Tuple constructor change from 0.5 to 0.6

[sbromberger]

0.5:

julia> immutable Edge
           src::Int
           dst::Int
       end

julia> src(e::Edge) = e.src
src (generic function with 1 method)

julia> dst(e::Edge) = e.dst
dst (generic function with 1 method)

julia> import Base.convert

julia> convert(::Type{Tuple}, e::Edge) = (src(e), dst(e))
convert (generic function with 597 methods)

julia> e = Edge(1,2)
Edge(1,2)

julia> Tuple(e)
(1,2)

0.6:

julia> struct Edge
         src::Int
         dst::Int
       end

julia> src(e::Edge) = e.src
src (generic function with 1 method)

julia> dst(e::Edge) = e.dst
dst (generic function with 1 method)

julia> import Base.convert

julia> convert(::Type{Tuple}, e::Edge) = (src(e), dst(e))
convert (generic function with 700 methods)

julia> e = Edge(1,2)
Edge(1, 2)

julia> Tuple(e)
ERROR: MethodError: no method matching start(::Edge)
Closest candidates are:
  start(::SimpleVector) at essentials.jl:259
  start(::Base.MethodList) at reflection.jl:559
  start(::ExponentialBackOff) at error.jl:107
  ...
Stacktrace:
 [1] Tuple(::Edge) at ./tuple.jl:198

Not sure whether I missed something in NEWS again, but here we are.

[sbromberger]

@yuyichao could you let me know how to fix this?

[yuyichao]

#15516

[yuyichao]

Well, you didn't define the constructor so define it if you want it.

[sbromberger]

Apparently Tuple requires a constructor instead of convert.

[JeffBezanson]

I think the new Tuple constructors could probably be methods of convert instead. I'll try it; I don't expect it to break anything.

[JeffBezanson]

Hmm, on second thought I guess it didn't seem like a good idea to allow convert from arbitrary iterables to tuples. We generally use constructors for making collections from other iterators, and convert only between types that are fairly similar.

[sbromberger]

@JeffBezanson thanks. Does that mean that convert(::Type{Pair}, e::Edge) (which works, incidentally, in 0.5 and 0.6) is better off being a constructor? (I confess to being unclear about when to use convert and when to use a constructor.)

[JeffBezanson]

When to define convert is a judgment call; the key thing is that it's called implicitly on assignment, so the types should be similar enough that that wouldn't cause surprises.

[musm]

FWIW I also ran into this ref: https://discourse.julialang.org/t/tuple-constructor-on-0-6-with-convert/2358

[sbromberger]

@musm - thanks. I'm going to close this out, I guess, since it looks like it was deliberate.

I'd really prefer "one way to do it" and not have to worry about judgement calls, but I understand that's not always possible. I'll default to explicit constructors, which means importing/extending Base.Pair and Base.Tuple, which seems ... wrong, somehow.

[JeffBezanson]

It's interesting to think about merging convert and constructors. It has certainly come up before. The main issue is that some types naturally have idempotent constructors, and others don't. For example Int(Int(x)) === Int(x) always, but tuple(tuple(x)) !== tuple(x). However, obviously tuple is a separate function, not the same as Tuple. So we could get away with this if we had a rule that non-idempotent constructors must be separate functions, and not types themselves. We seem to do this a lot already, e.g. tuple and [ ]. This could potentially fix #15120. It would be interesting to review what non-idempotent constructors exist in packages.

The other, lesser, issue, is the potential for surprises or performance traps when an expensive "conversion" happens. It makes sense to be able to construct a Tuple from a generator, say, but do we want to be able to store a generator into an array of tuples and have it converted? Some of those cases can be addressed by separate functions, like parse and collect, so they don't have to be either constructors or conversions.

[andyferris]

I think the general problem with idempotent constructors is with containers that nest. You can create a tuple(tuple(...)) and also static array SVector(SVector(1,2,3)). I'd like to be able to do both nested static arrays and be able to convert static arrays, but getting rid of convert so that I have to create a new svector function (and similarly for any other container type) doesn't seem like a net win to me.

Base.Array is immune since we use different syntax to populate the container (such as [[1,2,3]]), and just like many other mutable containers we use a constructor to allocate them and then mutate them with setindex! or push! or whatever. However, immutable containers are just as valid and they must be populated by the constructors.