#15457: (~) and (!) are parsed inconsistently in types (plus documentation warts) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 (Parser) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- (Spawned from https://ghc.haskell.org/trac/ghc/ticket/10056?replyto=41#comment:41) `~` and `!` are slightly special in the parser to allow strict annotations and lazy annotations (with `-XStrict`) in data types. As a result, we initially parse all uses of `~`/`!` as bangs, and we use a post-parsing pass (`mergeOps`/`splitTilde`) to figure out which uses of `~` are actually meant to refer to the equality type constructor. There's a couple of unsatisfying things here: 1. `splitTilde` handles `~`, but not `!`. This means that any use of `!` as a type operator will not work, as evidenced by this GHCi session: {{{ λ> type a ! b = Either a b <interactive>:1:6: error: Malformed head of type or class declaration: a !b }}} We should update `splitTilde` to handle `!` as well. (And perhaps give that function a new name to reflect its more ambitious goals.) 2. `Note [Parsing ~]` is supposed to explain all of this hullabaloo, but it does a rather poor job of it. Let's add some more prose to it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15457 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler