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, 7 insertions, 0 deletions

diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs
index 8ba0eefb34..19b0667ceb 100644
--- a/compiler/GHC/Utils/Monad.hs
+++ b/compiler/GHC/Utils/Monad.hs
@@ -11,6 +11,7 @@ module GHC.Utils.Monad
         , zipWith3M, zipWith3M_, zipWith4M, zipWithAndUnzipM
         , mapAndUnzipM, mapAndUnzip3M, mapAndUnzip4M, mapAndUnzip5M
         , mapAccumLM
+        , liftFstM, liftSndM
         , mapSndM
         , concatMapM
         , mapMaybeM
@@ -164,6 +165,12 @@ mapSndM f xs = go xs
     go []         = return []
     go ((a,b):xs) = do { c <- f b; rs <- go xs; return ((a,c):rs) }
 
+liftFstM :: Monad m => (a -> b) -> m (a, r) -> m (b, r)
+liftFstM f thing = do { (a,r) <- thing; return (f a, r) }
+
+liftSndM :: Monad m => (a -> b) -> m (r, a) -> m (r, b)
+liftSndM f thing = do { (r,a) <- thing; return (r, f a) }
+
 -- | Monadic version of concatMap
 concatMapM :: Monad m => (a -> m [b]) -> [a] -> m [b]
 concatMapM f xs = liftM concat (mapM f xs)