Fat comma
syntactic construction in computer programming
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. In CoffeeScript and TypeScript, the fat comma is used to declare a function that is bound to this.
Subtleties
ALGOL 60
The ALGOL "fat comma" is semantically identical to the comma. 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:
Perl
The "fat comma" forces the word to its left to be interpreted as a string.
Thus, where this would produce a run-time error under strict (barewords are not allowed):
the following use of the fat comma would be legal and idiomatic:
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.
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