[Haskell-cafe] Re: Monads and Functions sequence and sequence_
Ivan Lazar Miljenovic
ivan.miljenovic at gmail.com
Sat Oct 30 01:38:32 EDT 2010
On 30 October 2010 16:30, Mark Spezzano <mark.spezzano at chariot.net.au> wrote:
> Not exactly. If you use the type with Maybe Int like so:
>
> sequence [Just 1, Nothing, Just 2]
>
> then the result is Nothing.
>
> Whereas sequence [Just 1, Just 2, Just 3] gives
>
> Just [1, 2, 3]
>
> Why?
>
> I assume there's special implementations of sequence and sequence_ depending on the type of monad used. If it's a sequence_ [putStrLn "hello", putStrLn "goodbye"] then this prints out hello and goodbye on separate lines.
>
> It seems to work differently for different types.
The definition of the monad. In the Maybe monad, as soon as you get a
Nothing the entire thing returns Nothing.
sequence [ma,mb,mc] = do { a <- ma; b <- mb; c <- mc; return [a,b,c] }
= ma >>= \ a -> mb >>= \ b -> mc >>= \ c -> return [a,b,c]
However, for Maybe:
instance Monad Maybe where
...
Nothing >>= f = Nothing
Just x >>= f = f x
...
So yes, the behaviour of the Monad is dependent upon the Monad.
--
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com
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