Re: Solving stuck type families with a TC plugin

Thanks Richard! Matt pointed me in the same direction, and generating givens seems to work. Planning on releasing this solving tyfams stuff as a small library soon. Cheers! On Mon, Aug 5, 2019 at 10:36 AM Richard Eisenberg wrote:

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 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!

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