Strange dispatch with `@pure`

[andyferris]

I was trying to understand the behavior of @pure (as well as @generated) and how they relate to the recent fix to #265, and came across this strange case.

I'm not sure why the following code doesn't result in a missing method error on foo on the last line:

julia> foo(::Int) = 1
foo (generic function with 1 method)

julia> Base.@pure bar(x::ANY) = foo(x)
bar (generic function with 1 method)

julia> bar(42)
1

julia> bar(42.0)
1

The error is (correctly) present on Julia v0.5.

[StefanKarpinski]

$100 says @vtjnash tells you this function isn't pure.

[andyferris]

ROFL

[vtjnash]

Do I get that $100? Because that is indeed the correct answer. We inferred that if bar has a return value it must be 1. Since it is marked @pure, we can strengthen that say that its return value is always 1. Also, since it is marked @pure, we don't need to consider what might happen if the method table changes, so the compiler is free to disregard any operation that might invalidate that answer.

[yurivish]

What does @pure mean, exactly?

[Keno]

@pure means I am @vtjnash and I certify that this function doesn't break any of the implicit assumptions I use in the compiler ;)

[andyferris]

Thanks @vtjnash for the explanation of what the compiler has done. Please note that this behaviour doesn't occur without the ::ANY, and there were no method table changes.

Also contrast that with this situation:

julia> foo(::Int) = 1
foo (generic function with 1 method)

julia> foo(::Float64) = 2
foo (generic function with 2 methods)

julia> Base.@pure bar(x::ANY) = foo(x)
bar (generic function with 1 method)

julia> bar(42)
1

julia> bar(42.0)
2

julia> bar(true)
ERROR: MethodError: no method matching foo(::Bool)
Closest candidates are:
  foo(::Float64) at REPL[2]:1
  foo(::Int64) at REPL[1]:1
Stacktrace:
 [1] bar(::Any) at ./REPL[3]:1

To me, it seems undesirable that the different cases here have different results (in terms of sometimes throwing the MethodError and sometimes giving a spurious result). I'm pretty sure I can follow what Jameson is saying the compiler is doing with inference, the lack of dependency edges for @pure functions, and also that @pure is not exported for general usage - but despite all of that, it seems that @pure is currently fraught with dangers and complications which I hope that we don't expect everyday users to be happy with having to learn.

Also, since it is marked @pure, we don't need to consider what might happen if the method table changes, so the compiler is free to disregard any operation that might invalidate that answer.

I think this is specifically where I am hung-up (and probably disagree). Historical issues on github show that @pure was implemented for the purpose of constant propagation.

It seems of most utilitarian value if a @pure method is just a simple and plain method which inference is allowed to evaluate if it has constant inputs. To me, that doesn't imply anything distinct about dependency edges for @pure methods as opposed to functions which are @inlined.

[vtjnash]

currently fraught with dangers and complications which I hope that we don't expect everyday users to be happy with having to learn.

Correct. I don't expect anyone to learn them (I certainly haven't :P). I am happy to tell people just to avoid it.

was implemented for the purpose of constant propagation.

It sometimes gets used for that, but I suspect that it's not really correct. It was implemented to slightly generalize some tfunctions so that Core.Inference could move out of Base. I hope someday to finally be rid of it. But it currently functions as a stop gap.

[andyferris]

I am happy to tell people just to avoid it.

I hope someday to finally be rid of it. But it currently functions as a stop gap.

OK, I hadn't seen an opinion like this before - is this the consensus, then?

Again, I realise it was an unexported feature, but @pure has served as a very useful and powerful construct in v0.5 - for really nice constant (value) propagation, (slightly) more complex operations on types, and for creating API's that are a bit prettier (e.g. StaticArray uses it to allow users to type Size(3) instead of Size{(3,)}()). It would be a shame to lose its functionality and/or prior reliability without some replacement. Having a keyword which says "inference is free to evaluate this function when it encounters it at compile-time with constant inputs" seems incredibly useful (to me).

[c42f]

Oddities like this remind me of C-like UB - if you do the wrong thing "anything can happen, and frequently does". This seems at odds with the most julia semantics, where if the compiler can't figure it out at compile time you get the expected behavior, but at runtime.

Not that I mind if pure and generated go away, as long as there's a nice replacement for what we had in 0.5, which was, in my opinion quite pragmatic and fairly easy to understand: if you invoke a pure function on a new type before telling the compiler all the information about that type, you get the wrong thing. If you do it after providing the compiler with all the information, you get the right thing. It seems that the possible uses of pure in 0.6 have changed dramatically, perhaps to the extent that it should be called something else.

[andyferris]

Oddities like this remind me of C-like UB

Right, I'd generally prefer that language authors work hard (go out of their way) to throw errors instead of allow users to type things which result in undefined behavior... (says someone who has never authored a language).

[vtjnash]

where if the compiler can't figure it out at compile time you get the expected behavior, but at runtime

yes. That's exactly what the compiler does, if you don't mark something as "ignore this" (aka @pure). This was largely all hashed out in the original PR (#13555) and issue (#414)

If you do it after providing the compiler with all the information, you get the right thing

That was never what it did. The v0.6 semantics haven't changed, it's just gotten slightly better at benefitting from the information it has already computed.

throw errors instead of allow users to type things which result in undefined behavior

I'd be happy to replace Base.@pure with macro pure(f); error("function $f is not pure"); end. It would even be entirely correct 😛

I'm not convinced this particular optimization is particularly beneficial, so perhaps it should be disabled. It was introduced by Jeff in as a small piece of #16837.

[c42f]

Perhaps, then, @pure never meant what I thought it did. However, take this as a data point: I found the behavior of @pure in 0.5 easy to understand and useful in practice, regardless of the underlying semantic you actually intended when implementing it! In 0.6 it seems dramatically more difficult to understand and less practically useful.

[timholy]

I've been (and still am) confused about the meaning of @pure too. @vtjnash, can you write up a short docstring and a longer explanation for devdocs? Given that other languages also have pure annotations, I don't think this feature is going away; given the subtleties and potential for abuse, it should be a high priority to make sure it's clearly documented.

[timholy]

And as a good-faith exchange, I just added some documentation (#20712) that someone else indirectly requested (https://discourse.julialang.org/t/using-cartesianrange-flattening-before-collect/2150). I just wanted to make sure you didn't have any excuses for avoiding this task 😄.