#4220: EmptyDataDecls + DeriveFunctor == Panic!
---------------------------------+------------------------------------------
Reporter: conal | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.14.1
Component: Compiler | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Compile-time crash
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Changes (by JulesBean):
* cc: ju...@… (added)
Comment:
For `Foldable` it suffices to define `foldMap` :
{{{
foldMap :: Monoid m => (a -> m) -> t a -> m
}}}
If you "don't have" anything of type `a` either because it's an
EmptyDataDecl or because it's a constructor which doesn't mention the type
variable, then the only possible result is `mempty`.
{{{
foldMap _ _ = mempty
}}}
That suffices to define an instance, but for efficiency(?) you could add
the other methods directly.
{{{
fold :: Monoid m => t m -> m
fold _ = mempty
foldMap :: Monoid m => (a -> m) -> t a -> m
foldMap _ _ = mempty
foldr :: (a -> b -> b) -> b -> t a -> b
foldr _ start _ = start
foldl :: (a -> b -> a) -> a -> t b -> a
foldl _ start _ = start
foldr1 :: (a -> a -> a) -> t a -> a
foldr1 _ _ = error "Foldable.foldr1"
foldl1 :: (a -> a -> a) -> t a -> a
foldl1 _ _ = error "Foldable.foldl1"
}}}
For `Traversable` it suffices to define `traverse`:
{{{
traverse :: Applicative f => (a -> f b) -> t a -> f (t b)
}}}
Similar reasoning - if you don't have any `a` then you can't apply the
function given, and there can be no applicative effects to consider
because no value `f b` can be constructed. So, rather like in the functor
case, you do a constructor by constructor conversion, wrapped in `pure`
for no effects.
{{{
traverse _ (T2 a) = pure (T2 a) -- n.b. these two (T2 a) expressions are
at a different type
}}}
By the same process as for Functor you might choose to go for the under-
the-hood more efficient `pure . unsafeCoerce#`, or alternatively you can
"delegate" the efficiency hack to the Functor instance and use `pure .
fmap undefined`.
Does that all sound reasonable?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4220#comment:7>
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