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  • #12704

Closed
Open
Opened Oct 14, 2016 by Edward Z. Yang@ezyangDeveloper

Check if constraint synonym satisfies functional dependencies

When we resolve #12679 (closed), it will be possible to implement an abstract type class using a constraint synonym. However, in my implementation, I didn't implement functional dependency checking:

signature H where
  class F a b | a -> b

-- This should be an invalid implementation
module H where
  type F a b = (Eq a, Eq b)

The check is annoying fiddly: we have to descend into the constraint synonym, collect all the implied fundeps, and then see if we have the ones we need, so I didn't implement it. Maybe some time.

Trac metadata
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Version 8.1
Type Bug
TypeOfFailure OtherFailure
Priority low
Resolution Unresolved
Component Compiler (Type checker)
Test case
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Architecture
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Reference: ghc/ghc#12704