CFGExample.hs

{-# OPTIONS -XDoRec -XDeriveDataTypeable #-}

module Example where

import Text.Earley
import Data.Dynamic
import Control.Applicative

data E = Nat Int
       | Add E E deriving (Show, Typeable)

grammar :: Grammar (Rule Char E)
grammar = do
  nat <- rule "NAT"
         [ fmap (\_ -> 0) (term '0')
         , fmap (\_ -> 1) (term '1')
         ]
  rec expr <- rule "EXPR"
              [ fmap Nat $ var nat
              , pure (\x _ y -> Add x y)
                <*> var expr
                <*> term '+'
                <*> var expr
              ]
  return expr