diff options
Diffstat (limited to 'compiler/Language/Haskell/Syntax/Expr.hs')
| -rw-r--r-- | compiler/Language/Haskell/Syntax/Expr.hs | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/compiler/Language/Haskell/Syntax/Expr.hs b/compiler/Language/Haskell/Syntax/Expr.hs index 44c731735c..5ac3e33bdf 100644 --- a/compiler/Language/Haskell/Syntax/Expr.hs +++ b/compiler/Language/Haskell/Syntax/Expr.hs @@ -1533,7 +1533,10 @@ data HsMatchContext p = FunRhs -- ^ A pattern matching on an argument of a -- function binding - { mc_fun :: LIdP p -- ^ function binder of @f@ + { mc_fun :: LIdP (NoGhcTc p) -- ^ function binder of @f@ + -- See Note [mc_fun field of FunRhs] + -- See #20415 for a long discussion about + -- this field and why it uses NoGhcTc. , mc_fixity :: LexicalFixity -- ^ fixing of @f@ , mc_strictness :: SrcStrictness -- ^ was @f@ banged? -- See Note [FunBind vs PatBind] @@ -1560,6 +1563,21 @@ data HsMatchContext p | ThPatQuote -- ^A Template Haskell pattern quotation [p| (a,b) |] | PatSyn -- ^A pattern synonym declaration +{- +Note [mc_fun field of FunRhs] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The mc_fun field of FunRhs has type `LIdP (NoGhcTc p)`, which means it will be +a `RdrName` in pass `GhcPs`, a `Name` in `GhcRn`, and (importantly) still a +`Name` in `GhcTc` -- not an `Id`. See Note [NoGhcTc] in GHC.Hs.Extension. + +Why a `Name` in the typechecker phase? Because: +* A `Name` is all we need, as it turns out. +* Using an `Id` involves knot-tying in the monad, which led to #22695. + +See #20415 for a long discussion. + +-} + isPatSynCtxt :: HsMatchContext p -> Bool isPatSynCtxt ctxt = case ctxt of |