Solving stuck type families with a TC plugin

[Richard Eisenberg]

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 <sandy at 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!
> 
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