Monad

gistfile1.hs

import Prelude hiding( Left, Right )

data Heading = North | East | West | South deriving( Show, Read )
type Status = ( (Int, Int), Heading )
data Direction = Forward | Backward | Left | Right deriving( Read )

progress:: Direction -> Status -> Status
progress Forward  ((x, y), North) = (( x, y+1 ), North)
progress Backward ((x, y), North) = (( x, y-1 ), South)
progress Left     ((x, y), North) = (( x-1, y ), West )
progress Right    ((x, y), North) = (( x+1, y ), East )

progress Forward  ((x, y), East) = (( x+1, y ), East ) 
progress Backward ((x, y), East) = (( x-1, y ), West )
progress Left     ((x, y), East) = (( x, y+1 ), North)
progress Right    ((x, y), East) = (( x, y-1 ), South)

progress Forward  ((x, y), West) = (( x-1, y ), West )
progress Backward ((x, y), West) = (( x+1, y ), East )
progress Left     ((x, y), West) = (( x, y-1 ), South)
progress Right    ((x, y), West) = (( x, y+1 ), North)

progress Forward  ((x, y), South) = (( x, y-1 ), South)
progress Backward ((x, y), South) = (( x, y+1 ), North)
progress Left     ((x, y), South) = (( x+1, y ), East )
progress Right    ((x, y), South) = (( x-1, y ), West )


pf = progress Forward
pb = progress Backward
pl = progress Left
pr = progress Right


foo = pb $ pr $ pf $ pf $ pr $ pf $ pf $ ((0,0),North)

($>) = flip ($)
foof = ((0,0),North) $> pf $> pf $> pr $> pf $> pf $> pr $> pb

pf':: Monad m => Status -> m Status
pb':: Monad m => Status -> m Status
pl':: Monad m => Status -> m Status
pr':: Monad m => Status -> m Status
pf' = \x -> return( pf x )
pb' = \x -> return( pb x )
pl' = \x -> return( pl x )
pr' = \x -> return( pr x )

readStatus' = \x -> return( read x ::Status )
show' = \x -> return ( show x )

fooMaybe1 = Just ((0,0),North) >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'
fooMaybe2 = Nothing            >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'


pfm1 = \x -> Just Left >>= \y -> return( progress y x )
pfm2 = \x -> Nothing   >>= \y -> return( progress y x )

pfl1 = \x -> [Forward,Backward,Left,Right] >>= \y -> return( progress y x )
pfl2 = \x -> []                            >>= \y -> return( progress y x )


--readDirection = \x -> return( read x ::Direction)
pfio = \x-> getLine >>= \y-> return( read y ::Direction) >>= \y' -> return( progress y' x )





fooList1 = [((0,0),North)]
            >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'

fooList2 = [((0,0),North),((0,0),South),((0,0),East)]
--            >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'
            >>= pf' >>= pfl1 >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'

fooList3 = []
            >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'

fooIO1 = getLine >>= readStatus'
          >>= pf' >>= pfio >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'
--          >>= pf' >>= pf' >>= pr' >>= pf' >>= pf' >>= pr' >>= pb'
          >>= show'