[Haskell-cafe] Re: Are there standard idioms for lazy, pure error handling?

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.
> 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,

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