module Numl where
import Char

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sumProd b j k = b * j + k

unfoldr p f x = if p x then [] else y : unfoldr p f r
                where (r,y) = f x

unfoldl p f x = g x []
                where g x a = if p x then a else g r (y:a)
                              where (r,y) = f x





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int b = foldl (sumProd b) 0 . map digitToInt

str b = map intToDigit . unfoldl (==0) (`divMod` b)





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int' b = foldr (flip (sumProd b)) 0 . reverse . map digitToInt

str' b = map intToDigit . reverse . unfoldr (==0) (`divMod` b)





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[(bin,unbin), (dec,undec), (hex,unhex)] = map (\b -> (str b, int b)) [2,10,16]
























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data Expr a b = Lit a | Bop b (Expr a b) (Expr a b) | Var

folde f g v (Lit a)     = f a
folde f g v (Bop o l r) = g o (folde f g v l) (folde f g v r)
folde f g v  Var        = v

data Aopr = Add | Sub | Mul

folda a s m Add = a
folda a s m Sub = s
folda a s m Mul = m

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sem = folda (+) (-) (*)
syn = folda "+" "-" "*"

infx exp = folde show (inpar . syn) "x" exp
inpar o l r = concat ["(",l,o,r,")"]

eval = flip (folde id sem)

-- eval t n = folde id sema n t

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[(!+),(!-),(!*)] = map Bop [Add,Sub,Mul]

expr' = (Var !+ Lit 2) !* ((Lit 6 !- Var) !+ (Lit 3 !* Var))

-- 2x^2 + 10x + 12