Thanks, that makes sense. And the existing behaviour is in the compiler, so the surrounding parens are optional and can/should be stripped (except for Api Annotations) Alan On Mon, May 18, 2015 at 10:11 AM, Roman Cheplyaka wrote:

On 18/05/15 10:57, Alan & Kim Zimmerman wrote:

...

Hi all

I am working on D836 and have the following test case

data MaybeDefault v where SetTo4 :: forall v a. (( Eq v, Show v ) => v -> MaybeDefault v -> a -> MaybeDefault [a])

GHC 7.10.1 regards the return type of SetTo4 as `MaybeDefault [a]`

The question is, due to the parens, is the return type not the whole RHS?

i.e. Similar to how in the signature

map :: (a -> b) -> [a] -> [b]

the first paramater is a single function.

I am sure I am just confused here.

This is the wrong analogy, since (a -> b) in map's type is in the negative position.

The correct analogy would be the return type of

map :: (a -> b) -> ([a] -> [b])

You could argue it both ways. (But only one of them leads to the above declaration being correct, since the function type is not an instance of MaybeDefault.)

Roman