[Haskell-cafe] ANN: fixed-list -- A fixed length list library

[Job Vranish]

Is there anything wrong with increasing the context stack? That's what I do
when I run into an overflow, and so far I haven't had any problems with it.

Generally my uses of FixedList involve relatively short lists (vectors and
such) so it usually isn't a problem.

I could implement a more sophisticated way to keep track of the list lengths
that doesn't have this problem, but I really like the simplicity of the
current implementation.

- Job

On Sun, Mar 21, 2010 at 2:37 AM, Casey McCann <syntaxglitch at gmail.com>wrote:

> Job Vranish <job.vranish at gmail.com> wrote:
> > Its main advantages are:
> >  Very easy to use.
> >  Almost entirely Haskell98 (the non Haskell98 pieces are not critical,
> just
> > nice)
> >  The datatype is a member of  Foldable, Traverable, Applicative, Monad,
> > etc...
> >  Then length of the list is encoded in the type in a natural way.
>
> Unfortunately, it's very easy to get a context reduction stack
> overflow from GHC this way, which makes using such datatypes awkward
> for anything but very short lists. Explicit type annotations will
> often make things work, but at that point the type class isn't helping
> you anyway. For example, assuming the default stack size:
>
> import Data.FixedList
>
> fixed18 :: FixedList18 Int
> fixed18 = fromFoldable' [1..] -- this works
>
> fixed20 = 20 :. 20 :. fixed18 -- this works
>
> fixed22 :: FixedList22 Int
> fixed22 = 22 :. 22 :. fixed20 -- this only works with a type annotation
>
> show18 = show fixed18 -- this works
>
> -- this doesn't work:
> -- show20 = show fixed20
>
> show20' :: FixedList20 Int -> String
> show20' list20 = show list20
>
> show20 = show20' fixed20 -- this does work
>
> Using head and tail on longer lists fails likewise. I expect there's
> some way to make it work without simply increasing the stack depth,
> but I'm not sure how. Any thoughts?
>
> - C.
>
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