On Montag, 6. Juni 2011, 09:45, Patrick Browne wrote:
Are casts required to run the code below? If so why? Thanks, Pat
-- Idetifiers for objects class (Integral i) => IDs i where startId :: i newId :: i -> i newId i = succ i sameId, notSameId :: i -> i -> Bool -- Assertion is not easily expressible in Haskell -- notSameId i newId i = True sameId i j = i == j notSameId i j = not (sameId i j) startId = 1
instance IDs Integer where
-- are casts need here? sameId (newId startId::Integer) 3 sameId (3::Integer) (4::Integer) notSameId (3::Integer) (newId (3::Integer))
The type signatures (not casts) are needed if the compiler cannot determine the instance to use from the context. If you have e.g. a declaration foo :: Integer foo = whatever then sameId foo (newId 5) doesn't need a type signature since foo's type is known and determines the rest. Without such information, the compiler can't determine the instance it should use, so fails with an ambiguous type. [Some module might contain instance IDs Int where startId = 0 newId k = 3*k sameId i j = ((i `xor` j) .&. 7) == 0 or something, then what?]