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...