suggestion: use lazy pattern matching for Monoid instances of tuples

Petr Pudlák petr.mvd at
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?

   Best regards,

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.
> -Edward
> On Mon, Aug 19, 2013 at 3:29 AM, Petr Pudlák <petr.mvd at 
> <mailto:petr.mvd at>> 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
>     analyzer...)
>     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,
>       Petr

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