• thomasw's avatar
    Support wild cards in TH splices · 49373ffe
    thomasw authored
    - 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
    49373ffe
HsTypes.hs 38.7 KB