(Spawned from #10056?replyto=41\##15457) `~` 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.) 1. `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. <details><summary>Trac metadata</summary> | Trac field | Value | | ---------------------- | ----------------- | | Version | 8.4.3 | | Type | Bug | | TypeOfFailure | OtherFailure | | Priority | normal | | Resolution | Unresolved | | Component | Compiler (Parser) | | Test case | | | Differential revisions | | | BlockedBy | | | Related | | | Blocking | | | CC | | | Operating system | | | Architecture | | </details> <!-- {"blocked_by":[],"summary":"(~) and (!) are parsed inconsistently in types (plus documentation warts)","status":"New","operating_system":"","component":"Compiler (Parser)","related":[],"milestone":"8.8.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.4.3","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"(Spawned from https://ghc.haskell.org/trac/ghc/ticket/10056?replyto=41#comment:41)\r\n\r\n`~` 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.\r\n\r\nThere's a couple of unsatisfying things here:\r\n\r\n1. `splitTilde` handles `~`, but not `!`. This means that any use of `!` as a type operator will not work, as evidenced by this GHCi session:\r\n\r\n{{{\r\nλ> type a ! b = Either a b\r\n\r\n<interactive>:1:6: error:\r\n Malformed head of type or class declaration: a !b\r\n}}}\r\n\r\n We should update `splitTilde` to handle `!` as well. (And perhaps give that function a new name to reflect its more ambitious goals.)\r\n2. `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.","type_of_failure":"OtherFailure","blocking":[]} -->