Right, that sounds like a good idea to me, it's the same reason I added the synonym myself. WRT not needing an extension I guess this is part of a general pattern where you don't need extensions to use code that uses extensions. In this case though it seems a bit odd in that using the code that uses extensions also requires previously illegal syntax. But as long as it's intentional it's fine by me. But is the bit where the calling module needs FlexibleConstraints also intentional? On Mon, Feb 15, 2016 at 10:16 AM, Eric Seidel wrote:

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

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On Mon, Feb 15, 2016 at 12:57 PM, Eric Seidel wrote:

It looks like it is. According to [1], the context must have the form `C a` unless FlexibleContexts is enabled.

Works for me, thanks!