Re: Pattern Synonym Signature Confusion

On 01/10/2015 18:12, Sven Panne wrote:

2015-10-01 13:23 GMT+02:00 Matthew Pickering mailto:matthewtpickering@gmail.com>:

I think that the current state of pattern synonym signatures is quite confusing, especially regarding the constraints. [...]

Thanks to an off-list email from Matthew (thanks for that!) I found out that

pattern FOO = 1234 :: Int

behaves differently from

pattern FOO :: Int pattern FOO = 1234

In the former case one has to use ScopedTypeVariables, in the latter case it works without it. This is not really intuitive, although I'll have to admit that I've had only a cursory look at the "Typing of pattern synonyms" section in the GHC manual. But even after re-reading it, it's not really clear where the difference in the above example comes from.

So in a nutshell: +1 for the "quite confusing" argument.

Isn't that consistent with patterns in general, where something like f (5 :: Int) = 6 is only legal with ScopedTypeVariables on? Cheers, Ganesh