Class annotation on datatype ineffective in function
The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter.
module Main where
class (C a) data (C foo) => XY foo = X foo | Y foo
bar :: a -> XY a bar aFoo = X aFoo
main = return ()
I get:
$ ghc Test.hs
Test.hs:8:8: No instance for (C a) arising from use of `X' at Test.hs:8:8-10 Possible fix: add (C a) to the type signature(s) for `foo' In the expression: X a In the definition of `foo': foo a = X a
As suggested, this works:
bar :: (C a) => a -> XY a
Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)? Thanks, - Reto
Reto, You gave us a code snippet:
The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter.
module Main where
class (C a) data (C foo) => XY foo = X foo | Y foo
bar :: a -> XY a bar aFoo = X aFoo
main = return ()
And asked:
Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)?
Well, the compiler can infer this type, but it does not even try to do so, because you yourself explicitly gave a type signature for bar. So, then the compiler only checks whether bar indeed has the claimed type. Here, it does not, because the type you gave is too general. If you would have omit the signature, the compiler would have inferred the right type, i.e., including the class constraint. If you look at type signatures as machine-checkable documentation, then the compiler here pointed you at a flaw in your documentation. Cheers, Stefan
Reto Kramer wrote:
The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter.
class (C a) data (C foo) => XY foo = X foo | Y foo
bar :: a -> XY a bar aFoo = X aFoo
As suggested, this works:
bar :: (C a) => a -> XY a
Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)?
Hi Reto - If you'd not given any signature at all the compiler would have inferred the correct type for bar, but since you gave an explicit signature, the compiler had no option but to complain that you missed out the C a constraint. (ie if you decide to provide a signature you must give the full signature including constraints since the compiler won't add anything to them - partial signatures are not (yet) supported) Regards, Brian. -- http://www.metamilk.com
participants (3)
-
Brian Hulley -
Reto Kramer -
Stefan Holdermans