Well, the value of par x y is identical to that of y, so any expression which you could use to semantically distinguish pseq from seq using par could be rewritten into one which did so without involving par. If the way in which we're telling programs apart involves performance characteristics then it may already be possible to distinguish seq from pseq. It somewhat comes down to whether the implementation of the language is clever enough to notice when compiling seq x y any cases where it might be better to finish evaluating y first and simply evaluate x before making the result of that first evaluation available. GHC does do this rearranging, so probably someone can come up with a good example there. On Apr 29, 2016 5:38 PM, "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 wrote:

...

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...

...

[2]:

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

...

Regards, Takenobu

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Hi Prime, This is additional information to organize my brain. This issue also occurs in single thread. Especially, when they have side effects. seq exp1 exp2 Because compiler can always re-order two expressions in accordance with seq's denotational semantics. Regards, Takenobu 2016-04-30 16:11 GMT+09:00 Takenobu Tani :

Hi Jose and Cale,

Thank you for clear and detailed explanation.

short summary:

* `seq` used to eliminate/manage space leaks * `pseq` used to specify order of evaluation

* `seq` is a bad name, but well established. * If we introduce parallelism to standard, we need `pseq` or some method.

It's depending on whether or not corresponding to the parallelism. I learned a lot. Thank you very much.

Regards, Takenobu

2016-04-30 8:17 GMT+09:00 Cale Gibbard :

...

Well, the value of par x y is identical to that of y, so any expression which you could use to semantically distinguish pseq from seq using par could be rewritten into one which did so without involving par.

If the way in which we're telling programs apart involves performance characteristics then it may already be possible to distinguish seq from pseq. It somewhat comes down to whether the implementation of the language is clever enough to notice when compiling seq x y any cases where it might be better to finish evaluating y first and simply evaluate x before making the result of that first evaluation available. GHC does do this rearranging, so probably someone can come up with a good example there. On Apr 29, 2016 5:38 PM, "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 wrote:

...

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...

...

[2]:

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

...

Regards, Takenobu

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Hi Carter, You are right. I had mixed case. In single thread case, I implicitly assumed IO context rather than pure parallel context. Thank you for explanation. Regards, Takenobu 2016-04-30 23:20 GMT+09:00 Carter Schonwald :

On Sat, Apr 30, 2016, 10:16 AM Takenobu Tani wrote:

...

Hi Prime,

This is additional information to organize my brain.

This issue also occurs in single thread. Especially, when they have side effects.

seq exp1 exp2

Because compiler can always re-order two expressions in accordance with seq's denotational semantics.

Regards, Takenobu

That requires / presumes a none idempotent use of unsafe perform io in those sub expressions right?

...

2016-04-30 16:11 GMT+09:00 Takenobu Tani :

...

Hi Jose and Cale,

Thank you for clear and detailed explanation.

short summary:

* `seq` used to eliminate/manage space leaks * `pseq` used to specify order of evaluation

* `seq` is a bad name, but well established. * If we introduce parallelism to standard, we need `pseq` or some method.

It's depending on whether or not corresponding to the parallelism. I learned a lot. Thank you very much.

Regards, Takenobu

2016-04-30 8:17 GMT+09:00 Cale Gibbard :

...

Well, the value of par x y is identical to that of y, so any expression which you could use to semantically distinguish pseq from seq using par could be rewritten into one which did so without involving par.

If the way in which we're telling programs apart involves performance characteristics then it may already be possible to distinguish seq from pseq. It somewhat comes down to whether the implementation of the language is clever enough to notice when compiling seq x y any cases where it might be better to finish evaluating y first and simply evaluate x before making the result of that first evaluation available. GHC does do this rearranging, so probably someone can come up with a good example there. On Apr 29, 2016 5:38 PM, "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 wrote:

...

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...

...

[2]:

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

...

Regards, Takenobu

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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:

...

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...

...

[2]:

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

...

Regards, Takenobu

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_______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org javascript:; http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime

...

...

...

...

José Manuel Calderón Trilla writes:

Is this the reason it wasn't standardized this way to begin with? Miranda's `seq` is like `pseq` (as was `seq` in other lazy language implementations) so it's not as if there wasn't precedent.

Note that this has been discussed previously, but another incarnation of the Prime committee: https://mail.haskell.org/pipermail/glasgow-haskell-users/2006-November/01148... -- John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2