module PTree where

import Text.PrettyPrint.HughesPJ

data P sym
  = PSucceed
  | PToken [sym]
  | PApply (P sym)
  | PSeq (P sym) (P sym)
  | PLabel String (P sym)
  | PUnit (P sym)
  | PChain (P sym)
  deriving (Eq, Show, Read)

type PAlgebra s r = (r, [s] -> r, r -> r, r -> r -> r, String -> r -> r, r -> r, r -> r)

foldP :: PAlgebra s r -> P s -> r
foldP (suc, tok, app, seq, lab, unit, chn) = f where
  f PSucceed = suc
  f (PToken ss) = tok ss
  f (PApply p) = app (f p)
  f (PSeq p q) = seq (f p) (f q)
  f (PUnit p) = unit (f p)
  f (PChain p) = chn (f p)

extractSource :: P s -> [s]
extractSource = foldP ([], id, id, (++), flip const, id, id)

ppp :: P Char -> Doc
ppp PSucceed = text "PSucceed"
ppp (PToken xs) = text (show xs)
ppp (PApply p) = text "<$>" $$ indent (ppp p)
ppp (PSeq p q) = text "<*>" $$ indent (ppp p) $$ indent (ppp q)
ppp (PLabel n p) = text n $$ indent (ppp p)

indent = nest 2