Why does this Haskell statement work in GHCi but fail to compile?

Question

In GHCi the following works just dandy:

let (≠) = (/=)

But trying to compile this from a source file results in an error:

(≠) = (/=)

Here is the error, for reference:

"Ambiguous type variable 'a0' arising from a use of '/=' prevents the constraint '(Eq a0)' from being solved."

What am I doing wrong?

Try adding a type signature? `(≠) :: Eq a => a -> a -> Bool`? — Adam Smith, Dec 24 '18 at 07:50
Facepalm. Yes, that's it! Please make this an answer so I can accept it? — crisis.sheep, Dec 24 '18 at 07:57
`:set -XNoMonomorphismRestriction` in GHCi will load without error — assembly.jc, Dec 24 '18 at 08:00

score 6 · Accepted Answer · answered Dec 24 '18 at 07:58

6

You'll need to add a type signature, otherwise Haskell is trying to assign one specific type but gets confused due to the ambiguous type.

(≠) :: Eq a => a -> a -> Bool
(≠) = (/=)

answered Dec 24 '18 at 07:58

Adam Smith

6

"Gets confused" is maybe not the most accurate way to phrase it, as that would imply that GHCi, which doesn't "get confused", is somehow more intelligent than GHC. The relevant keyword here is the monomorphism restriction. – sepp2k Dec 24 '18 at 09:04
@sepp2k that's true, this answer was badly phrased. – Adam Smith Dec 24 '18 at 09:06

1 Answers1