Question

basic.hs:

areaCircle :: Floating -> Floating
areaCircle r = pi * r * r

Command:

*Main> :l basic.hs 
[1 of 1] Compiling Main             ( Sheet1.hs, interpreted )

Sheet1.hs:2:15:
    Expecting one more argument to `Floating'
    In the type signature for `areaCircle':
      areaCircle :: Floating -> Floating
Failed, modules loaded: none.

I see that areaCircle :: Floating a => a -> a loads as expected. Why is the above version not acceptable?

Answer 1

Because your version does not actually supply a type. Floating is a type class. If you want to allow any Floating, then Floating a => a -> a is correct. Otherwise you can try either Float -> Float or Double -> Double.

Just to flesh it out a bit more: Floating a => a -> a says not only that your function accepts any Floating type, but that it returns the same type it is passed. This must be true even if you narrow the type. For example, you would not be able to use Float -> Double without doing some additional conversions