Hi Sandy, I think the problem is that you're generating *Wanted* constraints. A Wanted is something that has not yet been proven, but which you would like to prove. If you have a metavariable a0, then created a Wanted `a0 ~ Bool` will work: you want to prove that, so GHC just unifies a0 := Bool. But anything more complicated than a unification variable will run into trouble, as GHC won't know how to prove it.
Instead, create Givens. With these, you are providing the evidence to GHC that something holds -- exactly what you want here. Also, there shouldn't be a need to use unsafeTcPluginTcM or runTcSDeriveds here: just use newGiven (or newWanted) from the TcPluginM module, and return these constraints (perhaps wrapped in mkNonCanonical) from your plugin function. I hope this helps! Richard > On Aug 4, 2019, at 1:06 PM, Sandy Maguire <sa...@sandymaguire.me> wrote: > > Hi all, > > I'm attempting to use a plugin to solve a generic > > type family CmpType (a :: k) (b :: k) :: Ordering > > by doing some sort of arbitrary hashing on `a` and `b` and ensuring they're > the same. > > In the past, I've been successful at getting GHC to unify things by emitting > new wanted CNonCanonical cts. This sort of works: > > > mkWanted :: TyCon -> CompareType -> TcPluginM Ct > mkWanted cmpType cmp = do > (ev, _) <- unsafeTcPluginTcM > . runTcSDeriveds > $ newWantedEq > (cmpTypeLoc cmp) > Nominal > (cmpTypeType cmp) > (doCompare cmp) > pure $ CNonCanonical ev > > > Which is to say that this will compile: > > > foo :: Proxy 'EQ > foo = Proxy @(CmpType 2 2) > > > So far so good! However, this acts strangely. For example, if I ask for bar > with the incorrect type: > > > bar :: Proxy 'GT > bar = Proxy @(CmpType 2 2) > > > I get the error: > > • Couldn't match type ‘CmpType 2 2’ with ‘'GT’ > Expected type: Proxy 'GT > Actual type: Proxy (CmpType 2 2) > > when I would expect > > • Couldn't match type ‘'EQ’ with ‘'GT’ > > > This is more than just an issue with the error messages. A type family that > is stuck on the result of CmpType, even after I've solved CmpType via the > above! > > > type family IsEQ (a :: Ordering) :: Bool where > IsEQ 'EQ = 'True > IsEQ _ = 'False > > zop :: Proxy 'True > zop = Proxy @(IsEQ (CmpType 2 2)) > > > • Couldn't match type ‘IsEQ (CmpType 2 2)’ with ‘'True’ > Expected type: Proxy 'True > Actual type: Proxy (IsEQ (CmpType 2 2)) > > > Any suggestions for what I might be doing wrong, and how to convince GHC to > make `zop` work properly? Thanks! > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
