Lua implementation of Maybe Monad.

maybeMonad.lua

-- The Maybe Monad:

local NOTHING = { val = nil }

maybe = {}
maybe.__index = maybe

-- Constructors:
maybe.just = function(val)
    local obj = setmetatable({ val = val }, maybe)
    return obj
end


setmetatable(NOTHING, maybe)
maybe.nothing = function()
    return NOTHING
end


maybe.fromVal = function(val)
    if val == nil then
        return maybe.nothing()
    else
        return maybe.just(val)
    end
end


-- Monad implementation:
maybe.bind = function(self, f)
    if self == NOTHING then
        return NOTHING
    else
        return f(self.val)
    end
end
maybe.__mul = maybe.bind

maybe.pure = maybe.just


-- Utilities:
maybe.isNothing = function(self)
    return self == NOTHING
end


function printMaybe(maybeObj)
    print(showMaybe(maybeObj))
end


function showMaybe(maybeObj)
    if maybeObj == maybe.nothing() then
        return 'Nothing'
    else
        return 'Just ' .. tostring(maybeObj.val)
    end
end


--##############################################################################
-- Monadic helpers for string operations:
maybeStr = {}

function maybeStr.wrap(f, ...)
    local args = {...}
    local ret = function(s)
        local res = f(s, unpack(args))
        if res == nil then
            return maybe.nothing()
        else
            return maybe.just(res)
        end
    end
    return ret
end

maybeStr.match = function(pat) return maybeStr.wrap(string.match, pat) end


--##############################################################################
-- Usage example:

maybeLen = maybe.just('abacate')                  -- Just 'abacate'
    :bind(maybeStr.wrap(string.match, '([ab]+)')) -- Just 'aba'
    :bind(maybeStr.wrap(string.gsub, 'b', ''))    -- Just 'aa'
    :bind(maybeStr.wrap(string.len))              -- Just 2

printMaybe(maybeLen)


maybeLen = maybe.nothing()                        -- Nothing
    :bind(maybeStr.wrap(string.match, '([ab]+)')) -- Nothing
    :bind(maybeStr.wrap(string.gsub, 'a', ''))    -- Nothing
    :bind(maybeStr.wrap(string.len))              -- Nothing

printMaybe(maybeLen)


maybeLen = maybe.just('abacate')                  -- Just 'abacate'
    :bind(maybeStr.wrap(string.match, '(%d+)'))   -- Nothing
    :bind(maybeStr.wrap(string.gsub, 'b', ''))    -- Nothing
    :bind(maybeStr.wrap(string.len))              -- Nothing

printMaybe(maybeLen)


maybeLen = maybe.just('abacate')                  -- Just 'abacate'
    * maybeStr.match('([abc]+)')                  -- Just 'abaca'
    * maybeStr.wrap(string.gsub, 'b', '')         -- Just 'aaca'
    * maybeStr.wrap(string.len)                   -- Just 4

printMaybe(maybeLen)


maybeLen = maybe.just('abacate')                                         -- Just 'abacate'
    * function(s) return maybe.fromVal(string.match(s, '([abc]+)')) end  -- Just 'abaca'
    * function(s) return maybe.fromVal(string.gsub(s, 'b', '')) end      -- Just 'aaca'
    * function(s) return maybe.just(#s) end                              -- Just 4

printMaybe(maybeLen)