LANGUAGE ConstraintKinds not needed to use ConstraintKinds?

[Eric Seidel]

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?
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