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

David Menendez dave at zednenem.com
Tue Dec 1 11:09:29 EST 2009

On Tue, Dec 1, 2009 at 5:29 AM, Heinrich Apfelmus
<apfelmus at quantentunnel.de> wrote:
> Duncan Coutts wrote:
>> On Mon, 2009-11-30 at 06:08 +0000, Malcolm Wallace wrote:
>>> However, if you really want to terminate the stream at
>>> the first error, and to reflect this in the type, then I guess you can
>>> define your own list type:
>>> data ListThenError e a = Cons a (ListThenError e a)
>>>                         | Error e
>>> Of course this has the disadvantage that then your consumer must
>>> change to use this type too.
>> I've been using this list type quite a lot recently. It's in the 'tar'
>> package for example. It comes with variants of the standard functions
>> foldl, foldr, unfoldr that take into account the error possibility.
>> At some point we should probably make a package to standardise and
>> document this lazy error handling idiom.
> 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.

This is almost a composition of a non-determism monad transformer with
an exception monad.

Specifically, "LogicT (Either e) a" is (almost) isomorphic with

data NX e a = Cons a (NX e a) | Nil | Error e

reflect :: NX e a -> LogicT (Either e) a
reflect (Cons a r) = return a `mplus` reflect r
reflect Nil = mzero
reflect (Error e) = lift (Left e)

reify :: LogicT (Either e) a -> NX e a
reify m = j $ runLogicT m (\a -> Right . Cons a . j) (Right Nil)
    where j = either Error id

Dave Menendez <dave at zednenem.com>

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