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Comment by jordigh | Hacker News Reader

> This method is really inefficient.

That's what I've noticed with Haskellers. They have a tendency to write really slow code and don't have a clear idea of the execution model behind what they write down.

A while ago I asked in #haskell for translations into Haskell of the following C++ code:

http://codepad.org/sQTXhqC2

This should be easy. The algorithm is right in front of you. There should not be a lot of thinking in transcribing an algorithm into Haskell. But most of them wrote down something with a fold which turned what is an O(m*n) algorithm in time and O(n) in space into something like O(m^n) in space and time. The only one who got close to the C++ implementation was one who used a lot of State monads.

Then, of course, there was someone who pointed me out to a radically different algorithm written in Haskell that I think is only O(n+m) in both space and time. I haven't been able to understand this other algorithm yet.

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The responses you got on IRC don't necessarily represent Haskell best practice (or even good practice). Most people probably have better things to do than translate random toy programs from C++.

Certain highly stateful algorithms may be difficult to translate into Haskell, in the same way that certain highly structural or lazy algorithms may be difficult to translate to C++.

I'm curious if your time complexity analyses are accurate, given that you (admittedly) did not understand the faster algorithm. You could be getting tripped up by laziness.

In general, I've found that most production Haskell code is quite fast, in both big-O and absolute terms.

Spoken like a true Haskell fanboy. Why don't you give it a shot, OP has posted the code.

Shriram Krishnamurti to the rescue! Here's a memoized version in scheme: http://blog.racket-lang.org/2012/08/dynamic-programming-vers...

> They have a tendency to write really slow code and don't have a clear idea of the execution model behind what they write down.

Of course, this isn’t unique to Haskellers. ;)

Granted, reasoning about performance in Haskell is still a relatively young art, and there aren’t many of us who are experienced with it.

If edit distance is bounded there appear to be algorithms that are linear in the input size:

http://en.wikipedia.org/wiki/Edit_distance and http://blog.notdot.net/2010/07/Damn-Cool-Algorithms-Levensht...

There is a set of simple patterns/recipes most people use in Haskell that corresponds to their kind of math intuition Haskell seems to support. It doesn't have anything to do with efficiency though, for that you need to be an expert and you still won't be 100% sure the compiler really optimizes the execution the way you intended.

> you still won't be 100% sure the compiler really optimizes the execution the way you intended.

You can be sure if you read the generated Core and/or assembly code.

And if you're willing to do that on every upgrade of the compiler or anything that may affect the compiler's exact execution path.

Performance regressions in GHC are taken seriously as far as I've seen. It's not that big of a deal since you should test/profile your program on any compiler upgrade.

If you see any regressions, just post it to the ghc mailing list and it will probably be solved for you since it's much needed regression feedback/data.

That's good to know. Though I will note that I don't use Haskell. (Haven't had the time to properly learn it, and keep getting frustrated by minor things when I try to dabble with it.)

That being said, I used to think that about Java as well - and then the whole substring thing happened.

The Haskell Wiki has what seems like a clean and efficient implementation:

https://wiki.haskell.org/Edit_distance

Yeah, it's got a completely different algorithm along the diagonal of the matrix. That's the O(m+n) algorithm. I still haven't been able to understand it.

But that's not the point. The point is that Haskellers have a hard time translating an algorithm that's right in front of them and instead of writing down an O(m*n) algorithm they instinctively do O(m^n). This whole "many-worlds" approach of the original article is also leading to a very inefficient algorithm.

The abstractions Haskellers prefer lead them towards slow code.

    Haskellers have a hard time translating an algorithm that's right in front of them

That's no surprise: it is an open problem if this is possible in general. Some time ago Pippenger [1] showed that some programs cannot be transliterated into pure strict functional languages without suffering a slowdown. Later, Bird et al [2] show that the example given by Pippenger to show the slowdown really depends on strictness, and can be transliterated into a pure lazy functional language without asymptotic slowdown.

As of May 2015, nobody knows if lazy functional language can solve all problems with the same asymptotic time complexity as stateful languages.

[1] N. Pippenger, Pure Versus Impure Lisp.

[2] R. Bird, G. Jones, O. De Moor, More haste, less speed: lazy versus eager evaluation.

It's an open problem whether it can be translated into a pure, lazy language without an asymptotic slowdown. It's not at all an open problem as to whether it can be implemented in Haskell. The algorithm is mutating vectors - the direct translation is to use ST, and will have the same asymptotic behavior as the C++ version.

So you are saying Haskell is not a pure, lazy language?

Why is that surprising? Haskell (as typically implemented; I'm slightly less confident about the details of the report) is lazy and pure by default, but that can be violated locally or more globally if you ask for it loud enough. So the language, as a whole, is not pure in the sense required for those theoretical questions to really apply.

Sure, if you use some of the unsafe stuff ... but what about the safe core of Haskell, do you consider that pure?

What's needed here is ST. It permits local O(1) mutable vectors, while enforcing referential transparency when viewed from outside the runST.

ST is clearly not pure in the sense that theory requires to make "can we implement this algorithm with the same complexity" an interesting question. It seems wrong to consider it part of "the unsafe stuff", however - it doesn't have the same kind of unenforced caveats as unsafePerformIO and unsafeCoerce and similar.

Separately, "the unsafe stuff" is still part of the language as used, and while it's certainly better practice to avoid it when you can, it's silly to exclude it when asking whether it's possible to do something - they are sometimes appropriate.

By ST you mean Haskell's StateT monad transfomer?

Are you suggesting to define purity by the inability to define O(1) mutable vector-like operations? That would be an interesting proposal. But I wonder if it captures the conceptual content of the informal concept of purity. Maybe a time complexity restriction should be provable from purity?

"By ST you mean Haskell's StateT monad transfomer?"

No, I mean ST. An entirely different monad: http://hackage.haskell.org/package/base-4.7.0.0/docs/Control...

"Are you suggesting to define purity by the inability to define O(1) mutable vector-like operations?"

"Define", no, as that certainly doesn't encompass the full scope of purity. I think mutability runs counter to most people's notion of purity, particularly when they say "Haskell isn't pure". Of course, we can fake mutability at a cost of O(log n), which is what makes the O(1) notable.

If you do think ST is "pure" then it's not an open question whether you can implement update-heavy array based algorithms in a pure language without algorithmic penalty - you clearly can.

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

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).

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.

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.

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).

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.

> 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!

"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."

Ah, if you'd made that assumption explicit I would not have objected.

I don't see anywhere the querent suggesting that his translation demand invoked any notion of purity. If it did, then certainly he was asking a harder question than he presented, although even in that case a naive translation could have trivially produced an O(m n lg(n)) solution and O(m^n) should quite reasonably be considered failure.

I think the issue in the main is that relatively novice Haskell developers are likely to be the most eager to try and translate arbitrary code on demand in #haskell.

"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.

What do "pure" and "lazy" mean to you?

Lazy is easy: it's call-by-name evaluation, i.e. (lambda x.M)N -> M[N/x] with caching, ie. each argument is evaluated at most once.

Purity is more subtle. I don't think there's an agreed upon definition. [1] essentially ties purity to the equivalency of call-by-name, call-by-need and call-by-value evaluation orders (ignoring divergence and errors). I'm not totally sure that is the right definition.

[1] A. Sabry, What is a Purely Functional Language?

The most proximate question here, with respect to purity, is "can you do O(1) mutation (outside of the narrow case of replacing a thunk with its result)?" The theory is far more interesting when we say "no", but the answer in the case of Haskell is "yes".

With those definitions pure and lazy languages can certainly do mutation and have effects, viz. the IO and ST monads.

It seems a bit of a broad generalisation to say "Haskellers have a hard time translating an algorithm" based on who happened to be on the IRC channel at the time and decided to answer a challenge from a random C++ programmer who turned up.

Anyway, Haskell is perfectly decent at implementing your algorithm, and imperative algoritms in general. He's a more-or-less direct translation

    http://lpaste.net/132519

There's extra noise related to wrapping and unwrapping mutable cells which I could tidy up if I had the time, otherwise it's essentially identical.

In any case the author of the post in question, Bartosz Milewski, is an expert in C++ and not particularly an expert in Haskell, so your point seems misdirected.

> It seems a bit of a broad generalisation to say "Haskellers have a hard time translating an algorithm" based on who happened to be on the IRC channel at the time and decided to answer a challenge from a random C++ programmer who turned up.

I'm not a "C++ programmer". I'm just a programmer. I happened to have the algorithm written in C++, but I could quite easily have written it in pseudocode.

I got about 6 responses in 20 minutes. Four of those were O(m^n). Yes, I think this is quite indicative that Haskellers have a hard time writing performant code.

And of course the root of this conversation was mistaken in the first place. The algorithm Bartosz is presenting is the b!/(b-n)! algorithm that just examines each permutation once.

If, as per your profile, you are someone "who really just wants us all to get along", I would recommend against picking stupid fights like this.

The blog post explicitly says that the C++ is a translation of Haskell code, and links to the (efficient) Haskell original.

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