-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Andrew Coppin wrote:
Take a look at the following:
data Writer o v = Writer [o] v
instance Monad (Writer o) where return v = Writer [] v
(Writer os v) >>= f = let (Writer os' v') = f v in Writer (os ++ os') v'
write :: o -> Writer o () write o = Writer [o] ()
writes :: [o] -> Writer o () writes os = Writer os ()
By this code, I define a new monad where I can "write" stuff into a big list. (Although I notice this is going to get slower and slower as the list grows...)
Try the dlist library instead of plain lists, for the speed issue.
For example,
test = do write 1 write 2 write 3
yields "Writer [1,2,3] ()".
Now, how do I implement the following?
data Reader i v = Reader [i] v
instance Monad (Reader i) where return = ??? (>>=) = ???
read :: Reader o o
such that a Reader is created with an initial list, and the read function fetches 1 element out of that list. That is, the expression "x <- read" will take the head element of the list and put it into x, keeping the tail to be read later.
(Oh yeah - and apparently that clashes with Prelude.read. Oh well!)
You could call it Unwriter/unwrite since it's in some way the opposite in time of Writer (and since Reader monad conventionally refers to one where the input state isn't consumed when it's read).
I can't figure out how to implement this... The closest I managed was to make a Reader object also contain a function that tells (>>=) what to do to the Reader object you're binding against... But that seems to be horribly buggy.
you could try this (I haven't tested it but it looks okay to me) newtype Unwriter i v = Unwriter ([i] -> (v, [i])) instance Monad (Unwriter i) where return v = \_ -> v (Unwriter m) >>= f = Unwriter $ \i -> let (v, i') = m i in let Unwriter m' = f v in m' i' unwrite :: Unwriter i i - --well, what do you want it to do when there is nothing more left in the input list? Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGWZERHgcxvIWYTTURAtN4AJ4zXKL97Pg9jttb27hPxJiPhibpjQCgp4AA GWsftHiS6OL1VKINkZW1bGc= =3T/G -----END PGP SIGNATURE-----