Some functional utilities

fn.lua

-- functional programming utilities
-- usage: local fn = require "fn"

NAME = "fn"

local M = { }

-- Invokes the reducer function for each element in the collection with the accumulator.
-- The initial value of the accumulator is initial. The function is invoked for each
-- element in the enumerable with the accumulator. The result returned by the
-- function is used as the accumulator for the next iteration. The function
-- returns the last accumulator.
-- This function is a building block for a lot of other utilities.
local function reduce(collection, initial, reducer)
    local accumulator = initial
    for k, v in pairs(collection) do
        accumulator = reducer({key = k, value = v}, accumulator)
    end
    return accumulator
end

-- returns the collection with its items transformed by the mapper function
-- mapper example:
-- local doubleUp = function (kv)
--   return kv.value * 2
-- end
local function map(collection, mapper)
    return reduce(collection, {}, function (kv, acc)
        table.insert(acc, mapper(kv))
        return acc
    end)
end

-- return a collection containing only items where the predicate function returns true
-- predicate example:
-- local isEven = function (kv)
--   return (k.value % 2) == 0
-- end
local function filter(collection, predicate)
    return reduce(collection, {}, function (kv, acc)
        if predicate(kv) then
            table.insert(acc, kv.value)
        end
        return acc
    end)
end

-- opposite of filter
local function reject(collection, predicate)
    return reduce(collection, {}, function (kv, acc)
        if not predicate(kv) then
            table.insert(acc, kv.value)
        end
        return acc
    end)
end

M.reduce = reduce
M.map = map
M.filter = filter
M.reject = reject

return M