[Haskell-cafe] Re: Are there standard idioms for lazy, pure error
wren ng thornton
wren at freegeek.org
Wed Dec 2 19:43:48 EST 2009
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 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).
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