类型签名错误Haskell

Question

This is probably a dumb question but I would like to understand type signatures better and why I am getting the following error when I run my code. Code is below.

area :: Double d => d -> d
area x = x^2 * 3.14

The error I am getting is below.

Double is applied to too many type arguments
In the type signature for `area': area :: Double d => d -> d

Answer 1

The signature you want is Double -> Double .

Double d => d -> d says “I take a value of any type d , and return a value of that same type, provided that d has an instance of a typeclass called Double ”. The compiler is looking for a typeclass called Double but there is no such typeclass; instead it finds a type called Double , giving an error.

With some extensions (such as TypeFamilies or GADTs ) you can write this type like so:

(d ~ Double) => d -> d

This says “I take a value of any type d , and return a value of that same type, provided that d is equal to Double ”. That's just a roundabout way of saying Double -> Double ; if you write a function of this type, the compiler will actually expand it out to Double -> Double :

> :set -XTypeFamilies
> let f :: (d ~ Double) => d -> d; f x = x
> :t f
f :: Double -> Double

More technically, the error you've encountered is a kind error—kinds are the “types of types” and are used to check things like giving the correct number of type parameters to a type. Because you give a type parameter to Double , GHC infers that it should be a typeclass like Eq or Ord that takes 1 type as an argument (kind * -> Constraint ), but Double is a plain type that takes no arguments (kind * ). You can see the kinds of common types & typeclasses in GHCi with the :kind or :k command to get a better understanding of them:

> :k Double
Double :: *

> :k Maybe
Maybe :: * -> *

> :k Maybe Double
Maybe Double :: *

> :k Eq
Eq :: * -> Constraint