Hi. Num is not a superclass of Int. Int is a type whereas Num is a type of a type. The fix would be sigmund :: Num a => a -> a ... This signature means "sigmund has type a to a where a is constrained to be a type variable of type Num" (so Num is the type of the type variable a) On Wed, Apr 29, 2020 at 12:41 PM Alexander Chen wrote:

Hi,

superclass.hs

myX = 1 :: Int

sigmund :: Int -> Int sigmund x = myX

prelude> :l superclass.hs [1 of 1] Compiling Main Ok, one module loaded.

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superclass.hs

myX = 1 :: Int

sigmund :: Num -> Num sigmund x = myX

prelude> :l superclass.hs typed_checked.hs:3:13: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num | 3 | sigmund :: Num -> Num | ^^^

typed_checked.hs:3:20: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num

===================================================================================

I would think since Num is a superclass of Int defining the type in the superclass would be OK, also in the error message it refers to (see black) what does that refer to?

thanks!

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