Improve type inference on pfield and hrecField

[MangoIV]

Problem

E.g. In examples/Examples/Api.hs, if you remove pfromData without adding a type signature to the declaration, GHC chooses the wrong instance of the class, i.e.

  let signatories = pfield @"signatories" # ctx.txInfo
  pure $
    pif
      (pelem # pdata ph # {- pfromData -} signatories)
      -- Success!
      (pconstant ())
      -- Signature not present.
      perror

errors with

    • Couldn't match type: PBuiltinList (PAsData PPubKeyHash)
                     with: PAsData PPubKeyHash
        arising from a functional dependency between:
          constraint ‘Plutarch.DataRepr.Internal.FromData.PFromDataable
                        (Plutarch.DataRepr.Internal.PUnLabel
                           (Plutarch.DataRepr.Internal.HList.Utils.IndexList
                              @PLabeledType
                              (Plutarch.DataRepr.Internal.PLabelIndex
                                 "signatories"
                                 (Plutarch.DataRepr.Internal.Field.PFields (PAsData PTxInfo)))
                              (Plutarch.DataRepr.Internal.Field.PFields (PAsData PTxInfo))))
                        (PAsData (PAsData PPubKeyHash))’
            arising from a use of ‘pfield’
          instance ‘Plutarch.DataRepr.Internal.FromData.PFromDataable
                      a (PAsData a)’
            at <no location info>
    • In the first argument of ‘(#)’, namely ‘pfield @"signatories"’
      In the expression: pfield @"signatories" # ctx.txInfo
      In an equation for ‘signatories’:
          signatories = pfield @"signatories" # ctx.txInfo
    |
165 |     let signatories = pfield @"signatories" # ctx.txInfo
    |                       ^^^^^^

Solution until this is fixed

There are two good solutions:

• continue using pfromData in those cases
• add a type signature as such:

        let signatories :: Term s (PBuiltinList (PAsData PPubKeyHash))
            signatories = pfield @"signatories" # ctx.txInfo
         in pif
              (pelem # pdata ph # signatories)
              -- Success!
              (pconstant ())
              -- Signature not present.
              perror

Other information

• this issue is a side effect of #235: automatic pfromData #270

[TotallyNotChase]

Another example:-

test :: Term s (PScriptContext :--> PAddress)
test = plam $ \x -> unTermCont $ do
  txInps <- tcont $ pletFields @'["inputs"] $ pfield @"txInfo" # x
  let
    g = phead # hrecField @"inputs" txInps
  let a = pfield @"resolved" # g
  pure $ pfield @"address" # a

This snippet gives out a bunch of errors. If you help out GHC a bit by providing an annotation on g-

test :: Term s (PScriptContext :--> PAddress)
test = plam $ \x -> unTermCont $ do
  txInps <- tcont $ pletFields @'["inputs"] $ pfield @"txInfo" # x
  let
    g :: Term _ (PAsData PTxInInfo)
    g = phead # hrecField @"inputs" txInps
  let a = pfield @"resolved" # g
  pure $ pfield @"address" # a

It still fails typecheck on hrecField @"inputs" txInps, with this error-

Couldn't match type ‘PBuiltinList (PAsData PTxInInfo)’
               with ‘PAsData PTxInInfo’
  arising from a use of ‘hrecField’

I have no idea how it managed to infer that hrecField returns a PAsData PTxInInfo, when in reality, it should only be able to return either PAsData (PBuiltinList (PAsData PTxInInfo)) or PBuiltinList (PAsData PTxInInfo).

The way to fix this is to provide an explicit type on the hrecField call itself-

phead # (hrecField @"inputs" txInps :: Term _ (PBuiltinList (PAsData PTxInInfo)))

Curiously, if I try to isolate it out a bit into this snippet-

test' :: HRec '[Labeled "inputs" (Term s (PAsData (PBuiltinList (PAsData PTxInInfo))))] -> Term s (PAsData PTxInInfo)
test' x =
  let
    g :: Term _ (PAsData PTxInInfo)
    g = phead # hrecField @"inputs" x -- but this works???
  in g

It no longer errors. What ????

[TotallyNotChase]

I changed the title to be a bit more general since we have the PR merged now.