The general form for a function type in Unison is I ->{A} O, where I is the input type of the function, O is the output type, and A is the set of ability requirements of the function. More generally, this can be any comma-separated list of types, like I ->{A1,A2,A3} O.
A function type in Unison like A -> B is really syntactic sugar for a type A ->{e} B where e is some set of abilities, possibly empty. A function that definitely requires no abilities has a type like A ->{} B (it has an empty set of abilities).