#10098: Refactor wild card renaming -------------------------------------+------------------------------------- Reporter: | Owner: thomasw thoughtpolice | Status: new Type: bug | Milestone: 7.12.1 Priority: normal | Version: 7.11 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: #9922 Test Case: | Blocking: | Differential Revisions: Phab:D613 | -------------------------------------+------------------------------------- Imported from #9922, comment #5: --------------------- Austin: let's merge this to 7.10.1. Thomas: I'm not very happy with the way that wild-card error reporting is done. I think we discussed this before, but left it on one side until it was all working. Which it now is. What I suggest is this: Things I dislike: * I dislike the `checkParitalTypeSignature` and `checkNoPartialType` stuff in `RdrHsSyn`. The only checks that belong in `RdrHsSym` are ones that prevent you building a syntax tree at all. All other checks are best done later, when good error reporting is easier, and you can recover from errors. * I particularly hate the `RnTypes.extractWildcards` stuff. It's like a whole extra renaming pass over the type, changing `HsWildCardTy` to `HsNamedWildCardTy` with an `Exact` `RdrName` in it. Yuk! Here is a possible plan: * Remove all the checking from `RdrHsSyn`, unless we can't build a syntax tree without it. * Collapse `HsWildcardTy` and `HsNamedWildcardTy` into one, with a boolean (or a `Named`/`Anonymous` flag) to distinguish. * Provide a specialised version of `rnLHsType`, perhpas `rnLHsTypeWithWildCards`, that does the inital pass to find the named wildcards and bring them into scope. This version is called in the places where you can have a type with wildcards, namely * `TypeSig` * `ExprWithTySig` * `rnHsBndrSig` * `rnLHsTypeWithWildCards` can work like this: 1. Collect all the named wildcards, and bring them into scope. This is a simple, ''pure'' function. 2. Call `rnLHsType`. When `rnLHsType` finds an anonymous wildcard, just make up a fresh name, rather than looking it up. 3. Collect all the wildcards (named or anonymous) to get a `[Name]`; again a pure function 4. Return the renamed type and the wildcard names That makes three passes, but each is simple. In fact (1) and (4) could perhaps be the same function, with a boolean flag to say which wildcards to return. * All this means that when `rnLHsType` is called directly (not via `rnLHsTypeWithWildCards`) on a type like `_ -> Int`, it will succeed, generateing a fresh name for the `_`. That's fine. In `tc_hs_type` we will find it is not in scope, so we can say "Unexpected wildcard in type", and the enclosing location information will nail down the details. * There are, I think, three places where `HsWithBndrs` is used: `HsDecls.HsTyPats`, `HsDecls.RuleBndr`, `HsPat.SigPatIn`. In the latter two I think that wildcards should be legal; in the first not so. (Do you have tests for all three?) So the caller of `rnHsBndrSig` should check for empty wildcards in the cases where there shouldn't be any. I this this is just in type/data family patterns. I have not throught throught the extra-constraints wild card, but I think a similar plan should work. Does this make sense? Might you do it? (To HEAD, of course.) Thanks Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10098 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler