Return specific type within Haskell
compiler-errors, haskell, type-systems, typeclass, types
Solution
A function with signature `Num a => a` is expected to work for any type in the class `Num`. The implementation `5.0 :: Double` just works for one type, not for all types of the class, so the compiler complains.
An example of a generic function would be:
square :: (Num a) => a -> a
square x = x * x
This works for any type that is a `Num`. It works for doubles, integers and whatever other numbers you want to use. Because of that it can have a generic type signature that just requires the parameter to be in class `Num`. (Type class `Num` is necessary because the function uses multiplication with `*`, which is defined there)
Problem
I have a pretty general question about Haskell's type system. I'm trying to become familiar with it, and I have the following function: ``` getN :: Num a => a getN = 5.0 :: Double ``` When I try to compile this, I get the following error: ``` Couldn't match expected type `a' against inferred type `Double' `a' is a rigid type variable bound by the type signature for `getN' at Perlin.hs:15:12 In the expression: 5.0 :: Double In the definition of `getN': getN = 5.0 :: Double ``` As I understand this, the function is set up to "return" a type in the class Num. Double is in this class (http://www.zvon.org/other/haskell/Outputprelude/Num_c.html), so I would have expected that it would be okay to "return" a Double in this case. Can someone explain this please?