Multiple dispatchers in JS

multiple-dispatchers.js

var slice = [].slice

function but_last(seq) {
    return seq.slice(0, -1)
}

function last(seq) {
    return seq[seq.length - 1]
}

function match_predicates(predicates, args) { var weight
    function pass(predicate, index) {
        return predicate(args[index], index, args, predicates) }

    function match_all() {
        return predicates.every(function(predicate, index) {
            if (pass(predicate, index))  return (weight += (predicate.specificity || 1)), true
            return false }) }

    weight = 0
    if (match_all())  return weight
}

function match_handler(fn, args) { var handler, handlers
    function specificity(lhs, rhs) {
        return lhs[0] - rhs[0] }

    handlers = fn.signatures.map(function(signature) { var predicates, code
        predicates = but_last(signature)
        code       = last(signature)
        return [ match_predicates(predicates, args)
               , code ] })

    handlers = handlers.filter(function(handler){ return handler[0] })
    handler = last(handlers.sort(specificity))
    return handler && handler[1]
}

function make_generic(fn) {
    function exec() { var args, callee
        args   = slice.call(arguments)
        callee = match_handler(exec, args) || exec.fallback
console.log('using ' + callee.toString(), 'for arguments ', args)
        return callee.apply(this, args)
    }
    exec.signatures = []
    exec.fallback   = fn
    return exec }


// -----------------------------------------------------------------------------
function class_of(obj)                { return {}.toString.call(obj)               }
function numericp(obj)                { return !isNaN(obj)                         }
function nump(obj)                    { return class_of(obj) === '[object Number]' }
function restp(obj, idx, args, preds) { return args.length >= preds.length
                                            && idx === preds.length - 1            }

nump.specificity     = 100
numericp.specificity = 10
restp.specificity    = 1


var sum = make_generic(function() { return 0 })
sum.signatures = [
    [ numericp
    , function(num){ return +num }]

   ,[ nump, numericp
    , function(lhs, rhs) { return lhs + sum(rhs) }]

   ,[ numericp, nump
    , function(lhs, rhs) { return sum(lhs) + rhs }]

   ,[ numericp, numericp
    , function(lhs, rhs) { return sum(lhs) + sum(rhs) }]

   ,[ nump, nump
    , function(lhs, rhs) { return lhs + rhs }]

   ,[ numericp, numericp, restp
    , function(lhs) { return sum(lhs, sum.apply(this, slice.call( arguments, 1))) }]
]

// tests
function test(expr, result, expected) {
    console.log( result === expected? '[OK]   ' : '[FAIL] '
               , expr + ' => ' + result + ' === ' + expected
               , '\n\n') }

test('sum(undefined   )', sum(undefined   ), 0)
test('sum(null        )', sum(null        ), 0)
test('sum([3]         )', sum([3]         ), 3)
test('sum(-3          )', sum(-3          ), -3)
test('sum("3"         )', sum("3"         ), 3)
test('sum(2, 3        )', sum(2, 3        ), 5)
test('sum(2, "3"      )', sum(2, "3"      ), 5)
test('sum("2", "3"    )', sum("2", "3"    ), 5)
test('sum("2", [4], 3 )', sum("2", [4], 3 ), 9)