Fun with Ruby Symbol Expressions

Posted 24 May 2011 to ruby, metaprogramming and has Comments

Groupon released an interesting extension to the Symbol#to_proc method named symbol_expressions over a year ago (I didn’t notice it until recently). It allows you to compose procs based on combinations of existing methods. For instance, to split and then join strings:

["foo", "bar"].map(&:split['']+:join['_'])
# => ["f_o_o", "b_a_r"]

I thought this was nifty, but the syntax is a bit odd (brackets are not generally used as argument list boundaries). Additionally, this sort of Proc composition is something a Proc should know how to create, but it doesn’t make sense to have a Symbol keeping track of a list of other Symbols that have been “added” to it (especially via an internal array class). It just seems like a bit of a hack to have Symbols acting as lists of other Symbols.

Based on these ideas, I reduced the symbol_expressions lib to the following lines:

With this little bit of code (which simply prefixes argument lists with a **

** symbol), you can now do stuff like this:

# composition using Proc (rather than Symbols that have lists of Symbols in them)
splitjoin = Proc.from_sym(:split | '', :join | " ", :upcase)
splitjoin.call "what"
# => "W H A T"

["foo", "bar"].map(&splitjoin)
# => ["F O O", "B A R"]

["foo", "bar"].map(&:split | '')
# => [["f", "o", "o"], ["b", "a", "r"]]

Fun stuff. Ruby consistently amazes me with its expressiveness.