Re: [Haskell-cafe] Is there a generic way to detect "mzero"?

On Tue, Mar 27, 2012 at 11:03 AM, Antoine Latter wrote:

On Mon, Mar 26, 2012 at 4:25 PM, Ting Lei wrote:

...

Hi Antoine and Tobias (and everyone else),

Thanks a lot for your answers. They are really helpful

Can you please show me how to use the (Eq m) constraint to do this?

Also, my general question (probably novice-level) is that in monadic programming, you can convert not necessarily monadic codes into monadic ones. I know for many cases, it is impossible to do the reverse conversion, e.g. you can't make a function involving real IO operations into a pure code. In other cases, for example, I may need to using things like Nothing as the "null" value as in other programming languages, just to represent a special "missing" value outside the regular type. Is mzero a reasonable replacement for this or is there any reasonable (abstract) approximation in Haskell for doing this? (Like "null", I need the ability to detect it.)

I think using 'Maybe' (with Nothing) is perfect for this - this function should come in handy:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Maybe....

Ting, It's often not good style to check Nothing explicitly, rather, it's better to use monads to thread it through automatically. If you have many functions that return a Maybe, then you can chain them together using do syntax: frobnicate = do foo <- function1 bar <- function2 foo return (bar + 1) If any of the functions in the chain return Nothing, then the monad will short circuit and the whole expression will result in Nothing. The <- acts like an automatic null check. Chris

Antoine

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