Replace HsImplicitBndrs with HsOuterTyVarBndrs

This refactors the GHC AST to remove `HsImplicitBndrs` and replace it with
`HsOuterTyVarBndrs`, a type which records whether the outermost quantification
in a type is explicit (i.e., with an outermost, invisible `forall`) or
implicit. As a result of this refactoring, it is now evident in the AST where
the `forall`-or-nothing rule applies: it's all the places that use
`HsOuterTyVarBndrs`. See the revamped `Note [forall-or-nothing rule]` in
`GHC.Hs.Type` (previously in `GHC.Rename.HsType`).

Moreover, the places where `ScopedTypeVariables` brings lexically scoped type
variables into scope are a subset of the places that adhere to the
`forall`-or-nothing rule, so this also makes places that interact with
`ScopedTypeVariables` easier to find. See the revamped
`Note [Lexically scoped type variables]` in `GHC.Hs.Type` (previously in
`GHC.Tc.Gen.Sig`).

`HsOuterTyVarBndrs` are used in type signatures (see `HsOuterSigTyVarBndrs`)
and type family equations (see `HsOuterFamEqnTyVarBndrs`). The main difference
between the former and the latter is that the former cares about specificity
but the latter does not.

There are a number of knock-on consequences:

* There is now a dedicated `HsSigType` type, which is the combination of
  `HsOuterSigTyVarBndrs` and `HsType`. `LHsSigType` is now an alias for an
  `XRec` of `HsSigType`.
* Working out the details led us to a substantial refactoring of
  the handling of explicit (user-written) and implicit type-variable
  bindings in `GHC.Tc.Gen.HsType`.

  Instead of a confusing family of higher order functions, we now
  have a local data type, `SkolemInfo`, that controls how these
  binders are kind-checked.

  It remains very fiddly, not fully satisfying. But it's better
  than it was.

Fixes #16762. Bumps the Haddock submodule.

Co-authored-by: Simon Peyton Jones <simonpj@microsoft.com>
Co-authored-by: Richard Eisenberg <rae@richarde.dev>
Co-authored-by: Zubin Duggal <zubin@cmi.ac.in>

1 files changed, 4 insertions, 2 deletions

diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index 2c2b9fc3bb..025f82fa08 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -1689,7 +1689,8 @@ defineMacro overwrite s = do
           ioM = nlHsTyVar (getRdrName ioTyConName) `nlHsAppTy` stringTy
           body = nlHsVar compose_RDR `mkHsApp` (nlHsPar step)
                                      `mkHsApp` (nlHsPar expr)
-          tySig = mkLHsSigWcType (nlHsFunTy stringTy ioM)
+          tySig = mkHsWildCardBndrs $ noLoc $ mkHsImplicitSigType $
+                  nlHsFunTy stringTy ioM
           new_expr = L (getLoc expr) $ ExprWithTySig noExtField body tySig
       hv <- GHC.compileParsedExprRemote new_expr
 
@@ -1757,7 +1758,8 @@ getGhciStepIO = do
       ghciM = nlHsTyVar (getRdrName ghciTyConName) `nlHsAppTy` stringTy
       ioM = nlHsTyVar (getRdrName ioTyConName) `nlHsAppTy` stringTy
       body = nlHsVar (getRdrName ghciStepIoMName)
-      tySig = mkLHsSigWcType (nlHsFunTy ghciM ioM)
+      tySig = mkHsWildCardBndrs $ noLoc $ mkHsImplicitSigType $
+              nlHsFunTy ghciM ioM
   return $ noLoc $ ExprWithTySig noExtField body tySig
 
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