Test.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE ConstraintKinds #-}

type Value = String
type FromJSON = Read
fromJSON :: FromJSON a => Value -> a
fromJSON = read

type family Result a where
    Result (a -> b) = Result b
    Result b = b

class ParseArgs a where
    parseArgs :: a -> [Value] -> Maybe (Result a)

instance (Result a ~ a) => ParseArgs a where
    parseArgs x [] = Just x
    parseArgs _ _ = Nothing

instance (FromJSON a, ParseArgs b) => ParseArgs (a -> b) where
    parseArgs f [] = Nothing
    parseArgs f (x:xs) = parseArgs (f (fromJSON x)) xs 
    
add :: Int -> Int -> Int
add x y = x+y

five :: Maybe Int
five = parseArgs add ["2","3"]