[Haskell-cafe] Short circuiting and the Maybe monad
abhay.parvate at gmail.com
Tue May 13 01:13:33 EDT 2008
Yes, I had always desired that the operator >>= should have been right
associative for this short cut even when written without the 'do' notation.
On Tue, May 13, 2008 at 3:39 AM, John Hamilton <jlhamilton at gmail.com> wrote:
> I'm trying to understand how short circuiting works with the Maybe monad.
> Take the expression n >>= f >>= g >>= h, which can be written as
> (((n >>= f) >>= g) >>= h) because >>= is left associative. If n is
> Nothing, this implies that (n >>= f) is Nothing, and so on, each nested
> sub-expression easily evaluating to Nothing, but without there being a
> quick way to short circuit at the beginning.
> Now take the example
> do x <- xs
> y <- ys
> z <- zs
> return (x, y, z)
> which I believe desugars like
> xs >>= (\x -> ys >>= (\y -> zs >>= (\z -> return (x, y, z))))
> Here the associativity of >>= no longer matters, and if xs is Nothing the
> whole expression can quickly be determined to be Nothing, because Nothing
> >>= _ = Nothing. Am I looking at this correctly?
> - John
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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