Re: Evaluation order control between two expressions

Order of evaluation can be very important for memory use. So I can imagine cases where seq would run out of memory, but pseq would not. I would argue that pseq is almost always what you want, but seq has a nicer denotational semantics. -- Lennart On Friday, April 29, 2016, José Manuel Calderón Trilla wrote:

Hello Takenobu,

Great question, this is actually a pretty interesting issue! It isn't out of scope at all.

The first thing to think about is the following thought experiment:

Without the presence of side-effects, how can you tell the difference between a `seq` that conforms to the Haskell report and one that evaluates it's first argument before its second?

If your answer involves `unsafePerformIO` then you're cheating ;)

Even if your first argument to `seq` is an IO action it won't get executed because `seq` only evaluates to WHNF. It might be possible to construct a program that allows you to observe the difference, but in the general case I don't see how you could. I'd be very interested to be shown otherwise though!

Now in a parallel program things change. When we use `pseq` it's because we don't want two threads to collide when trying to evaluate the same expression. Let's look at an example:

x `par` y `seq` x + y

As you noted, the semantics of `seq` doesn't actually guarantee that `y` will be evaluated before `x + y`. But this only matters because we've used `par` and introduced threads (via an effect!) and therefore the possibility of collision. We can avoid this by using `pseq` instead.

So, both `seq` and `pseq` both allow the programmer to express *operational* concerns, `seq` is used mostly to eliminate/manage space leaks, and `pseq` is used to specify order of evaluation. Those concerns sometimes overlap, but they are different!

It could be argued (and I would agree) that `seq` is a bad name; a better name might have been something like `synch` [1]. That being said, unless we add parallelism to the standard (and even then) I am not sure it would be wise to change the operational behavior of `seq`. It's current behavior is well established, and if you're writing sequential Haskell code where order of evaluation matters, it's probably better to reach for a different tool (IMO). However, if parallelism is introduced then I'd fight for `pseq` to be part of that (as you suggest).

I hope that sheds some light on the issue.

Cheers,

Jose

[1]: John Hughes introduced a `synch` combinator in his thesis, but it had very different semantics, so maybe that's a reason it was avoided? Someone with more knowledge of the history can probably shed more light on this.

On Thu, Apr 28, 2016 at 6:56 PM, Takenobu Tani javascript:;> wrote:

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Dear Community,

Apologies if I'm missing context.

Does Haskell 2020 specify evaluation order control by `pseq`?

We use `pseq` to guarantee the evaluation order between two expressions. But Haskell 2010 did not specify how to control the evaluation order between two expressions. (only specified `seq` in Haskell 2010 section 6.2 [1]. but `seq` don't guarantee the order. [2])

I think it's better to explicitly specify `pseq` as standard way.

Already discussed? or out of scope?

[1]:

https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1260006...

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[2]:

https://www.schoolofhaskell.com/user/snoyberg/general-haskell/advanced/evalu...

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Regards, Takenobu

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