Do eager instantation in terms

This patch implements eager instantiation, a small but critical change
to the type inference engine, #17173.  The main change is this:

  When inferring types, always return an instantiated type
  (for now, deeply instantiated; in future shallowly instantiated)

There is more discussion in
https://www.tweag.io/posts/2020-04-02-lazy-eager-instantiation.html

There is quite a bit of refactoring in this patch:

* The ir_inst field of GHC.Tc.Utils.TcType.InferResultk
  has entirely gone.  So tcInferInst and tcInferNoInst have collapsed
  into tcInfer.

* Type inference of applications, via tcInferApp and
  tcInferAppHead, are substantially refactored, preparing
  the way for Quick Look impredicativity.

* New pure function GHC.Tc.Gen.Expr.collectHsArgs and applyHsArgs
  are beatifully dual.  We can see the zipper!

* GHC.Tc.Gen.Expr.tcArgs is now much nicer; no longer needs to return
  a wrapper

* In HsExpr, HsTypeApp now contains the the actual type argument,
  and is used in desugaring, rather than putting it in a mysterious
  wrapper.

* I struggled a bit with good error reporting in
  Unify.matchActualFunTysPart. It's a little bit simpler than before,
  but still not great.

Some smaller things

* Rename tcPolyExpr --> tcCheckExpr
         tcMonoExpr --> tcLExpr
* tcPatSig moves from GHC.Tc.Gen.HsType to GHC.Tc.Gen.Pat

Metric Decrease:
    T9961

Reduction of 1.6% in comiler allocation on T9961, I think.

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/GHC/Tc/Gen/Splice.hs b/compiler/GHC/Tc/Gen/Splice.hs
index 87b23a8b27..830e17abd4 100644
--- a/compiler/GHC/Tc/Gen/Splice.hs
+++ b/compiler/GHC/Tc/Gen/Splice.hs
@@ -286,7 +286,7 @@ tcPendingSplice m_var (PendingRnSplice flavour splice_name expr)
   = do { meta_ty <- tcMetaTy meta_ty_name
          -- Expected type of splice, e.g. m Exp
        ; let expected_type = mkAppTy m_var meta_ty
-       ; expr' <- tcPolyExpr expr expected_type
+       ; expr' <- tcCheckExpr expr expected_type
        ; return (PendingTcSplice splice_name expr') }
   where
      meta_ty_name = case flavour of
@@ -616,7 +616,7 @@ tcNestedSplice pop_stage (TcPending ps_var lie_var q@(QuoteWrapper _ m_var)) spl
        ; meta_exp_ty <- tcTExpTy m_var res_ty
        ; expr' <- setStage pop_stage $
                   setConstraintVar lie_var $
-                  tcMonoExpr expr (mkCheckExpType meta_exp_ty)
+                  tcLExpr expr (mkCheckExpType meta_exp_ty)
        ; untypeq <- tcLookupId unTypeQName
        ; let expr'' = mkHsApp
                         (mkLHsWrap (applyQuoteWrapper q)
@@ -639,7 +639,7 @@ tcTopSplice expr res_ty
        -- Top level splices must still be of type Q (TExp a)
        ; meta_exp_ty <- tcTExpTy q_type res_ty
        ; q_expr <- tcTopSpliceExpr Typed $
-                          tcMonoExpr expr (mkCheckExpType meta_exp_ty)
+                          tcLExpr expr (mkCheckExpType meta_exp_ty)
        ; lcl_env <- getLclEnv
        ; let delayed_splice
               = DelayedSplice lcl_env expr res_ty q_expr
@@ -676,7 +676,7 @@ runTopSplice (DelayedSplice lcl_env orig_expr res_ty q_expr)
             captureConstraints $
               addErrCtxt (spliceResultDoc zonked_q_expr) $ do
                 { (exp3, _fvs) <- rnLExpr expr2
-                ; tcMonoExpr exp3 (mkCheckExpType zonked_ty)}
+                ; tcLExpr exp3 (mkCheckExpType zonked_ty)}
        ; ev <- simplifyTop wcs
        ; return $ unLoc (mkHsDictLet (EvBinds ev) res)
        }
@@ -709,7 +709,7 @@ tcTopSpliceExpr :: SpliceType -> TcM (LHsExpr GhcTc) -> TcM (LHsExpr GhcTc)
 -- Note that set the level to Splice, regardless of the original level,
 -- before typechecking the expression.  For example:
 --      f x = $( ...$(g 3) ... )
--- The recursive call to tcPolyExpr will simply expand the
+-- The recursive call to tcCheckExpr will simply expand the
 -- inner escape before dealing with the outer one
 
 tcTopSpliceExpr isTypedSplice tc_action