Pure function notation

[si14]

There are a lot of optimization opportunities coming from the ability to distinguish pure functions from unpure. The most important one (IMO) is deforestation: when you have something like sum(transpose(transpose(generate_random_matrix(M, N)) * SCALAR)), you can optimize away an entire matrix construction if you know that sum consumes matrix, generate_random_matrix builds one and all functions that was applied to generated matrix are pure. So it would be handy to have an ability to say that the particular function is pure.

[ViralBShah]

We have had a fair amount of discussion in the past about this. It's good to have this issue to discuss so that it remains on the priority list.

[StefanKarpinski]

@si14: do you mean the ability to annotate a function as pure or for the compiler to determine that the function is pure?

[StefanKarpinski]

Oh, nevermind. I just read the issue subject and it's clear. I once proposed procedure for impure and function for pure, but it was deemed that it would drive users coming from many programming languages and backgrounds completely crazy.

[JeffBezanson]

Yeah, it's good to be able to declare functions pure (optional, unchecked declaration). Even C has that now.

[ViralBShah]

I guess the interface could be @pure function.... What happens if functions marked pure are actually not pure? Would the program run slower, or would it give wrong answers, or undefined behaviour?

-viral

On 20-Feb-2012, at 11:53 PM, JeffBezanson wrote:

Yeah, it's good to be able to declare functions pure (optional, unchecked declaration). Even C has that now.

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#414 (comment)

[ivanmantova]

If we can't be sure the procedure is indeed pure, we wouldn't be able to optimize it without risking (potentially really bad) undefined behavior.

As for possible optimizations, something like C++ AMP comes to mind.

[StefanKarpinski]

Pure functions would only be allowed to call other pure functions — calling and impure function would be an error (probably run-time, but maybe compile-time). That way you can be sure a function is pure. Each intrinsic would need to be marked as pure or impure; likewise, one would want to be able to indicate whether each ccall is pure or impure. Of course one could lie or be mistaken there, in which case all hell could break loose, but that's what you get for lying.

[si14]

@StefanKarpinski
The question here is balance between let's say Haskell (where all of your functions have controlled side effects and they are reflected in signatures) and JS (where you don't have any control on types). Your architecture looks sane enough (quite easy to implement and a lot of errors would be handled at compile time). Moreover, it can be expanded to an arbitrary "effect system" when you can annotate functions with "effects" like "memory allocation", "mutates arguments" or "can throw exception", with effects propagating through the call stack (the only problem here is that such feature would probably cause a lot of noise in the source file without proper effect inference — maybe one can generate "intermediate" file with all inferred types and effects shown so programmer can check if everything is right). There are a lot of possibilities in the effect system world, for example you can warn your user if he puts some "allocating" function in the tight loop (and this is clearly bad for performance).

[si14]

Here is an example of what I mean: #249 — this "!" is in fact an "effect".

[si14]

And here is an example of "deforestation" optimization: http://arma.sourceforge.net/ . This library uses C++ templates to implement such optimization, and as you can see here http://arma.sourceforge.net/speed.html it's fast.

[StefanKarpinski]

Armadillo is really, really interesting. We've talked a lot about doing things like this to avoid creation of temporaries when executing linear algebra code. The main limiting factor actually has been a collective aversion to doing things that are not dead obvious and dead simple — I'd much rather give the programmer the ability to write this easily in an explicit way than to have the compiler do it automatically in a clever way that might not be obvious to the programmer.

At some point (a while ago), I was arguing for the ! at the end of function names to be enforced — effectively what we're talking about with pure versus impure. The idea was that all functions without a ! at the end would be required to be pure while functions with ! at the end would be allowed to be impure. This idea didn't go over too well, however, I think largely because it was deemed to draconian to force people to put ! everywhere that a side-effect might happen. I wasn't even talking about I/O either, just things that affect global state.

[si14]

@StefanKarpinski , the thing that you are talking about is somewhat different from doing IO, for example. There are different aspects of "purity" and it would be better to have a uniform way to denote different aspects of function behaviour. Let me show you an example in a form of pseudocode.

def first(&a)[mutates]:
    a += 1

def second()[io]:
    print "hello world"

def third()[???]:
    a = 1
    first(a)
    second()

Let's think about this ???. Should we propogate "mutates" effect to the third function? Clearly not. What about "io" effect? Looks like so, because if we now that function will make some IO we can't optimize it away because of unused return, for example. Both first and second are impure, but their impurities are different and so does optimization that can be done on them.
Another examples of impurity are memory allocation or exceptions that can be thrown. And, again, that should be handled by compiler in a different ways.

[StefanKarpinski]

Annotating things in that much detail is a non-starter. It's way too much book-keeping to foist on the programmer. The language has to be usable by people who don't know and don't care what different kinds of functional impurity are. On the other hand, if the compiler can compute and track all of these things for the programmer, then it's cool.

[si14]

@StefanKarpinski Of course compiler can infer effects, it's the whole point :) Moreover, compiler should do it even if programmer explicitly annotate function in some way to control if programmer was mistaken. Look, rules for inferring effects are kinda easy (though different for different effects): some will simply propagate through functions (like "allocation" one — you can't build a function that uses allocating function and don't allocate itself), some will be little harder to infer (like "mutates it's arguments" — "outer" function will be "mutative" only if it passes it's arguments to "mutative" function), but it's definitely useful.
BTW, we can meet in IRC to discuss in more realtime fashion :)

[StefanKarpinski]

I'll hop on IRC at some point, but can't today. I'm on vacation in Buenos Aires...

[o-jasper]

What about 'semipure' functions. For instance, memoized functions;

memoize_hash = dict((Number,),(Number,))
function memoized_function(x::Number)
   val = get(memoize_hash,x, nothing) #hmm, in the real world, maybe you want to round x instead
   if isnothing(val)
     val = very_expensive_function(x)
     assign(memoize_hash,x, val)
   end
   return val
end

It isn't thread-safe, but f(memoized_function(x),memoized_function(x)) would be better off converted to tmp=memoized_function(x); f(tmp,tmp), it isn't really destructive either.

[diegozea]

Maybe a silly comment (sorry if it is), but... Maybe @pure can check for not I/O operations, for only use arguments and variables defined on the function or const if are global and for the absent of assignments or applications of mutating functions into the arguments and trow and error if something of this is break. Is there more cases of side effects?

@StefanKarpinski I hope did you enjoy your vacations here? :)

[stevengj]

This discussion is getting out of control. It's way more important to have an unchecked pure annotation first; you can worry about compiler checking later. If the user annotates an impure function as "pure" then the results are simply undefined.

The ability to annotate pure functions is incredibly important in numerical applications in order to implement constant-folding optimizations, because constant expressions like float64(1//3) or sin(3) are very common in numerical code.

Hence pure should mean, "with constant arguments, this function may be safely evaluated at compile-time." (This is essentially the usual meaning of pure function, of course, not counting memory allocation as a side-effect.)

(For the same reason, I suspect that this needs to be a keyword recognized by the parser, not a macro, so that the pure annotation can be passed through to the compiler.)

[stevengj]

(By the way, it seems to me that @si14's original example is not pure, because pseudo-random-number generation both depends on and modifies global state, assuming generate_random_matrix is pseudorandom. But I suppose "random_matrix" was only meant in the informal sense of "some arbitrary constant matrix".)

[si14]

@stevengj you are definitely right about purity of generate_random_matrix :)

[StefanKarpinski]

I think we should learn from Rust's experience and just close this issue:

http://thread.gmane.org/gmane.comp.lang.rust.devel/3674/focus=3855

If this couldn't be made to work in Rust, which is statically compiled and relatively much more willing to make its users just through hoops (their target: professional programmers who write large, complex systems software; our target: scientists), then there's no way this will ever fly in Julia.

[stevengj]

Note that Rust apparently made everything pure by default and required a keyword for non-pure functions, according to that post, which proved unworkable for them. That would definitely not fly for our audience, I agree. Nor, as I argued above, should we worry about compiler-enforced purity (any more than Julia enforces @inbounds).

But there have been plenty of working examples of optional pure annotations for functions in other languages. For example, __attribute__((pure)) in gcc. Why is gcc such a terrible model to follow?

[stevengj]

It would be a shame for x::FloatingPoint + (2//3) to always be slow in Julia because the floating-point conversion of 2//3 cannot be performed at compile-time.

[JeffBezanson]

Impure-by-default with a pure keyword would certainly be the way to go. But the other points in the Rust discussion are relevant: for example [1,2] is pretty darn pure, but can't be evaluated fully at compile time since it creates a new mutable data structure on each call.

[stevengj]

@lindahua, +1 on a "safe" mode. (I would prefer "safe" to "debug".)

[StefanKarpinski]

@StefanKarpinski, following Jeff's suggested definition of pure, then it has to be per-method and not per-generic function. sin(::Float64) is pure, but sin(::Array{Float64}) is not (because it allocates a new array).

Yes, that's kind of my point. When writing any code that's generic, you can very easily find yourself in a situation where you have to split the exact same definition into pure and impure versions based on very subtle differences. Nobody is realistically going to do that. At that point, purity is a property that's better off automatically inferred rather than manually annotated. The only thing standing in the way of pretty decent bottom-up inference of purity, at least in situations where we've figured out exactly what method gets called, is annotation of pure vs. impure ccalls.

[JeffBezanson]

That is a good point; f(x) = sin(x) could not be declared pure.

[GlenHertz]

Doesn't sin(::Float64) allocate memory too (64 bytes)? If that is the criteria then purity wouldn't be that common.

[JeffBezanson]

That's the beauty of ===. It takes care of most of these issues if you just
go by my definition above. Allocating memory is not considered a side
effect by itself; the caller has to be able to observe the difference.
Also in most uses sin will not allocate anything; it is probably boxing
some values due to the repl.
On Dec 22, 2013 10:18 PM, "Glen Hertz" [email protected] wrote:

Doesn't sin(::Float64) allocate memory too (64 bytes)? If that is the
criteria then purity wouldn't be that common.

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[RauliRuohonen]

I think Jeff's rule is sort of right and sort of wrong - it's basically what the compiler should pretend the "pure" declaration means, with the understanding that some functions declared pure will not actually obey the rule. The point is to enable optimizations, so when the programmer declares a function to be pure, it should mean that the compiler is free to evaluate it as many times as it wishes whenever it wishes for a particular argument, and at any time return any one of those evaluations in whatever arbitrary way, and the programmer asserts that the program won't break because of it. It should also be method-specific, not generic-function-specific (this is an optimization, so the narrow claim is safer and sometimes the wider claim won't even be possible).

It is entirely reasonable to declare sin(::Array{Float64})) pure. The result won't always be a new copy depending on CSE optimizations, but then again, that's not anything new with array expressions. In e.g. Python if you do A[1:3], you won't get a copy either, and people accept that for performance reasons, using A[1:3].copy() when they really want a copy. That's not in principle any different from sin(A) not returning a new unshared array, and you can still use sin(A).copy() if desired. I'm not saying that the base sin() function should do this, just that it is not unreasonable to declare the implementation of such a function pure. If the base version does not do that, it is easy enough to implement a module with pure-declared wrappers of functions when such semantics are desired. If the compiler simply accepts the pure declaration at face value without any checking, it leaves the users the ability to implement functions with that kind of API. Even Haskell has the unsafePerformIO escape hatch for when you want something to be declared pure but not really technically be so.

[JeffBezanson]

There's a difference between a function like A[1:3] reliably referencing the same underlying data, vs. a function that may or may not return a new array based on what the compiler did.

[RauliRuohonen]

True, but still there is a common use case where you'd want a pure-declared sin(::Array{Float64})) to enable CSE and compile-time evaluation, and it is entirely safe. This happens when you write entirely pure mathematical functions, where by "pure" I mean pure-as-in-Haskell. You don't modify anything, so you don't care whether something is a reference or a copy. You could have a module with pure-declared versions of functions like sin() designed for that use case. If the final result of such a computation is an array (instead of e.g. a scalar) and you want to ensure that it is a new one, then you can do a copy as a final step to hide any optimization effects.

Basically what I'm trying to say that the simplest implementation of "pure" is probably the most useful one: the programmer asserts that the compiler can do its optimizations with no checking, and that's it, much like it is with @inbounds.

[StefanKarpinski]

Haskell is a completely off-base model here since you can't semantically modify anything. In Julia – or any language with semantically mutable arrays – deciding arbitrarily whether to return a copy or the same array is totally nuts.

[stevengj]

With #13555, there is now an (unchecked, unexported) @pure annotation that allows functions to be evaluated at compile-time, but it needs several compiler optimizations to be hooked into it in order to be generally useful (e.g. CSE, loop-invariance, constant propagation, and to interact properly with inlining).

It would also be nice to have a julia --safe flag to turn off this and other unsafe annotations. Or maybe just --pure=no.

[stevengj]

@vtjnash, maybe we need a followup issue to track @pure optimizations?

Update: created #14324.