Left and right numbered entries in a stack.

StackPair.elm

module StackPair where

import Either as E
import Either (Either (..), lefts, rights)

type Numbered a = {a | n: Int}

type Entry a = Either (Numbered a) (Numbered a)
type Entries a = [Entry a]

{-
    add a new entry to left or right stack
-}
add: Entry a -> Entries a -> Entries a
add = (::)

{- 
    Heights of a numberd stack
    stackHeights [1,4,7] = [0,1,5,12]
-}
stackHeights : [Numbered a] -> [Int]
stackHeights s = scanl (\na x -> x + na.n) 0 (reverse s)

{- 
    heightsOf lefts [Left 1, Right 3, Left 9] == [0,1,10]
-}
heightsOf : (Entries a -> [Numbered a]) -> Entries a -> [Int]
heightsOf onSide = onSide >> stackHeights

-- This incorrect version causes the 0.13 compiler to spin indefinitely
-- heightsOf : (Entries a -> [a]) -> Entries a -> [Int]
-- heightsOf onSide = onSide >> stackHeights

main : Element
main = []
        |> add (Left {n=1})
        |> add (Right {n=3})
        |> add (Right {n=5})
        |> add (Left {n=4})
        |> add (Left {n=7})
        |> heightsOf lefts
        |> asText