[Haskell-cafe] Capped lists and |append|

[Henning Thielemann]

John Millikin schrieb:
> Earlier today I uploaded the capped-list package; I didn't think there
> would be any interest, since it's a relatively trivial data structure,
> but already there's been three emails and an IRC convo about it.
>
> In short, this is Heinrich Apfelmus's "Train" type from
> <http://www.haskell.org/pipermail/haskell-cafe/2009-December/069895.html>,
> which showed up in a thread I posted regarding lazy error handling
> <http://www.haskell.org/pipermail/haskell-cafe/2009-November/069825.html>.
> The structure of a Train / CappedList (I like my name better) is:
>
>     data Train a b = Wagon a (Train a b) | Loco  b
>     data CappedList cap a = Next a (CappedList cap a) | Cap cap
>
> Since uploading, there's been a big problem pointed out to me
> regarding this structure, namely the possible definitions of |append|.
> Because the list terminus is itself a value, but isn't / shouldn't be
> the same type as the elements, either obvious implementation will drop
> it.
>
>     append :: CappedList cap a -> CappedList cap a -> CappedList cap a
>     append (Cap 0) (Cap 1) = -- either (Cap 0) or (Cap 1), but
> information has been lost
>
>   

For exception handling I need
   append :: CappedList (Maybe exception) a -> CappedList (Maybe 
exception) a -> CappedList (Maybe exception) a

where 'append' appends the second list only, if the first list did not 
end with an exception.