There's a lot of well thought out stuff in Haskell, but getting familiar with
it all seems like a huge task.
Thanks for the help.
Michael
--- On Tue, 4/21/09, Ross Mellgren <rmm-hask...@z.odi.ac> wrote:
From: Ross Mellgren <rmm-hask...@z.odi.ac>
Subject: Re: [Haskell-cafe] Getting the x out
To: "michael rice" <nowg...@yahoo.com>
Cc: haskell-cafe@haskell.org
Date: Tuesday, April 21, 2009, 8:53 PM
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>
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