id x = x
add1 x = x + 1

Reversible = Trait {
    .reverse = Self.() -> Self.RevReturnType
    .RevReturnType = Type
}
Rev4Bool = Patch Bool, Impl=Reversible
Rev4Bool
    .reverse self = not self
    .RevReturnType = Bool
# Str already has the 'reverse' method
Rev4Str = Patch Str, Impl=Reversible
Rev4Str
    .RevReturnType = Str
Rev4Nat = Patch Nat, Impl=Reversible
Rev4Nat
    .reverse self = -self
    .RevReturnType = Neg

reverse x = x.reverse() #  |R <: Reversible| R -> R.RevReturnType
assert False == reverse True
assert "olleh" == reverse "hello"
assert -5 == reverse 5

# rank-1
# : {(T -> U, (T, T, T)) -> (U, U, U) | T, U: Type}
triple_map|T| f, (l: T, c: T, r: T) = f(l), f(c), f(r)

assert triple_map(id, (1, 2, 3)) == (1, 2, 3)
assert triple_map(add1, (1, 2, 3)) == (2, 3, 4)

# rank-2
# : {(F, (T, U, V)) -> (W, X, Y) | F, T, U, V, W, X, Y: Type; F <: T -> W; F <: U -> X; F <: V -> Y}
triple_map2|T, U, V| f, (l: T, c: U, r: V) = f(l), f(c), f(r)

assert triple_map2(id, (True, "a", 5)) == (True, "a", 5)
assert triple_map2(reverse, (True, "hello", 5)) == (False, "olleh", -5)