{-# LANGUAGE MultiParamTypeClasses, LambdaCase #-}

module FreeVars (FreeVars(..), fvApp, fvIf, fvFun, fvLet) where

import qualified Data.Set as Set
import Data.Set (Set)
import Data.Foldable

class Ord b => FreeVars a b where
    freeVars :: a -> Set b

fvApp :: FreeVars e t => e -> e -> Set t
fvApp f a = Set.unions (map freeVars [f, a])

fvIf :: FreeVars e t => e -> e -> e -> Set t
fvIf p c a = Set.unions (map freeVars [p, c, a])

fvFun :: FreeVars e t => t -> e -> Set t
fvFun p b = Set.delete p (freeVars b)

fvLet :: (FreeVars e t, Foldable f) => (Set t, f e) -> e -> Set t
fvLet (bVs, bBs) b = Set.difference
    (Set.union (freeVars b) (foldr (Set.union . freeVars) Set.empty bBs))
    (Set.fromList (toList bVs))