Typeclasses in lua

fun.lua

local fun = {}

--[[

fun.lua, tiffany's typeclass implementation in lua

Copyright (c) 2013 tiffany <tiffany@stormbit.net>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Usage:

--To create a trait
fun.trait "SomeTypeclass" {
  doSomething = function() doSomethingNoIMeanReallyDoIt() end,
  overLoadMe = fun.stub,
  someConst = 4
}

-- an implementation of that trait for numbers
fun.impl("number", "SomeTypeclass") {
  overLoadMe = function(self, x) print("hi, "..x) end,
}

-- casting a value into a typeclass, if someval is a number it'll use the previously defined impl
local val = cast("SomeTypeclass", someval)

-- calls overLoadMe with a properly set "self"
val:overLoadMe("you")

]]

fun.traits = {}

function fun.stub()
    error "Stub function!"
end

function fun.trait(name)
    return function(tab)
        fun.traits[name] = tab
    end
end

function fun.typename(v)
    if type(v) == "userdata" then
        if type(getmetatable(v).__tname) == "string" then
            return getmetatable(v).__tname
        end
        for k, v in pairs(debug.getregistry()) do
            if v == getmetatable(v) then
                getmetatable(v).__tname = k
                return k
            end
        end
        return "userdata"
    else
        return type(v)
    end
end

function fun.impl(T, trait)
    if not fun.traits[trait] then
        error("No such trait "..tostring(trait))
    end
    if not fun.traits[trait].impls then
        fun.traits[trait].impls = {}
    end
    local impls = fun.traits[trait].impls
    return function(tab)
        local function ignored(k)
            return k == "impls"
        end
        impls[T] = {}
        for k, v in pairs(fun.traits[trait]) do
            if not ignored(k) then
                if not tab[k] then
                    error("Incomplete typeclass: No such value "..tostring(k))
                end
                if type(v) ~= type(tab[k]) then
                    error("Type mismatch for member "..tostring(k)..": Expected "..fun.typename(v)..", got "..fun.typename(tab[k]))
                end
                if type(v) == "function" then
                    local oldfun = tab[k] or v
                    impls[T][k] = function(self, ...)
                        return oldfun(self.value, ...)
                    end
                else
                    impls[T][k] = tab[k]
                end
            end
        end
        setmetatable(impls[T], {__newindex=function() error "Attempt to modify impl" end})
    end
end

function fun.cast(trait, val)
    if not fun.traits[trait] then
        error("No such trait "..tostring(trait))
    end
    if not fun.traits[trait].impls then
        error("No impls of trait "..tostring(trait))
    end
    if not fun.traits[trait].impls[fun.typename(val)] then
        error("No impl of trait "..tostring(trait).." for type "..fun.typename(val))
    end
    local instance = {}
    for k, v in pairs(fun.traits[trait].impls[fun.typename(val)]) do
        instance[k] = v
    end
    instance.value = val
    return instance
end

function fun.partialapp(fn, ...)
    if type(fn) ~= "function" then
        error "Expected function"
    end
    local t = {...}
    return function(...)
        return fn(unpack(t), ...)
    end
end

-- (.) :: (b -> c) -> (a -> b) -> a -> c
-- f . g = \x -> f (g x)
function fun.compose(...)
    local t = {...}
    return function (...)
        local last = {...}
        for i = #t, 1, -1 do
            last = {t[i](unpack(last))}
        end
        return unpack(last)
    end
end

-- map :: (a -> b) -> [a] -> [b]
function fun.map(fn, t)
    if type(fn) ~= "function" then
        error "Expected function"
    end
    if type(t) ~= "table" then
        error "Expected table"
    end
    local r = {}
    for _, v in pairs(t) do
        r[#r + 1] = fn(v)
    end
    return r
end

function fun.fold(fn, acc, t)
    if type(fn) ~= "function" then
        error "Expected function"
    end
    for _, v in pairs(t) do
        acc = fn(acc, v)
    end
    return acc;
end

function fun.filter(fn, t)
    if type(fn) ~= "function" then
        error "Expected function"
    end
    if type(t) ~= "table" then
        error "Expected table"
    end
    local r = {}
    for _, v in pairs(t) do
        if fn(v) then
            r[#r+1] = v
        end
    end
    return r
end

fun.trait "Functor" {
    fmap = function(self, fn) end
}

fun.impl("table", "Functor") {
    fmap = function(self, fn) fun.map(fn, self) end
}

return fun