#14316: Precedence of type-level operators -------------------------------------+------------------------------------- Reporter: lspitzner | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: duplicate | Keywords: precedence | TypeOperators DataKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10056, #10059, | Differential Rev(s): #10431 | Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #10056, #10059, #10431 Comment: There are two questions here: 1. What are the precedences of data constructors that are promoted to types via `DataKinds`? This has a simple answer: they have the same precedences as their value-level counterparts. Since `(:)` is `infixr 5` at the value level, `'(:)` is also `infixr 5` at the type level. 2. What is the precedence of `(~)`? Alas, this is a simple question with an ugly answer. For unfortunate historical reasons, `(~)` is not a proper type operator, but rather a specially parsed symbol that happens to //look// like a type operator. As such, it does not have a well defined precedence (#10056), and you can't even ask GHCi for its precedence with `:info` (#10059). This is obviously not a great situation, since it leads to all sorts of confusing parser oddities like the one observed here. The plan laid out in #10056 will hopefully make the story a little more palatable. The plan is (roughly): * Remove as much special treatment for `(~)` from the parser as possible * Define `(~)` as a plain old type operator somewhere in `base`. For backwards compatibility purposes, this will probably need to always be in scope (otherwise, it'd be possible to hide it via Haskell's module system, which is something that you can't do with `(~)` today). * Give `(~)` a precedence of `infix -1` (i.e., lower than `(->)`). * Enable the use of `(~)` via a new language extension (`-XEqualityConstraints` has been proposed in #10431). For backwards compatibility, `-XGADTs` and `-XTypeFamilies` would also continue to allow the use of `(~)`. Given that this ticket is essentially just a symptom of #10056, I'll close this as a duplicate in favor of #10056. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14316#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler