{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, TypeFamilies #-}

-- generates arguments to functions
module GenArgs where

import Control.Monad.Random
import Data.Proxy

-- bnch represents a function whose arguments can be generated,
-- resulting in a "NFValue"
class GenArgs rnd bnch where
  type ResultOf bnch
  genArgs :: bnch -> rnd (ResultOf bnch)

instance (Generatable rnd a, GenArgs rnd b,
          Monad rnd, ResultOf b ~ ResultOf (a -> b))
  => GenArgs rnd (a -> b) where
  type ResultOf (a -> b) = ResultOf b
  genArgs f = do
    x <- genArg
    genArgs $ f x

-- a parameter that can be generated using a particular monad
class Generatable rnd arg where
  genArg :: rnd arg

instance {-# Overlappable #-} (Random a, MonadRandom rnd) => Generatable rnd a where
  genArg = getRandom

instance (Monad rnd) => Generatable rnd (Proxy a) where
  genArg = return Proxy