Hi Adam, Hi Eric,

At least for what I want to use them for [1] it would be nice if there was an easy way to say:

If you are stuck on solving "Constraint a b TypeC", then you should pick, e.g.:

- "a ~ TypeA" and "b ~ TypeB" (What I am actually trying to say is: use the instance "Constraint TypeA TypeB TypeC")

- "a ~ b"

Currently I am producing equality constraints, like this:

mkDerivedTypeEqCt :: TcTyVar -> TcType -> TcPluginM Ct

mkDerivedTypeEqCt tyVar ty = do

(_, lclEnv) <- getEnvs

return $ CTyEqCan

{ cc_ev = CtDerived -- :: CtEvidence

{ ctev_pred = ty -- :: TcPredType

-- This matches type-wise, but I have no idea what actually belongs here.

, ctev_loc = mkGivenLoc topTcLevel (UnifyForAllSkol [tyVar] ty) lclEnv -- :: CtLoc

-- Again no idea what actually belongs here:

-- topTcLevel :: TcLevel

-- To what does this relate? I guess top level

-- is ok for equality constraints

-- (UnifyForAllSkol [tyVar] ty) :: SkolemInfo

-- This one matches what we have at disposal (no idea if it is the right one).

-- lclEnv :: TcLclEnv

-- I just use the only one I know.

}

, cc_tyvar = tyVar -- :: TcTyVar

, cc_rhs = ty -- :: TcType

, cc_eq_rel = NomEq -- :: EqRel

-- Alternative would be ReprEq. Whats the difference?

}

Which seems to be working, but still leaves a lot of open questions (see comments).

Maybe my problems using the API are more related to missing documentation, then lack of API functionality.

Jan

[1]: https://mail.haskell.org/pipermail/ghc-devs/2015-February/008414.html

On Wed, 27 May 2015 07:13:37, Eric Seidel wrote:

Hi Adam,

I like the addition of the new* functions for creating constraints, that
should make for a much nicer API than dealing directly with the
CtEvidence constructors!

I'm not so convinced however about embedding arbitrary CoreExprs in
EvTerms. First of all, it feels a bit strange to generate CoreExprs
before the desugarer (and we would have to add a `MonadThings TcPluginM`
instance to generate Integer and String CoreExprs).

But more importantly, based on your wiki page [1], it sounds like what
we really want is a nice API for creating dictionaries.

Eric

[1]:
https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker#EmbeddingCoreExprinEvTerm

On Wed, May 27, 2015, at 01:33, Adam Gundry wrote:
> Hi devs,
>
> I thought I should flag up some proposed changes relating to typechecker
> plugins, which Christiaan, Iavor and I have been discussing. The quick
> summary:
>
> * make it possible for plugins to create constraints (Phab:D909);
>
> * make it easier for plugins to define special type families;
>
> * embed CoreExpr in EvTerm.
>
> For more details, see the wiki page:
> https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker#Post-7.10changestoTcPluginMAPI
>
> Questions/review/comments very welcome.
>
> Adam
>
>
> --
> Adam Gundry, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/
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