ruby using the “&:methodname” shortcut from array.map(&:methodname) for hash key strings rather than methodname

Question

Most ruby developers know how to save a few keystrokes by doing something like this:

array.map(&:methodname)

rather than

Answer 1

You can do this with a lambda:

extract_keyname = ->(h) { h[:keyname] }
ary_of_hashes.map(&extract_keyname)

This tends to be more useful if the block's logic is more complicated than simply extracting a value from a Hash. Also, attaching names to your bits of logic can help clarify what a chain of Enumerable method calls is trying to do.

You can also have a lambda which returns a lambda if you're doing this multiple times:

extract = ->(k) { ->(h) { h[k] } }
ary_of_hashes.map(&extract[:keyname])
ary_of_hashes.map(&extract[:other_key])

or a lambda building method:

def extract(k)
  ->(h) { h[k] }
end

ary_of_hashes.map(&extract(:keyname))
ary_of_hashes.map(&extract(:other_key))

Answer 2

& before a statement in a line of ruby is a shortcut of calling to_proc.

On a symbol to_proc looks for the method on the "context" and calls uses that as reference.

also:

• Ruby ampersand colon shortcut
• What does map(&:name) mean in Ruby?
• what is the functionality of "&: " operator in ruby?