Re: [Haskell-cafe] Getting the x out

Hi Neil, I just tried out Hoogle for the first time and it works great. It seems there are a lot of resources available for Haskell; it just takes time to find out about them and how they work. Thanks. Michael --- On Wed, 4/22/09, Neil Mitchell wrote: From: Neil Mitchell Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" Cc: "Tony Morris" , haskell-cafe@haskell.org Date: Wednesday, April 22, 2009, 1:26 AM Hi It's not too hard. You wanted a function that converted Maybe a -> a, you just Hoogle for it: http://haskell.org/hoogle/?hoogle=Maybe+a+->+a Thanks Neil On Wed, Apr 22, 2009 at 2:07 AM, michael rice wrote:

Got it! I figured there must be some way to unpack it.

My goodness, there are so many functions I'm not even aware of. Has anyone ever counted them all?

Thanks.

Michael

--- On Tue, 4/21/09, Tony Morris wrote:

From: Tony Morris Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" Cc: haskell-cafe@haskell.org Date: Tuesday, April 21, 2009, 8:54 PM

You mean, the x out of *Maybe* x even. In the very literal sense, the assumption that there is an x in "Maybe x" is false -- there may not be one since it is maybe, but not necessarily, x. IT's a bit like the use of null that you might have seen in other languages where you might have a value or you might have null. What you can do however, is say "give me the x if there is one, otherwise, use this value".

This is the fromMaybe function.

Prelude Data.Maybe> let safeDivision x y = if y == 0 then Nothing else Just (x/y) Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 5) 5.0 Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 0) 45.0

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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