suggestion: use lazy pattern matching for Monoid instances of tuples

Petr Pudlák petr.mvd at
Mon Aug 19 09:29:24 CEST 2013

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,
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