Parser, renamer, type checker for @a-binders (#17594)

GHC Proposal 448 introduces binders for invisible type arguments (@a-binders) in various contexts. This patch implements @-binders in lambda patterns and function equations:

  {-# LANGUAGE TypeAbstractions #-}

  id1 :: a -> a
  id1 @t x = x :: t      -- @t-binder on the LHS of a function equation

  higherRank :: (forall a. (Num a, Bounded a) => a -> a) -> (Int8, Int16)
  higherRank f = (f 42, f 42)

  ex :: (Int8, Int16)
  ex = higherRank (\ @a x -> maxBound @a - x )
                         -- @a-binder in a lambda pattern in an argument
                         -- to a higher-order function

Syntax

To represent those @-binders in the AST, the list of patterns in Match now uses ArgPat instead of Pat:

  data Match p body
     = Match {
         ...
-        m_pats  :: [LPat p],
+        m_pats  :: [LArgPat p],
         ...
   }

+ data ArgPat pass
+   = VisPat (XVisPat pass) (LPat pass)
+   | InvisPat (XInvisPat pass) (HsTyPat (NoGhcTc pass))
+   | XArgPat !(XXArgPat pass)

The VisPat constructor represents patterns for visible arguments, which include ordinary value-level arguments and required type arguments (neither is prefixed with a @), while InvisPat represents invisible type arguments (prefixed with a @).

Parser

In the grammar (Parser.y), the lambda and lambda-cases productions of aexp non-terminal were updated to accept argpats instead of apats:

  aexp : ...
-        | '\\' apats '->' exp
+        | '\\' argpats '->' exp
         ...
-        | '\\' 'lcases' altslist(apats)
+        | '\\' 'lcases' altslist(argpats)
         ...

+ argpat : apat
+        | PREFIX_AT atype

Function left-hand sides did not require any changes to the grammar, as they were already parsed with productions capable of parsing @-binders. Those binders were being rejected in post-processing (isFunLhs), and now we accept them.

In Parser.PostProcess, patterns are constructed with the help of PatBuilder, which is used as an intermediate data structure when disambiguating between FunBind and PatBind. In this patch we define ArgPatBuilder to accompany PatBuilder. ArgPatBuilder is a short-lived data structure produced in isFunLhs and consumed in checkFunBind.

Renamer

Renaming of @-binders builds upon prior work on type patterns, implemented in 2afbddb0, which guarantees proper scoping and shadowing behavior of bound type variables.

This patch merely defines rnLArgPatsAndThen to process a mix of visible and invisible patterns:

+ rnLArgPatsAndThen :: NameMaker -> [LArgPat GhcPs] -> CpsRn [LArgPat GhcRn]
+ rnLArgPatsAndThen mk = mapM (wrapSrcSpanCps rnArgPatAndThen) where
+   rnArgPatAndThen (VisPat x p)    = ... rnLPatAndThen ...
+   rnArgPatAndThen (InvisPat _ tp) = ... rnHsTyPat ...

Common logic between rnArgPats and rnPats is factored out into the rn_pats_general helper.

Type checker

Type-checking of @-binders builds upon prior work on lazy skolemisation, implemented in f5d3e03c.

This patch extends tcMatchPats to handle @-binders. Now it takes and returns a list of LArgPat rather than LPat:

  tcMatchPats ::
              ...
-             -> [LPat GhcRn]
+             -> [LArgPat GhcRn]
              ...
-             -> TcM ([LPat GhcTc], a)
+             -> TcM ([LArgPat GhcTc], a)

Invisible binders in the Match are matched up with invisible foralls in the type. This is done with a new clause in the loop worker of tcMatchPats:

  loop :: [LArgPat GhcRn] -> [ExpPatType] -> TcM ([LArgPat GhcTc], a)
  loop (L l apat : pats) (ExpForAllPatTy (Bndr tv vis) : pat_tys)
    ...
    -- NEW CLAUSE:
    | InvisPat _ tp <- apat, isInvisibleForAllTyFlag vis
    = ...

In addition to that, tcMatchPats no longer discards type patterns. This is done by filterOutErasedPats in the desugarer instead.