Monads

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s.6.3.6, p.90, RR, Class Monad:
class Monad m where
   (>>=)  :: m a -> (a -> m b) -> m b
   (>>)   :: m a -> m b -> m b
   return :: a -> m a
   fail   :: String -> m a

   m >> k  =  m >>= \ _ -> k
   fail s  =  error s

Think of m a as being the (type of) a "step" in a sequence of operations, and of >> as being an operator, like `;' in an imperative language, which connects a sequence of steps together. >>= is like >> except that the former's right operand takes a parameter of type a.

Note that >> is defined in terms of >>= for brevity and consistency; they behave in essentially the same way. Below we see that >>= must be (essentially) associative and that therefore >> must be associative, i.e. p>>(q>>r) = (p>>q)>>r.

". . . Instance of Monad should [obey]":
return a >>= k          = k a
m >>= return            = m
m >>= (\x -> k x >>= h) = (m >>= k)>>= h  --~associative±
fmap f xs               = xs >>= return . f

"... lists, Maybe and IO are all instances of Monad. .... for IO, the fail method invokes error."
  >>= >> return fail
Monad
defn
(>>=) :: m a -> (a->m b) -> mb (>>) :: m a -> m b -> m b return :: a -> m a fail :: String -> m a
  m >> k = m >>= \_ -> k --default   fail s = error s --default
instances...
IO (>>=) =... --~magic --"-- return =... --~magic fail s = error s
[]
list
m >>= k = concat(map k m) --"-- return x = [x] fail s = []
Maybe (Just x) >>= k = k x
Nothing >>= k = Nothing
--"-- return = Just --~success fail s = Nothing

Example: [m.hs]
module Main where
main =
    putStrLn "-- Monad experiments, 11/2005 --"


-- example 1.1                                          m = list
 >> (putStrLn.show)                               -- equiv to...
      ( ["anna", "carol"] >>= \x ->        -- do x <- [..., ...]
        ["bill", "dave" ] >>= \y ->        --    y <- [..., ...]
        return (x,y)                       --    return (x,y)
      )                           --[(a,b), (a,d), (c,b), (c,d)]

-- example 1.2                                          m = list
 >> (putStrLn.show)
      ( ["anna", "carol"] >>= \x ->
        concat [ return [x],                  -- blob with TV
                 ["bill", "dave" ] >>= \y ->  -- a date?
                 if x > y then fail "bad vibe" else return [x,y]
               ]
      )                            --[[a],[a,b],[a,d],[c],[c,d]]


-- example 2                                           m = Maybe
 >> let sqrt x = if x >= 0 then Just(x**0.5) else Nothing
    in (putStrLn.show)
         ( sqrt 4 >>= \x ->
           sqrt 9 >>= \y ->
           return (x+y) )                            -- Just 5.0


-- example 3                                              m = IO
 >> return "print-me" >>= putStrLn
 >> fail "-- stopping by invoking IO's fail --"
-- ------------------------------------------------LA--11/2005--

RR = revised report, CUP, 2003.


7/2002, 11/2005
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© L. Allison   http://www.allisons.org/ll/   (or as otherwise indicated),
Faculty of Information Technology (Clayton), Monash University, Australia 3800 (6/'05 was School of Computer Science and Software Engineering, Fac. Info. Tech., Monash University,
was Department of Computer Science, Fac. Comp. & Info. Tech., '89 was Department of Computer Science, Fac. Sci., '68-'71 was Department of Information Science, Fac. Sci.)
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