based on https://stackoverflow.com/questions/38603014/type-signature-for-function-with-possibly-polymorphic-arguments

Poly.idr

module Poly

fT : (img:Type->Type) -> Type
fT img = (a:Type) -> (a -> img a)

g : {a:Type} -> {b:Type} -> (img : Type -> Type) -> (fT img) -> a -> b -> (img a, img b)
g {a} {b} _ f x y = ((f a) x, (f b) y)

f1 : a -> List a
f1 x = [x]

x1 : Int
x1 = 42

c1 : Char
c1 = 'c'

f2 : Int -> Int
f2 x = 3*x

x2 : Int
x2 = 5

t1 : (List Int, List Char)
t1 = g List f1 x1 c1 -- SCOPE ERROR