> what is the "default" structure if you don't specify any

similar to:

display::Show a => a -> String

display = show

fa::(Applicative f, Num a) => f a

fa = pure 1

f and a are bounded by Applicative and Num, so to say. No default. Or, it is typed however type is a bit broader than *

> In this case it defaults f to IO and a to Int,

does it though?

Prelude> let a1 = pure 1

Prelude> let a2 = pure 1

Prelude> a1 == a2

<interactive>:20:1: error:

• Ambiguous type variable ‘a0’ arising from a use of ‘a1’

prevents the constraint ‘(Num a0)’ from being solved.