Re: [Haskell-cafe] Question on a common pattern

On Mon, 2011-03-14 at 17:56 +0100, Yves Parès wrote:

If you have only one alternative, then you can simply do:

Opt1 <- someIO

E.g., if you are _sure_ that foo returns always a 'Just' within a monad you can perfectly do :

Just x <- foo

Please beware - it is not exactly the same as with case if your assumption is wrong:

(do Just x <- return Nothing; [x]) [] (do x <- return Nothing; case x of Just x -> [x]) *** Exception: <interactive>:1:26-48: Non-exhaustive patterns in case

For IO and many monads it is the same but it is not necessary for any monad. Regards