[Haskell-cafe] Getting the x out

[Neil Mitchell]

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 <nowgate at yahoo.com> 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 <tonymorris at gmail.com> wrote:
>
> From: Tony Morris <tonymorris at gmail.com>
> Subject: Re: [Haskell-cafe] Getting the x out
> To: "michael rice" <nowgate at yahoo.com>
> Cc: haskell-cafe at 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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>
> --
> Tony Morris
> http://tmorris.net/
>
>
>
>
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