[Haskell-cafe] lazy logging (was: Are there standard idioms for lazy, pure error handling?)

[Daniel Fischer]

Am Dienstag 01 Dezember 2009 21:00:13 schrieb Daniel Fischer:
> Am Dienstag 01 Dezember 2009 20:21:27 schrieb Evan Laforge:
> > This is only peripherally related, but I also have a lot of list
> > functions that can possibly be an error, but usually processing can
> > continue.  So they tend to return [Either Error Result].  I have
> > another function thus:
> >
> > -- A foldr version is not lazy enough and overflows the stack.
>
> try
>
> foldr (\e ~(ls,rs) -> case e of { Left l -> (l:ls,rs); Right r -> (ls,r:rs)
> }) ([],[])
>
> with the lazy pattern, it should be lazy enough.

Yup. Tested now.

>
> > partition_either [] = ([], [])
> > partition_either (x:xs) =
> >     let (ls, rs) = partition_either xs
> >     in case x of
> >         Left l -> (l:ls, rs)
> >         Right r -> (ls, r:rs)
> >
> > I was a little surprised I couldn't find this in the standard
> > library... or maybe it is?
>
> Data.Either.partitionEithers

And that's not lazy enough, either.
Ticket: http://hackage.haskell.org/trac/ghc/ticket/3709

>
> Data.List.partition isLeft
>
> isLeft (Left _) = True
> isLeft _ = False

Not quite the same.