly Media, Inc.|isbn=0-596-00173-8|chapter=4: Values and Expressions|pages=66|quote=Whenever you are creating a list of key/value or name/value pairs, use the "fat comma" (=>) to connect the keys to their corresponding values.}} In CoffeeScript and TypeScript, the fat comma is used to declare a function that is bound to this.{{cite web|last=Ashkenas|first=Jeremy|title=Coffeescript Documentation: grammar.coffee|url=http://coffeescript.org/documentation/docs/grammar.html|access-date=11 December 2011|archive-url=https://web.archive.org/web/20120104174350/http://coffeescript.org/documentation/docs/grammar.html|archive-date=4 January 2012|url-status=dead}}{{Cite web|url=http://www.typescriptlang.org/docs/handbook/functions.html|title = Handbook – Functions}}
- a typical, idiomatic use of the fat comma in Perl
my %hash = (
first_name => "Larry",
last_name => "Wall",
);
Subtleties
=ALGOL 60=
The ALGOL "fat comma" is semantically identical to the comma.[http://www.masswerk.at/algol60/report.htm Revised Report on the Algorithmic Language Algol 60] by Peter Naur, et al. In particular, whether letter strings are used, and what their contents are, need not match between the definition of a function and its uses. The following are equivalent:
S(s-5, T, P)
S(s-5) t: (T) p: (P)
S(s-5) Temperature: (T) Pressure: (P)
=Perl=
The "fat comma" forces the word to its left to be interpreted as a string.[http://perldoc.perl.org/perlop.html#Comma-Operator perldoc.perl.org – perlop – Comma Operator]
Thus, where this would produce a run-time error under strict (barewords are not allowed):
%bad_example = ( bad_bareword, "not so cool" );
the following use of the fat comma would be legal and idiomatic:
%good_example = ( converted_to_string => "very monkish" );
This is because the token converted_to_string would be converted to the string literal "converted_to_string" which is a legal argument in a hash key assignment.
The result is easier-to-read code, with a stronger emphasis on the name-value pairing of associative arrays.
=PHP=
In PHP, the fat comma is termed a double arrow, and is used to specify key/value relationships when declaring an array. Unlike in Perl, the double arrow does not treat what comes before it as a bare word, but rather evaluates it. Hence, constants used with the double arrow will be evaluated:
$array = array("name" => "PHP", "influences" => array("Perl", "C", "C++", "Java", "Tcl"));
=Ruby=
In Ruby, the fat comma is the token to create hashes. Ruby 1.9 introduced a special syntax to use symbols as barewords.{{cite web|last=Galero|first=Michael|title=Ruby 1.9 Hash in Ruby 1.8|url=http://devblog.michaelgalero.com/2008/04/03/ruby-19-hash-in-ruby-18/|accessdate=3 April 2008}} In Ruby, the fat comma is called a hash rocket.{{cite web |last=Nash |first=Phil |title=I don't like the Ruby 1.9 hash syntax |url=http://logicalfriday.com/2011/06/20/i-dont-like-the-ruby-1-9-hash-syntax/ |work=Logical Friday |accessdate=13 July 2011 |url-status=dead |archiveurl=https://web.archive.org/web/20110625034400/http://logicalfriday.com/2011/06/20/i-dont-like-the-ruby-1-9-hash-syntax/ |archivedate=25 June 2011 }}
- Old syntax
old_hash = { :name => 'Ruby', :influences => ['Perl', 'Python', 'Smalltalk'] }
- New syntax (Ruby >= 1.9)
new_hash = { name: 'Ruby', influences: ['Perl', 'Python', 'Smalltalk'] }
Use as lambda functions
The fat arrow is used to declare single expression anonymous functions in JavaScript,{{cite web|url=https://dzone.com/articles/javascript-fat-city|title=Fat arrows in javascript}} and C sharp.{{cite web|url=https://joelholder.com/2013/07/19/hacking-cs-lambda-expressions-into-hash-rockets/|title=Hacking Sharp Lambda Expressions into Hash Rockets|date=20 July 2013}}