#10094: Template Haskell cannot represent type wildcards -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 7.10.1-rc1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #9879, #10548 | Differential Revisions: -------------------------------------+------------------------------------- Changes (by adamgundry): * owner: adamgundry => * related: #9879 => #9879, #10548 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10094#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10094: Template Haskell cannot represent type wildcards -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Template Haskell | Version: 7.10.1-rc1 Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #9879, #10548 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"49373ffe4cbc87b46468d2372e850138e151a9ae/ghc" 49373ffe/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="49373ffe4cbc87b46468d2372e850138e151a9ae" Support wild cards in TH splices - Declaration splices: partial type signatures are fully supported in TH declaration splices. For example, the wild cards in the example below will unify with `Eq a` and `a -> a -> Bool`, as expected: ``` [d| foo :: _ => _ foo x y = x == y |] ``` - Expression splices: anonymous and named wild cards are supported in expression signatures, but extra-constraints wild cards aren't. Just as is the case for regular expression signatures. ``` [e | Just True :: _a _ |] ``` - Typed expression splices: the same wildcards as in (untyped) expression splices are supported. - Pattern splices: TH doesn't support type signatures in pattern splices, consequently, partial type signatures aren't supported either. - Type splices: partial type signatures are only partially supported in type splices, specifically: only anonymous wild cards are allowed. So `[t| _ |]`, `[t| _ -> Maybe _ |]` will work, but `[t| _ => _ |]` or `[| _a |]` won't (without `-XNamedWildCards`, the latter will work as the named wild card is treated as a type variable). Normally, named wild cards are collected before renaming a (partial) type signature. However, TH type splices are run during renaming, i.e. after the initial traversal, leading to out of scope errors for named wild cards. We can't just extend the initial traversal to collect the named wild cards in TH type splices, as we'd need to expand them, which is supposed to happen only once, during renaming. Similarly, the extra-constraints wild card is handled right before renaming too, and is therefore also not supported in a TH type splice. Another reason not to support extra-constraints wild cards in TH type splices is that a single signature can contain many TH type splices, whereas it mustn't contain more than one extra-constraints wild card. Enforcing would this be hard the way things are currently organised. Anonymous wild cards pose no problem, because they start without names and are given names during renaming. These names are collected right after renaming. The names generated for anonymous wild cards in TH type splices will thus be collected as well. With a more invasive refactoring of the renaming, partial type signatures could be fully supported in TH type splices. As only anonymous wild cards have been requested so far, these small changes satisfying this request will do for now. Also don't forget that a TH declaration splices support all kinds of wild cards. - Extra-constraints wild cards were silently ignored in expression and pattern signatures, appropriate error messages are now generated. Test Plan: run new tests Reviewers: austin, goldfire, adamgundry, bgamari Reviewed By: goldfire, adamgundry, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1048 GHC Trac Issues: #10094, #10548 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10094#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler