Nicolas Pouillard wrote:
Excerpts from Heinrich Apfelmus's message of Tue Dec 01 11:29:24 +0100 2009:
I propose to (trivially) generalize this type to "list with an end"
data ListEnd a b = Cons a (ListEnd a b) | End b
because it may have other uses than just lazy error handling. For mnemonic value, we could call it a "train":
data Train a b = Wagon a (Train a b) | Loco b
[...]
This proposition looks quite nice and gently subsume the ListThenError type.
type ListThenError e a = Train a (Maybe e)
Anyone to put this on Hackage?
I rather like it too. The mnemonic version sounds a lot nicer than "ListEnd", though I'd probably call the constructors Cabin and Caboose. The nice thing about the generalization is that even though (Train a b) is very similar to ([a],b) it's not exactly isomorphic. There are differences in the strictness of generating them and I've often wanted something like Train. Wherever this ends up, it'd be pretty easy to do train-fusion in order to reduce the cost over using lists. If noone else wants to take it, I could probably find a few tuits to get it done. Though it looks like John Millikin already has failable-list up on Hackage, which differs only in also having a Nil to end with (which interferes with certain fusions, but not the major ones). -- Live well, ~wren