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

[Roelof Wobben]

Stefan Höck schreef op 10-11-2014 17:18:
>> I try to say that if the input list has only 1 item the outcome is the head
>> of that list.
> This is not how one typically thinks about folds. Look at the type of
> acc:
>
>    acc :: a -> Maybe a -> Maybe a
>
> acc is a function wich takes two arguments (ignoring currying): On is of
> type `a`. This is the element type of the list you fold over. The other
> is of type `Maybe a`. This is the value you accumulate in your fold.
> The initial value is `Nothing`. You start your fold literally with
> nothing, since you haven't visited any element in the list yet. Now the
> first element of the list (see below) is passed to acc. What do you do
> with it?
>
>    acc a Nothing = ???
>
> a is an element in your lit, Nothing is the value you have accumulated
> so far. What do you do? Ignore a? Keep it? If you ignore a, you return
> `Nothing`.
>
>    acc a Nothing = Nothing
>
> With this accumulator, the list will be traversed (ignoring lazy
> evaluation for the time being), all values will be
> ignored and your result will be Nothing, no matter what kind of list you
> fold over. Therefore, let's keep value `a`. In order to do that, we have
> to wrap it in a Just, otherwise the types won't match:
>
>    acc a Nothing = Just a
>
> This function will give the right result for empty lists (Nothing) and
> for lists with a single value where the value is wrapped in a Just. The
> function will throw an error however, if you pass it a list containing
> more than one element, because the pattern match is not exhaustive.
> We need to decide what happens, when we already have a
> value wrapped in a Just:
>
>    acc a Just b = ???
>
>

It makes sence except when I enter acc a Just B I see this error message :
ConstructorJustshould have 1 argument, but has been given none…