cps version of function reversing slice compared with Haskell's revCps.hs

revCps.go

package main

import "fmt"

// Haskell: data R a = R (R a) | E a
type R func() (R, []int)

// Haskell: Cont (R [a]) [a]
type Cont func([]int) (R, []int)

func step(x int, xs []int, k Cont) (R, []int) {
    fmt.Printf("Got x = %v, xs = %v, k = %v\n", x, xs, k)
    xs = append(xs, x)
    if k != nil {   // Preserve nil.
        // Haskell: R (k (x : xs))
        t := func() (R, []int) { // t is thunk.
            return k(xs)
        }
        return t, nil
    }
    return nil, xs
}

// Haskell: In fact, this is '>>= \zs -> shift (f zs)' in foldr, which is
// hidden behind 'm'. For reference, haskell shift implementation:
// shift :: ((a -> r) -> Cont r r) -> Cont r a
// shift g = Cont $ \k -> runCont (g k) id
func shift(f func([]int, Cont) (R, []int), k Cont) Cont {
    return func (xs []int) (R, []int) {
        return f(xs, k)   // Haskell's shift: runCont (g k) id
    }
}

func reverseCps(xs0 []int) (R, []int) {
    var m Cont  // Haskell: Cont (R [a]) [a]
    for _, x := range xs0 { // Haskell: foldr .. xs
        /*
        m = func (k Cont) Cont {    // Haskell: shift .. >>= m
            return func (xs []int) (R, []int) {
                return step(x, xs, k)   // Haskell: runCont (f k) id
            }
        }(m) // Capture current m
        */
        stepX := func (xs []int, k Cont) (R, []int) { return step(x, xs, k) } // Haskell: step x
        // It's not a real 'Cont' though, because there is no >>=. Monads
        // composition is done manually here. For real 'Cont' see cont.go.
        m = shift(stepX, m) // Haskell: \zs -> m =<< (shift (step x zs))
    }

    if m != nil {
        // Unwrap Cont.
        return m(nil)   // flip runCont id
    }
    return nil, []int{} // return . E
}

func runR(f R, zs []int) []int {
    for ; f != nil; f, zs = f() {    // Haskell: fix run
        fmt.Printf("Iterate..\n")
    }
    return zs
}

func main() {
    xs := []int{1, 2, 3, 4}
    fmt.Printf("%v\n", runR(reverseCps(xs)))
}