[Haskell-cafe] Getting the x out

[Ross Mellgren]

If you want to just get the value out, meaning you'll get a program  
error if it happens to be Nothing, then you can use  
Data.Maybe.fromJust. But usually, you'd want to preserve the Nothing.  
Applicative or Monad is pretty good for this:

import Control.Applicative

(3+) <$> safeDivision 10 5

the result will be Just 5.0 in this case, but if the division was  
incorrect it would be nothing.

If you want to do something else, you can either pattern match on it:

case safeDivision 10 5 of
   Just x -> -- do something with x
   Nothing -> -- do something else

or use some functions from Data.Maybe. Say you want to evaluate to 1  
instead of Nothing:

import Data.Maybe

fromMaybe 1 (safeDivision 10 5)

-Ross

On Apr 21, 2009, at 8:49 PM, michael rice wrote:

> How do I get the x out of Just x?
>
> Michael
>
> =============
>
> safeDivision :: Float -> Float -> Maybe Float
> safeDivision x y = if y == 0 then Nothing else Just (x/y)
>
> *Main Data.List> safeDivision 10 5
> Just 2.0
> *Main Data.List> 3 + (safeDivision 10 5)
>
> <interactive>:1:0:
>     No instance for (Num (Maybe Float))
>       arising from a use of `+' at <interactive>:1:0-22
>     Possible fix: add an instance declaration for (Num (Maybe Float))
>     In the expression: 3 + (safeDivision 10 5)
>     In the definition of `it': it = 3 + (safeDivision 10 5)
> *Main Data.List>
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090421/4f899c79/attachment.htm