Question

Why can’t I call Enumerable#reduce(sym) without parentheses like the following?

>> [1, 2, 3].reduce :+
?>

While using parentheses results in this:

>> [1, 2, 3].reduce(:+)
=> 6

Am I accidentally calling Enumerable#reduce {| memo, obj | block } instead?

Furthermore, why does this happen?

>> [1, 2, 3].reduce &:+
?> ^C
>> [1, 2, 3].reduce(&:+)
=> 6

Thanks a lot!

Answer 1

That seems to be an error in IRb’s parser. It works just fine if you try it in Pry, or on the commandline or in a file:

ruby -e"res = [1, 2, 3].reduce :+
p res"
# 6

Basically, IRb gets confused, thinks the + is a binary operator and is waiting for the second operand.