On Fri, Jan 8, 2010 at 6:38 PM, John Millikin
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
The order of the type parameters is important. In particular, Train is
a free monad, with (>>=) acting as a generalized append.
instance Monad (Train a) where
return = Loco
Loco a >>= f = f a
Wagon x t = Wagon x (t >>= f)
Monad instances for CappedList are also possible, but more complex:
instance Monoid cap => Monad (CappedList cap) where
return a = Next a mempty
Cap c >>= f = Cap c
Next a as >>= f = f a `mappend` (as >>= f)
(I can't check what you've uploaded because Hackage is down, so my
apologies if I'm telling you things you already know.)
--
Dave Menendez