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.
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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On 09/30/2015 08:10 PM, David Feuer wrote:
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.
Perhaps ScopedTypeVariables by default (or only?) would be relevant for the revamped[1] Haskell' committee? Would there be any actual downsides? It's always seemed to me that ScopedTypeVariables was the way it should have been from the start (given perfect foresight). Regards, [1] http://permalink.gmane.org/gmane.comp.lang.haskell.prime/3965
Edward Kmett seems to have some concerns about its implications for type
checker implementations. Personally, I have little interest in programming
in a Haskell that doesn't have a feature with the power to express what
ScopedTypeVariables can. That said, there are some weird corners in the
design relating to pattern signatures and in particular to pattern bindings.
On Sep 30, 2015 2:26 PM, "Bardur Arantsson"
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
On 09/30/2015 08:10 PM, David Feuer wrote: pattern
signatures with that extension enabled. Once upon a time there was a separate PatternSignatures extension, I believe, but not any more.
Perhaps ScopedTypeVariables by default (or only?) would be relevant for the revamped[1] Haskell' committee?
Would there be any actual downsides? It's always seemed to me that ScopedTypeVariables was the way it should have been from the start (given perfect foresight).
Regards,
[1] http://permalink.gmane.org/gmane.comp.lang.haskell.prime/3965
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2015-09-30 20:10 GMT+02:00 David Feuer
The Eq constraint is needed to support pattern matching, the raison d’être of pattern synonyms.
I was just a bit confused by the fact that normally you don't need an 'Eq a' constraint for pattern matching. But looking at the Haskell report, one can see that matching against numeric/char/string literals involves (==), so: \x -> case x of 1234 -> undefined :: (Eq a, Num a) => a -> r This makes sense now, and there is no magic involved. :-]
I'm pretty sure the reason you need ScopedTypeVariables for your second example is that GHC only allows pattern signatures with that extension enabled.
My main question is: Why is this the case? Is it really necessary in our simple example? The GHC docs should probably mention the magic behind this in the patter synonym section.
Once upon a time there was a separate PatternSignatures extension, I believe, but not any more.
Never heard of that one...
participants (3)
-
Bardur Arantsson -
David Feuer -
Sven Panne