{-# OPTIONS -XTypeFamilies -XFlexibleContexts -XRankNTypes #-}

-- | The Satisfy type class.
module Satisfy (
    -- * The Satisfy type class
    Satisfy(..), SatisfyA, SatisfyM,
    
    -- * Some combinators
    symbol, token, choice, oneOf
  ) where

import Control.Applicative
import Control.Monad
import Data.Traversable (sequenceA)
import Data.Foldable (asum)

-- | A parser consumes a list of symbols and produces a result on a valid parse.
class Functor p => Satisfy p where
  -- | The type of the input symbols.
  type Input p :: *

  -- | Recognise a symbol matching a predicate.
  satisfy :: (Input p -> Bool) -> p (Input p)

-- | A polymorphic applicative parser.
type SatisfyA s a = forall p. (Satisfy p, Alternative p, Input p ~ s) => p a

-- | A polymorphic monadic parser.
type SatisfyM s a = forall p. (Satisfy p, MonadPlus p, Input p ~ s) => p a

-- | Recognise a specific symbol.
symbol :: (Satisfy p, Eq (Input p)) => Input p -> p (Input p)
symbol = satisfy . (==)

-- | Recognise a specific string.
token :: (Satisfy p, Eq (Input p), Applicative p) => [Input p] -> p [Input p]
token = sequenceA . map symbol

-- | Try all alternatives in order. (A synonym for 'asum'.)
choice :: Alternative f => [f a] -> f a
choice = asum

-- | Recognise one of many symbols.
oneOf :: (Satisfy p, Alternative p, Eq (Input p)) => [Input p] -> p (Input p)
oneOf = choice . map symbol