Haskell's `otherwise` is a synonym for `_`?

Question

I ran across a piece of code recently that used Haskell's otherwise to pattern match on a list. This struck me as odd, since:

ghci> :t otherwise
otherwise :: Bool

So, I tried the following:

ghci> case [] of otherwise -> "!?"
"!?"

I also tried it with various other patterns of different types and with -XNoImplicitPrelude turned on (to remove otherwise from scope), and it still works. Is this supposed to happen? Where is this documented?

Answer 1

It's not equivalent to _, it's equivalent to any other identifier. That is if an identifier is used as a pattern in Haskell, the pattern always matches and the matched value is bound to that identifier (unlike _ where it also always matches, but the matched value is discarded).

Just to be clear: the identifier otherwise is not special here. The code could just as well have been x -> "!?". Also, since the binding is never actually used, it would make more sense to use _ to avoid an "unused identifier" warning and to make it obvious to the reader that the value does not matter.

Answer 2

Just since nobody has said it yet, otherwise is supposed to be used as a guard expression, not a pattern. case ... of pat | ... -> ... | otherwise -> ... Now its definition as True is important. – Reid Barton

An example:

fact n acc
  | n == 0 = acc
  | otherwise = fact (n-1) $! (acc * n)

Since otherwise is True, that second guard will always succeed.

Note that using otherwise in a pattern (as opposed to a guard) is likely to confuse people. It will also trip a name shadowing warning if GHC is run with the appropriate warnings enabled.