#10056: Inconsistent precedence of ~ -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 7.8.4 (Parser) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10059, #10431, | Differential Rev(s): Phab:D4876 #14316 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): An update on this ticket. It seems that the only remaining thing here is to wire in an appropriately low fixity for `(~)` (and `(~~)`, which GHC introduced after the creation of this ticket). As it turns out, this part is way, way easier than I originally thought. This patch suffices, in fact: {{{#!diff diff --git a/compiler/rename/RnFixity.hs b/compiler/rename/RnFixity.hs index f1bfb38..05d1b89 100644 --- a/compiler/rename/RnFixity.hs +++ b/compiler/rename/RnFixity.hs @@ -27,6 +27,8 @@ import Maybes import Data.List import Data.Function ( on ) import RnUnbound +import PrelNames ( eqTyConKey, heqTyConKey ) +import Unique {- ********************************************************* @@ -124,6 +126,9 @@ lookupFixityRn_help' name occ -- a>0 `foo` b>0 -- where 'foo' is not in scope, should not give an error (Trac #7937) + | name `hasKey` eqTyConKey || name `hasKey` heqTyConKey + = pure (True, Fixity NoSourceText (-2) InfixN) + | otherwise = do { local_fix_env <- getFixityEnv ; case lookupNameEnv local_fix_env name of { diff --git a/testsuite/tests/ghci/scripts/T10059.stdout b/testsuite/tests/ghci/scripts/T10059.stdo index 92fbb45..854f52a 100644 --- a/testsuite/tests/ghci/scripts/T10059.stdout +++ b/testsuite/tests/ghci/scripts/T10059.stdout @@ -1,4 +1,6 @@ class (a ~ b) => (~) (a :: k0) (b :: k0) -- Defined in ‘GHC.Types’ +infix -2 ~ (~) :: k0 -> k0 -> Constraint class (a GHC.Prim.~# b) => (~) (a :: k0) (b :: k0) -- Defined in ‘GHC.Types’ +infix -2 ~ }}} I'm giving this a precedence of -2, since in #15235, we decided to give `(->)` a precedence of -1, and the consensus in this ticket is that `(~)`/`(~~)` should have a lower precedence than everything else. Unfortunately, things are never as simple as they appear. Even with this patch, `(->)` will //still// have a lower precedence than `(~)` in practice. Why? Because saying that `(->)` has a precedence of -1 is a bit of a lie; in reality, it has a precedence closer to -∞, since `(->)` has special treatment in the parser, which causes it to bind more tightly than it ought to. Note that `(~)` is also treated specially in the parser, but there is a post-parsing pass which flattens uses of `(~)` to appear as ordinary type operators. Perhaps we should extend this treatment to `(->)` as well? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10056#comment:38 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler