Foldable, Functor, and Traversable deriving handle phantom types badly
Suppose we have
data Phantom a = Z | S (Phantom a)
We'd like to get something like
foldMap _ _ = mempty
fmap = coerce
traverse _ m = pure (coerce m)
But instead we actually pattern match all the way down! Basically, we want to treat "has a phantom role" and "does not occur" similarly.
Trac metadata
Trac field | Value |
---|---|
Version | 8.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | RyanGlScott |
Operating system | |
Architecture |