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SPJ and J Launchbury seem to think ST is pure. Looking over the PLDI paper, I get the impression that the key difference between ST and the usual State monad i

by mafribe 11y ago

SPJ and J Launchbury seem to think ST is pure. Looking over the PLDI paper, I get the impression that the key difference between ST and the usual State monad is (1) the ability to name state, and (2) the use of parametric polymorphism to prevent scope-extrusion of state. Of course ML has scope-extrusion of state, and one of my questions/examples upthread was about the ability of Haskell's monadically controlled effects to implement such scope extrusion without 'tricks'. All in all I feel that the notion of purity is controversial, insufficiently theorised, and that the community would benefit from a clearer understanding of the trade-offs involved in the different notions of purity

dllthomas 11y ago
You are infuriating. If you object to notions of purity, why did you bring it up in the first place? The first place it was used in this thread was your citing a paper entitled "Pure versus Impure Lisp". In that paper, it was used to mean prohibiting "set-car!" and "set-cdr!". ST provides the equivalent of "set-car!" and "set-cdr!", so in the presence of ST the result you were citing in the paper doesn't even apply to a strict version of Haskell. Please just stop making claims about Haskell until you have some sense of what you're talking about (or at least add a caveat warning others that you might not).
- mafribe 11y ago
  I don't object to purity, I want to know what it means. I didn't realise that this is not a well-understood concept. (I mentioned Sabry's definition of purity which I found surprising.) The original discussion started because somebody complained that Haskell programmers often appear unable to transliterate imperative algorithms. I assume that this complaint was about transliteration into pure Haskell (whatever that may mean exactly). Clearly transliteration into full Haskell with actual mutability is possible. I mentioned Pippenger's article to bring to the debate the idea that the question whether such transliteration is always possible is an open research problem.
  - tome 11y ago
    I don't think anyone knows a solid definition of purity. It certainly refers to at least two things (1. independence of order of evaluation, 2. no side effects) and informally speaking we often "know" what it is in a particular context, but writing down a formal definition is very hard. I'm certainly not satisfied with Sabry's definition. I don't like the idea that you have to have both a strict and a non-strict evaluation for your language.
    - mafribe 11y ago
      If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... Anyway, I'm a bit unhappy about the requirement of full evaluation order independence, because even the purest of pure languages (e.g. the untyped lambda-calculus or PCF) don't have full evaluation order independence. I guess you mean evaluation order independence modulo termination? My intuition is that pure means that all language expressions M can always be evaluated locally by just looking at the meaning of all subterms of M. In terms of Hoare logic (for total correctness) that would mean that the pre-condition speaks only about the termination of the free variables of M (sorry for being so terse, I think this can be made formal).
      - dllthomas 11y ago
        Usually this is the way words work - we notice a regularity in the environment we wish to refer to; we build a sense of it, usually motivated by some examples and counterexamples; we give it a handle; and we then try to find a definition that matches our intuition. There certainly seem to be regularities that "pure" is intended to point at in this context, but it definitely seems that we've not settled on a definition, and we may not even agree on examples. The notions of purity I was pointing at was any that entailed the restrictions in the paper you'd linked. There are restrictions of Haskell that are pure in those senses, but Haskell taken as a whole is not, even without invoking anything unsafe.
      - tome 11y ago
        > If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... They're about what purity means!
dllthomas 11y ago
"I get the impression that the key difference between ST and the usual State monad is (1) the ability to name state, and (2) the use of parametric polymorphism to prevent scope-extrusion of state." The key difference between ST and the State monad is that the State monad is simply sugar for a function call of the form (s -> (a, s)), whereas ST allows for actual mutability. This is what changes the asymptotic complexity. ST also does things to prevent references from leaking, and that's valuable and more useful in the context of ST, but that's irrelevant to our questions up-thread. "Of course ML has scope-extrusion of state, and one of my questions/examples upthread was about the ability of Haskell's monadically controlled effects to implement such scope extrusion without 'tricks'." That wasn't this thread. I still think scope extrusion - without asking for it - would be a bug not a feature; particularly so in the case of a multithreaded, lazily evaluated language with inlining and CSE - where you really have little control over what gets executed when if you're ducking the features that guide what gets executed when. This doesn't limit expressibility, because you do have the ability to ask for it, using those "tricks" that you keep excluding for no good reason. "How expressive is Haskell if it lacked those" is an interesting question, but not am immediately practical one, and you should be clear that that is the question you're asking.

by mafribe 11y ago

dllthomas 11y ago

You are infuriating. If you object to notions of purity, why did you bring it up in the first place? The first place it was used in this thread was your citing a paper entitled "Pure versus Impure Lisp". In that paper, it was used to mean prohibiting "set-car!" and "set-cdr!". ST provides the equivalent of "set-car!" and "set-cdr!", so in the presence of ST the result you were citing in the paper doesn't even apply to a strict version of Haskell. Please just stop making claims about Haskell until you have some sense of what you're talking about (or at least add a caveat warning others that you might not).

mafribe 11y ago
I don't object to purity, I want to know what it means. I didn't realise that this is not a well-understood concept. (I mentioned Sabry's definition of purity which I found surprising.) The original discussion started because somebody complained that Haskell programmers often appear unable to transliterate imperative algorithms. I assume that this complaint was about transliteration into pure Haskell (whatever that may mean exactly). Clearly transliteration into full Haskell with actual mutability is possible. I mentioned Pippenger's article to bring to the debate the idea that the question whether such transliteration is always possible is an open research problem.
- tome 11y ago
  I don't think anyone knows a solid definition of purity. It certainly refers to at least two things (1. independence of order of evaluation, 2. no side effects) and informally speaking we often "know" what it is in a particular context, but writing down a formal definition is very hard. I'm certainly not satisfied with Sabry's definition. I don't like the idea that you have to have both a strict and a non-strict evaluation for your language.
  - mafribe 11y ago
    If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... Anyway, I'm a bit unhappy about the requirement of full evaluation order independence, because even the purest of pure languages (e.g. the untyped lambda-calculus or PCF) don't have full evaluation order independence. I guess you mean evaluation order independence modulo termination? My intuition is that pure means that all language expressions M can always be evaluated locally by just looking at the meaning of all subterms of M. In terms of Hoare logic (for total correctness) that would mean that the pre-condition speaks only about the termination of the free variables of M (sorry for being so terse, I think this can be made formal).
    - dllthomas 11y ago
      Usually this is the way words work - we notice a regularity in the environment we wish to refer to; we build a sense of it, usually motivated by some examples and counterexamples; we give it a handle; and we then try to find a definition that matches our intuition. There certainly seem to be regularities that "pure" is intended to point at in this context, but it definitely seems that we've not settled on a definition, and we may not even agree on examples. The notions of purity I was pointing at was any that entailed the restrictions in the paper you'd linked. There are restrictions of Haskell that are pure in those senses, but Haskell taken as a whole is not, even without invoking anything unsafe.
    - tome 11y ago
      > If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... They're about what purity means!

dllthomas 11y ago

"I get the impression that the key difference between ST and the usual State monad is (1) the ability to name state, and (2) the use of parametric polymorphism to prevent scope-extrusion of state." The key difference between ST and the State monad is that the State monad is simply sugar for a function call of the form (s -> (a, s)), whereas ST allows for actual mutability. This is what changes the asymptotic complexity. ST also does things to prevent references from leaking, and that's valuable and more useful in the context of ST, but that's irrelevant to our questions up-thread. "Of course ML has scope-extrusion of state, and one of my questions/examples upthread was about the ability of Haskell's monadically controlled effects to implement such scope extrusion without 'tricks'." That wasn't this thread. I still think scope extrusion - without asking for it - would be a bug not a feature; particularly so in the case of a multithreaded, lazily evaluated language with inlining and CSE - where you really have little control over what gets executed when if you're ducking the features that guide what gets executed when. This doesn't limit expressibility, because you do have the ability to ask for it, using those "tricks" that you keep excluding for no good reason. "How expressive is Haskell if it lacked those" is an interesting question, but not am immediately practical one, and you should be clear that that is the question you're asking.