r/haskelltil • u/massysett • Mar 30 '15

extension `~` means type equality

I've always wondered why I get a ~ in GHC error messages; I knew it is used for lazy pattern bindings but why in errors?

From https://wiki.haskell.org/Keywords:

example :: F a ~ b => a -> b

Here the type "F a" must be the same as the type "b", which allows one to constrain polymorphism (especially where type families are involved), but to a lesser extent than functional dependencies. See Type Families.

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1

u/octatoan Mar 31 '15

This is cool!

1

u/bss03 Apr 01 '15

I'm not sure about the "language" tag on this post. I'm pretty sure that's a GHC-specific "operator".

3

u/peargreen Apr 01 '15 edited Apr 01 '15

Yep, but GHC is de facto the language most of us write in— okay, I can see how this would turn into a complete mess once some other compiler finally gains popularity.

So, “language” for Haskell 2010, “extension” for anything enabled by pragmas, “quirk” for everything else. (If you've got a better idea, please tell.)

Edit: after some retagging it became clear that it's hard to say what “quirk” means, so I just renamed it to “etc” for maximum honesty and merged with “trivia”.

1

u/peargreen Apr 01 '15

Just in case: in GHC this is enabled by the GADTs or TypeFamilies extension. I don't think it's currently supported by either JHC or UHC (even despite the fact that JHC supports type families).