The Eq constraint is needed to support pattern matching, the raison d’être
of pattern synonyms. I'm pretty sure the reason you need
ScopedTypeVariables for your second example is that GHC only allows pattern
signatures with that extension enabled. Once upon a time there was a
separate PatternSignatures extension, I believe, but not any more.
On Sep 30, 2015 2:04 PM, "Sven Panne"
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?
Cheers, S.
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