Fix #14875 by introducing PprPrec, and using it

Trying to determine when to insert parentheses during TH
conversion is a bit of a mess. There is an assortment of functions
that try to detect this, such as:

* `hsExprNeedsParens`
* `isCompoundHsType`
* `hsPatNeedsParens`
* `isCompoundPat`
* etc.

To make things worse, each of them have slightly different semantics.
Plus, they don't work well in the presence of explicit type
signatures, as #14875 demonstrates.

All of these problems can be alleviated with the use of an explicit
precedence argument (much like what `showsPrec` currently does). To
accomplish this, I introduce a new `PprPrec` data type, and define
standard predences for things like function application, infix
operators, function arrows, and explicit type signatures (that last
one is new). I then added `PprPrec` arguments to the various
`-NeedsParens` functions, and use them to make smarter decisions
about when things need to be parenthesized.

A nice side effect is that functions like `isCompoundHsType` are
now completely unneeded, since they're simply aliases for
`hsTypeNeedsParens appPrec`. As a result, I did a bit of refactoring
to remove these sorts of functions. I also did a pass over various
utility functions in GHC for constructing AST forms and used more
appropriate precedences where convenient.

Along the way, I also ripped out the existing `TyPrec`
data type (which was tailor-made for pretty-printing `Type`s) and
replaced it with `PprPrec` for consistency.

Test Plan: make test TEST=T14875

Reviewers: alanz, goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14875

Differential Revision: https://phabricator.haskell.org/D4688

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/typecheck/TcGenDeriv.hs b/compiler/typecheck/TcGenDeriv.hs
index 05c6276cb5..b94452059d 100644
--- a/compiler/typecheck/TcGenDeriv.hs
+++ b/compiler/typecheck/TcGenDeriv.hs
@@ -1709,7 +1709,8 @@ nlHsAppType e s = noLoc (HsAppType hs_ty e)
     hs_ty = mkHsWildCardBndrs $ nlHsParTy (typeToLHsType s)
 
 nlExprWithTySig :: LHsExpr GhcPs -> Type -> LHsExpr GhcPs
-nlExprWithTySig e s = noLoc (ExprWithTySig hs_ty e)
+nlExprWithTySig e s = noLoc $ ExprWithTySig hs_ty
+                            $ parenthesizeHsExpr sigPrec e
   where
     hs_ty = mkLHsSigWcType (typeToLHsType s)
 
@@ -1855,7 +1856,7 @@ mkFunBindSE arity loc fun pats_and_exprs
   = mkRdrFunBindSE arity (L loc fun) matches
   where
     matches = [mkMatch (mkPrefixFunRhs (L loc fun))
-                               (map parenthesizeCompoundPat p) e
+                               (map (parenthesizePat appPrec) p) e
                                (noLoc emptyLocalBinds)
               | (p,e) <-pats_and_exprs]
 
@@ -1876,7 +1877,7 @@ mkFunBindEC arity loc fun catch_all pats_and_exprs
   = mkRdrFunBindEC arity catch_all (L loc fun) matches
   where
     matches = [ mkMatch (mkPrefixFunRhs (L loc fun))
-                                (map parenthesizeCompoundPat p) e
+                                (map (parenthesizePat appPrec) p) e
                                 (noLoc emptyLocalBinds)
               | (p,e) <- pats_and_exprs ]