[Haskell-beginners] Using findIndex and then splitAt in Data.List

[David McBride]

Ah yeah, I messed up a bit.  Another way you can deal with this is using
the maybe function.  If you are absolutely positive that it can never fail,
it doesn't hurt to choose a default index of 0.

let gradeItemIndex = ...
let twoListsTuple = splitAt (maybe 0 id gradeItemIndex)

However if the code around it changes your assumption may become
invalidated in the future, and your code will explode.

On Fri, Feb 27, 2015 at 4:41 PM, Geoffrey Bays <charioteer7 at gmail.com>
wrote:

> David:
> Thanks for putting me on the right track. This works:
>
>     let twoListsTuple = case findIndex (\g -> (itemName g) == (itemName
> gradeItemP)) gradeItemList of
>         Nothing -> (gradeItemList, [])
>         Just x -> splitAt x gradeItemList
>
> I was not sure if the splitAt function would take a Just Int in place of a
> regular Int without complaint, but that seems to be the case.
> As it turns out, I can be certain from the context that the item will
> definitely be found in the list, but the case statement allows for the
> unpacking of the Maybe.
>
> Thanks,
>
> Geoffrey
>
> On Fri, Feb 27, 2015 at 4:16 PM, David McBride <toad3k at gmail.com> wrote:
>
>> You will probably do a case match on the result of findIndex to find out
>> whether you want to split there.  You will have to deal with the
>> possibility that the item you are searching for is not in the list.
>> Untested:
>>
>> let twoListsTuple = case findIndex (\g -> (itemName g) == (itemName
>> gradeItemP)) gradeItemList of
>>   Nothing -> (gradeItemList,[])
>>   Just newlists -> newlists
>>
>> On Fri, Feb 27, 2015 at 3:51 PM, Geoffrey Bays <charioteer7 at gmail.com>
>> wrote:
>>
>>> Hi.
>>> An elementary question here about two functions in Data.List:
>>>  how to use a value from findIndex which returns a Maybe Int,
>>> and then use that result in splitAt which takes a regular Int?
>>> This is in an IO() do
>>>
>>> Like so: (This is in an IO() do block)
>>>
>>>     let gradeItemIndex = findIndex (\g -> (itemName g) == (itemName
>>> gradeItemP)) gradeItemList
>>>     let twoListsTuple = splitAt  gradeItemIndex gradeItemList
>>>
>>> // does not compile obviously
>>>
>>> Many Thanks,
>>>
>>> Geoffrey
>>>
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>>
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>
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