Prelude> -- I have 2 functions: f and g
Prelude> f x y = x + y
Prelude> g x = 2*x
Prelude> f 2 3
5
To express $ f(x, g(y))*
with x=2 and y=3 this work well:
Prelude> f 2 (g 3)
8
why does the following return error ?
Prelude>
Prelude> f 2 g 3
<interactive>:19:1: error:
• Non type-variable argument in the constraint: Num (a -> a)
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. (Num a, Num (a -> a)) => a
Prelude>
f 2 g 3
is (because function application left-associative):
f 2 g 3 = ((f 2) g) 3
that's why you get this error - it expects g
there to be a Num
(as it is the parameter y
in f x y = x+y
and + :: Num a -> a -> a -> a
)
2
as a literal can be a value in every Num a
but GHC does not know a instance for Num
that is a function a -> a
.
Now the error itself talks about the context - basic Haskell cannot have a constraints of the form Num ((->) a a)
- but you could easily (and safely) circumvent this with the given extension ... then you should get the error with the type-class.