[Haskell-beginners] Are these soloutions all valid and a good use of Haskell

[Julian Birch]

Consider what happens if you apply the last function to xs.

On Monday, November 10, 2014, Roelof Wobben <r.wobben at home.nl> wrote:

> Of course the compiler is right.
>
> Im still struggeling.
>
> What I try to do is this
>
> 1) if acc is empty then a empty list so the output must be Nothing
> 2) if the input has only one item then it is the last so the output must
> be that item.
> 3) if the input has more then 1item then the last item is not reached so
> acc must have the value of the next item of the input file
>
> And I do not see how I can give acc the value of the next value of the
> input file maybe head xs
>
> Roelof
>
>
> Leza Morais Lutonda schreef op 10-11-2014 19:52:
>
>> Hi Roelof,
>>
>> On 10/11/14 10:10, Roelof Wobben wrote:
>>
>>> No problem .
>>>
>>> Im strugelling to make acc work.
>>>
>>> I try to say that if the input list has only 1 item the outcome is the
>>> head of that list.
>>> But doing
>>>
>>> acc = Just (head a)  or doing acc = Just (head acc) gives both that acc
>>> or a is not in scope
>>>
>> The `a` in the type signature `acc :: a -> Maybe a -> Maybe a` is not a
>> variable name, it is a type.
>>
>>>
>>> also doing acc x = Just (head x) gives a error messages that the types
>>> are not matching.
>>>
>> :t Just (head x)
>> Maybe a
>>
>> :t acc x
>> Maybe a -> Maybe a
>>
>> So, the compiler is right!
>>
>>
>>
>>> Roelof
>>>
>>>
>>>
>>>
>>
>>
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-- 
Sent from an iPhone, please excuse brevity and typos.
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