suggestion: use lazy pattern matching for Monoid instances of tuples
petr.mvd at gmail.com
Mon Aug 19 15:31:24 CEST 2013
This is exactly how I run into the Monoid problem. My original `foldr`
version works fine, but when I saw this Gabriel's post:
the monoid variant looked cleaner and nicer so I wanted to redesign my
idea using monoids as well. And that was precisely when I realized that
(,) isn't lazy enough.
Could you elaborate a bit when/how the asymptotic difference occurs?
Dne 08/19/2013 02:03 PM, Edward Kmett napsal(a):
> If you are looking into a tackling lazy foldr, I'd recommend also
> including or considering using foldMap as a basis. It can make an
> asymptotic difference for some folds. I sent Gabriel a version in that
> style. I'll dig up a copy and send it your way as well.
> On Mon, Aug 19, 2013 at 3:29 AM, Petr Pudlák <petr.mvd at gmail.com
> <mailto:petr.mvd at gmail.com>> wrote:
> Thank you all for the responses.
> Edward's objection is very serious, I didn't think of it.
> Because of it I retract the proposal, this would indeed create big
> problems. (I just wish someone invents an oracle strictness
> Instead, as suggested, I'll make a package with `newtype` wrappers
> for tuples that will provide the extra-lazy monoid semantics. Any
> ideas for what other type classes except `Monoid` (and
> `Semigroup`) could be included? Or perhaps even other data types
> except tuples?
> Dne 08/18/2013 11:21 PM, Gabriel Gonzalez napsal(a):
>> I'm guessing this proposal is related to this Stack Overflow
>> answer you gave:
>> Note that your solution is very similar to the solution in the
>> `foldl` package I just released (also based off of the same blog
>> post you got your solution from). The key differences are that:
>> * The `foldl` solution is for left folds and uses a strict tuple
>> internally to prevent space leaks
>> * Your solution is for right folds and uses an extra-lazy tuple
>> internally to promote laziness
>> This suggests to me that it would be better to keep this
>> extra-lazy tuple as an internal implementation detail of a
>> right-fold package that would be the lazy analogy of `foldl`,
>> rather than modifying the standard Haskell tuple.
> Yes, this is how I encountered the problem. If I have time I'll
> make a mirror package `foldr` based on extra-lazy tuples. (Or
> perhaps we could merge the ideas into a single package.)
> Best regards,
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