Typing pattern synonyms

Sven Panne svenpanne at gmail.com
Wed Sep 30 18:04:33 UTC 2015

The type of a pattern synonym like

   pattern FOO = 1234

seems to be '(Eq a, Num a) => a', which makes partially makes sense,
although it's not immediately clear to me where the 'Eq a' part comes from.
But probably that would be clear if I read the desugaring rules closely
enough. ;-) My real question is about:

   pattern FOO = 1234 :: Int

This doesn't compile out of the box, GHC seems to require
ScopedTypeVariables, too:

    Illegal type signature: `Int'
      Perhaps you intended to use ScopedTypeVariables
    In a pattern type-signature

Why is this the case? From a user perspective, the error is totally bogus,
there are no visible type variables at all. Can GHC be fixed to avoid
enabling ScopedTypeVariables?

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