IIRC ConstraintKinds is only required in the module that defines the type synonym, so your module T does not need it. My guess is that haskell-src-exts sees 'Log.Stack =>', in which we have nullary constraint instead of a unary constraint, and assumes that's bogus without MultiParamTypeClasses. But I'm not familiar with haskell-src-exts' internals, so that's just a wild guess :) FYI, as of RC2 we provide a type synonym type HasCallStack = (?callStack :: CallStack) in GHC.Stack, which is the recommended way to request a CallStack. We're hiding the implicit parameter from the docs and API, as the reliance on using the same name is a bit of a misfeature. We might also reimplement HasCallStack without implicit-params in the future, so in the interest of forward-compatibility I'd suggest using HasCallStack instead of your own implicit-parameters. Eric On Sun, Feb 14, 2016, at 16:32, Evan Laforge wrote:
I recently upgraded to ghc 8 and started using stacks via ImplicitParams. For that I wind up using 'type Stack = (?stack :: CallStack)' and so ContraintKinds (I see that in the future GHC will do this by default).
So now I can have a file like:
module T where import qualified Log as Log
f :: Log.Stack => IO () f = Log.warn "blah blah"
I noticed that now haskell-src-exts refuses to parse this file, saying 'MultiParamTypeClasses language extension is not enabled.'.
I assume it's a bug with haskell-src-exts in that it should require LANGUAGE ConstraintKinds instead, but then GHC itself doesn't want ConstraintKinds. Instead, it wants FlexibleContexts. From the docs, FlexibleContexts seems to be about the contexts in instance heads.
Is this intentional? I'll go ahead and make a bug for haskell-src-exts, but the ghc behaviour here seems odd as well. What extension should haskell-src-exts require to parse this? _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users