polymorphic_effects.tao

import "../lib/main.tao"

## A function that maps elements of a list using a function.
## The mapping function, `f`, may have a side-effect, `e`.
## This side-effect is propagated to the returned list
fn emap A, B, e : (A -> e ~ B) -> [A] -> e ~ [B] =
    | _, [] => []
    \ f, [x .. xs] => [x->f! .. xs->emap(f)!]

def main : io ~ () = do {
    ## Square every number in a list (i.e: no side effect)
    let x = [1, 2, 3]->emap(fn x => x * x);
    print("Squared list = " + x)!;

    ## Print every number in a list and double it as we do so
    let y = [1, 2, 3]
        -> emap(fn x => do {
            print(x->show)!; ## This is effectful!
            x + x
        })!;
    print("Doubled list = " + y)!;
}