Currency monad

gistfile1.coffee

# mv = [currency amount]
# v = "$ 123"

rates =
    '$': 8
    'E': 10
    'R': 0.25
    'U': 1
    'T': 123

unit = (mv) ->
    "#{mv[0]} #{(mv[1] / rates[mv[0]]).toFixed 2}"


bind = (mv, f) ->
    [currency, amount] = mv

    if currency in (Object.keys rates) and amount and not (isNaN amount)
        f (rates[currency] * (parseInt amount, 10))
    else
        "Bad value"

lift_value = (v) ->
    v.split ' '

lift2 = (f) ->
    (a, b, res_curr) ->
        ma = lift_value a
        mb = lift_value b

        bind(ma, (a) ->
                    bind(mb, (b) ->
                                unit([res_curr, f(a, b)])))

add = (a, b) -> a + b
sub = (a, b) -> a - b

cadd = lift2 add
csub = lift2 sub

doasync = (funcs...) ->


module.exports = {unit, bind, lift2, add, cadd, csub}