unsafeInterleaveST (and IO) is really unsafe
Timon Gehr wrote:
I am not sure that the two statements are equivalent. Above you say that the context distinguishes x == y from y == x and below you say that it distinguishes them in one possible run.
I guess this is a terminological problem. The phrase `context distinguishes e1 and e2' is the standard phrase in theory of contextual equivalence. Here are the nice slides http://www.cl.cam.ac.uk/teaching/0910/L16/semhl-15-ann.pdf Please see adequacy on slide 17. An expression relation between two boolean expressions M1 and M2 is adequate if for all program runs (for all initial states of the program s), M1 evaluates to true just in case M2 does. If in some circumstances M1 evaluates to true but M2 (with the same initial state) evaluates to false, the expressions are not related or the expression relation is inadequate. See also the classic http://www.ccs.neu.edu/racket/pubs/scp91-felleisen.ps.gz (p11 for definition and Theorem 3.8 for an example of a distinguishing, or witnessing context).
In essence, lazy IO provides unsafe constructs that are not named accordingly. (But IO is problematic in any case, partly because it depends on an ideal program being run on a real machine which is based on a less general model of computation.)
I'd agree with the first sentence. As for the second sentence, all real programs are real programs executing on real machines. We may equationally prove (at time Integer) that 1 + 2^100000 == 2^100000 + 1 but we may have trouble verifying it in Haskell (or any other language). That does not mean equational reasoning is useless: we just have to precisely specify the abstraction boundaries. BTW, the equality above is still useful even in Haskell: it says that if the program managed to compute 1 + 2^100000 and it also managed to compute 2^100000 + 1, the results must be the same. (Of course in the above example, the program will probably crash in both cases). What is not adequate is when equational theory predicts one finite result, and the program gives another finite result -- even if the conditions of abstractions are satisfied (e.g., there is no IO, the expression in question has a pure type, etc).
I think this context cannot be used to reliably distinguish x == y and y == x. Rather, the outcomes would be arbitrary/implementation defined/undefined in both cases.
My example uses the ST monad for a reason: there is a formal semantics of ST (denotational in Launchbury and Peyton-Jones and operational in Moggi and Sabry). Please look up ``State in Haskell'' by Launchbury and Peyton-Jones. The semantics is explained in Sec 6. Please see Sec 10.2 Unique supply trees -- you might see some familiar code. Although my example was derived independently, it has the same kernel of badness as the example in Launchbury and Peyton-Jones. The authors point out a subtlety in the code, admitting that they fell into the trap themselves. So, unsafeInterleaveST is really bad -- and the people who introduced it know that, all too well.
On 04/12/2013 10:24 AM, oleg@okmij.org wrote:
Timon Gehr wrote:
I am not sure that the two statements are equivalent. Above you say that the context distinguishes x == y from y == x and below you say that it distinguishes them in one possible run.
I guess this is a terminological problem.
It likely is.
The phrase `context distinguishes e1 and e2' is the standard phrase in theory of contextual equivalence. Here are the nice slides http://www.cl.cam.ac.uk/teaching/0910/L16/semhl-15-ann.pdf
The only occurrence of 'distinguish' is in the Leibniz citation.
Please see adequacy on slide 17. An expression relation between two boolean expressions M1 and M2 is adequate if for all program runs (for all initial states of the program s), M1 evaluates to true just in case M2 does. If in some circumstances M1 evaluates to true but M2 (with the same initial state) evaluates to false, the expressions are not related or the expression relation is inadequate.
In my mind, 'evaluates-to' is an existential statement. The adequacy notion given there is inadequate if the program execution is indeterministic, as I would have expected it to be in this case. (They quickly note this on slide 18, giving concurrency features as an example.)
See also the classic http://www.ccs.neu.edu/racket/pubs/scp91-felleisen.ps.gz (p11 for definition and Theorem 3.8 for an example of a distinguishing, or witnessing context).
Thanks for the pointer, I will have a look. However, it seems that the semantics the definition and the proof rely on are deterministic?
In essence, lazy IO provides unsafe constructs that are not named accordingly. (But IO is problematic in any case, partly because it depends on an ideal program being run on a real machine which is based on a less general model of computation.)
I'd agree with the first sentence. As for the second sentence, all real programs are real programs executing on real machines. We may equationally prove (at time Integer) that 1 + 2^100000 == 2^100000 + 1 but we may have trouble verifying it in Haskell (or any other language). That does not mean equational reasoning is useless: we just have to precisely specify the abstraction boundaries.
Which is really hard. I think equational reasoning is helpful because it is valid for ideal programs and it seems therefore to be a good heuristic for real ones.
BTW, the equality above is still useful even in Haskell: it says that if the program managed to compute 1 + 2^100000 and it also managed to compute 2^100000 + 1, the results must be the same. (Of course in the above example, the program will probably crash in both cases). What is not adequate is when equational theory predicts one finite result, and the program gives another finite result -- even if the conditions of abstractions are satisfied (e.g., there is no IO, the expression in question has a pure type, etc).
The abstraction bound is where exact reasoning necessarily stops.
I think this context cannot be used to reliably distinguish x == y and y == x. Rather, the outcomes would be arbitrary/implementation defined/undefined in both cases.
My example uses the ST monad for a reason: there is a formal semantics of ST (denotational in Launchbury and Peyton-Jones and operational in Moggi and Sabry). Please look up ``State in Haskell'' by Launchbury and Peyton-Jones. The semantics is explained in Sec 6.
InterleaveST is first referred to in chapter 10. As far as I can tell, the construct does not have specified a formal semantics.
Please see Sec 10.2 Unique supply trees -- you might see some familiar code. Although my example was derived independently, it has the same kernel of badness as the example in Launchbury and Peyton-Jones. The authors point out a subtlety in the code, admitting that they fell into the trap themselves.
They informally note that the final result depends on the order of evaluation and is therefore not always uniquely determined. (because the order of evaluation is only loosely specified.)
So, unsafeInterleaveST is really bad -- and the people who introduced it know that, all too well.
I certainly do not disagree that it is bad. However, I am still not convinced that the example actually shows a violation of equational reasoning. The valid outputs, according to the informal specification in chapter 10, are the same for both expressions.
Am 13.04.2013 00:37, schrieb Timon Gehr:
On 04/12/2013 10:24 AM, oleg@okmij.org wrote:
Timon Gehr wrote:
I am not sure that the two statements are equivalent. Above you say that the context distinguishes x == y from y == x and below you say that it distinguishes them in one possible run.
I guess this is a terminological problem.
It likely is.
The phrase `context distinguishes e1 and e2' is the standard phrase in theory of contextual equivalence. Here are the nice slides http://www.cl.cam.ac.uk/teaching/0910/L16/semhl-15-ann.pdf
The only occurrence of 'distinguish' is in the Leibniz citation.
Please see adequacy on slide 17. An expression relation between two boolean expressions M1 and M2 is adequate if for all program runs (for all initial states of the program s), M1 evaluates to true just in case M2 does. If in some circumstances M1 evaluates to true but M2 (with the same initial state) evaluates to false, the expressions are not related or the expression relation is inadequate.
In my mind, 'evaluates-to' is an existential statement. The adequacy notion given there is inadequate if the program execution is indeterministic, as I would have expected it to be in this case. (They quickly note this on slide 18, giving concurrency features as an example.)
See also the classic http://www.ccs.neu.edu/racket/pubs/scp91-felleisen.ps.gz (p11 for definition and Theorem 3.8 for an example of a distinguishing, or witnessing context).
Thanks for the pointer, I will have a look. However, it seems that the semantics the definition and the proof rely on are deterministic?
In essence, lazy IO provides unsafe constructs that are not named accordingly. (But IO is problematic in any case, partly because it depends on an ideal program being run on a real machine which is based on a less general model of computation.)
I'd agree with the first sentence. As for the second sentence, all real programs are real programs executing on real machines. We may equationally prove (at time Integer) that 1 + 2^100000 == 2^100000 + 1 but we may have trouble verifying it in Haskell (or any other language). That does not mean equational reasoning is useless: we just have to precisely specify the abstraction boundaries.
Which is really hard. I think equational reasoning is helpful because it is valid for ideal programs and it seems therefore to be a good heuristic for real ones.
BTW, the equality above is still useful even in Haskell: it says that if the program managed to compute 1 + 2^100000 and it also managed to compute 2^100000 + 1, the results must be the same. (Of course in the above example, the program will probably crash in both cases). What is not adequate is when equational theory predicts one finite result, and the program gives another finite result -- even if the conditions of abstractions are satisfied (e.g., there is no IO, the expression in question has a pure type, etc).
The abstraction bound is where exact reasoning necessarily stops.
I think this context cannot be used to reliably distinguish x == y and y == x. Rather, the outcomes would be arbitrary/implementation defined/undefined in both cases.
My example uses the ST monad for a reason: there is a formal semantics of ST (denotational in Launchbury and Peyton-Jones and operational in Moggi and Sabry). Please look up ``State in Haskell'' by Launchbury and Peyton-Jones. The semantics is explained in Sec 6.
InterleaveST is first referred to in chapter 10. As far as I can tell, the construct does not have specified a formal semantics.
Please see Sec 10.2 Unique supply trees -- you might see some familiar code. Although my example was derived independently, it has the same kernel of badness as the example in Launchbury and Peyton-Jones. The authors point out a subtlety in the code, admitting that they fell into the trap themselves.
They informally note that the final result depends on the order of evaluation and is therefore not always uniquely determined. (because the order of evaluation is only loosely specified.)
So, unsafeInterleaveST is really bad -- and the people who introduced it know that, all too well.
I certainly do not disagree that it is bad. However, I am still not convinced that the example actually shows a violation of equational reasoning. The valid outputs, according to the informal specification in chapter 10, are the same for both expressions.
A very interesting discussion, I may add my 2 cents: making unsafeInterleaveIO nondeterministic indeed seems to make it safe, more or less this was proved in our paper: http://www.ki.informatik.uni-frankfurt.de/papers/sabel/chf-conservative-lics... slides: http://www.ki.informatik.uni-frankfurt.de/persons/sabel/chf-conservative.pdf there we proposed an extension to Concurrent Haskell which adds a primitive future :: IO a -> IO a Roughly speaking future is like unsafeInterleaveIO, but creates a new concurrent thread to compute the result of the IO-action interleaved without any fixed order. We have shown that adding this primitive to the functional core language is 'safe' in the sense that all program equations of the pure language still hold in the extended language (which we call a conservative extension in the above paper) The used equality is contextual equivalence (with may- and a variant of must-convergence in the concurrent case). We also showed that adding unsafeInterleaveIO (called lazy futures in the paper..) - which delays until its result is demanded - breaks this conservativity, since the order of evaluation can be observed. Best wishes, David
On Mon, 2013-04-15 at 20:44 +0200, David Sabel wrote:
A very interesting discussion, I may add my 2 cents: making unsafeInterleaveIO nondeterministic indeed seems to make it safe, more or less this was proved in our paper:
http://www.ki.informatik.uni-frankfurt.de/papers/sabel/chf-conservative-lics... slides: http://www.ki.informatik.uni-frankfurt.de/persons/sabel/chf-conservative.pdf
there we proposed an extension to Concurrent Haskell which adds a primitive
future :: IO a -> IO a
Roughly speaking future is like unsafeInterleaveIO, but creates a new concurrent thread to compute the result of the IO-action interleaved without any fixed order.
That's very interesting to hear. It has always been my intuition that the right way to understand unsafeInterleaveIO is using a concurrency semantics (with a demonic scheduler). And whenever this "unsafeInterleaveIO is unsound" issue comes up, that's the argument I make to whoever will listen! ;-) That intuition goes some way to explain why unsafeInterleaveIO is fine but unsafeInterleaveST is right out: ST is supposed to be deterministic, but IO can be non-deterministic.
We have shown that adding this primitive to the functional core language is 'safe' in the sense that all program equations of the pure language still hold in the extended language (which we call a conservative extension in the above paper)
The used equality is contextual equivalence (with may- and a variant of must-convergence in the concurrent case).
Ok.
We also showed that adding unsafeInterleaveIO (called lazy futures in the paper..) - which delays until its result is demanded - breaks this conservativity, since the order of evaluation can be observed.
My conjecture is that with a concurrent semantics with a demonic scheduler then unsafeInterleaveIO is still fine, essentially because the semantics would not distinguish it from your 'future' primitive. That said, it might not be such a useful semantics because we often want the lazy behaviour of a lazy future. Duncan
Am 18.04.2013 15:17, schrieb Duncan Coutts:
On Mon, 2013-04-15 at 20:44 +0200, David Sabel wrote:
A very interesting discussion, I may add my 2 cents: making unsafeInterleaveIO nondeterministic indeed seems to make it safe, more or less this was proved in our paper:
http://www.ki.informatik.uni-frankfurt.de/papers/sabel/chf-conservative-lics... slides: http://www.ki.informatik.uni-frankfurt.de/persons/sabel/chf-conservative.pdf
there we proposed an extension to Concurrent Haskell which adds a primitive
future :: IO a -> IO a
Roughly speaking future is like unsafeInterleaveIO, but creates a new concurrent thread to compute the result of the IO-action interleaved without any fixed order. That's very interesting to hear. It has always been my intuition that the right way to understand unsafeInterleaveIO is using a concurrency semantics (with a demonic scheduler). And whenever this "unsafeInterleaveIO is unsound" issue comes up, that's the argument I make to whoever will listen! ;-)
That intuition goes some way to explain why unsafeInterleaveIO is fine but unsafeInterleaveST is right out: ST is supposed to be deterministic, but IO can be non-deterministic. I agree.
We have shown that adding this primitive to the functional core language is 'safe' in the sense that all program equations of the pure language still hold in the extended language (which we call a conservative extension in the above paper)
The used equality is contextual equivalence (with may- and a variant of must-convergence in the concurrent case). Ok.
We also showed that adding unsafeInterleaveIO (called lazy futures in the paper..) - which delays until its result is demanded - breaks this conservativity, since the order of evaluation can be observed. My conjecture is that with a concurrent semantics with a demonic scheduler then unsafeInterleaveIO is still fine, essentially because the semantics would not distinguish it from your 'future' primitive. Yes our result should hold for any scheduling.
That said, it might not be such a useful semantics because we often want the lazy behaviour of a lazy future. Yes I agree with that, too.
Best wishes, David
On Fri, Apr 12, 2013 at 3:37 PM, Timon Gehr
Please see Sec
10.2 Unique supply trees -- you might see some familiar code. Although my example was derived independently, it has the same kernel of badness as the example in Launchbury and Peyton-Jones. The authors point out a subtlety in the code, admitting that they fell into the trap themselves.
They informally note that the final result depends on the order of evaluation and is therefore not always uniquely determined. (because the order of evaluation is only loosely specified.)
If the final result depends on the order of evaluation, then the context in which the result is defined is not referentially transparent. If a context is referentially opaque, then equational reasoning "can fail" -- i.e., it is no longer a valid technique of analysis, since the axioms on which it depends are no longer satisfied: "It is necessary that four and four is eight" "The number of planets is eight" does not imply "It is necessary that the number of planets is eight", as "equational reasoning" (or, better put, "substitution of equals", the first order axiom for equality witnessing Leibniz equality) requires. In particular, quotation marks, necessity, and unsafeInterleaveST are referentially opaque contexts.
participants (5)
-
Alexander Solla -
David Sabel -
Duncan Coutts -
oleg＠okmij.org -
Timon Gehr