Generate function of given arity in Haskell using type numbers

Question

Assume I have encoded the natural numbers in Haskell types, and that I have a way of adding and subtracting from them:

data Zero
data Succ n
-- ...

I have seen various bits of code which create the appearance of variadic functions, such as this, which allows the following:

buildList "polyvariadic" "function" "wut?" :: [String]
-- ["polyvariadic","function","wut?"]

What I am wondering is whether I can build off of that to make a function which will only accept the number of arguments that corresponds to an instance of a type number. What I'm trying to do would look something like:

one = Succ Zero
two = Succ one
three = Succ two

threeStrings :: String -> String -> String -> [String]
threeStrings = buildList three

threeStrings "asdf" "asdf" "asdf"
-- => ["asdf","asdf","asdf"]

threeStrings "asdf"
-- type checker is all HOLY CHRIST TYPE ERROR

threeStrings "asdf" "asdf" "asdf" "asdf"
-- type checker is all SWEET JESUS WHAT YOU ARE DOING

I'm aware that this is pretty silly and that it's probably a waste of my time, but it seemed like something that would be fun for the weekend.

Answer 1

OK. Yes. Definitely, by threading a numeric type around the recursive instances.

First, some boilerplate:

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE EmptyDataDecls         #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FlexibleContexts       #-}
{-# LANGUAGE ScopedTypeVariables    #-}

Your nats:

data Zero
data Succ n

A recursive builder for the variadic functions, now with an n argument:

class BuildList n a r | r -> a where
    build' :: n -> [a] -> a -> r

A base case: stop when we get to Zero:

instance BuildList Zero a [a] where
    build' _ l x = reverse $ x:l

Otherwise, decrement by one and recurse:

instance BuildList n a r => BuildList (Succ n) a (a->r) where
    build' (_ :: Succ n) l x y = build' (undefined :: n) (x:l) y

Now, we only want to loop 3 times, so write that down:

build :: BuildList (Succ (Succ Zero)) a r => a -> r
build x = build' (undefined :: Succ (Succ Zero)) [] x

Done.

Testing:

> build "one" "two" "three" :: [[Char]]
["one","two","three"]

Any less or more are errors:

*Main> build "one" "two" "three" "four" :: [[Char]]

<interactive>:1:1:
    No instance for (BuildList Zero [Char] ([Char] -> [[Char]]))

*Main> build "one" "two" :: [[Char]]

<interactive>:1:1:
    No instance for (BuildList (Succ Zero) [Char] [[Char]])