Unclear documentation of `collect`? (similarity and difference with brackets)

General Usage
1

Hello,

I just came across the docstring of collect (Collections and Data Structures · The Julia Language) and I’ve got the feeling that the second usage example would need some more explanation, or is it just me?

julia> [x^2 for x in 1:8 if isodd(x)]
4-element Vector{Int64}:
  1
  9
 25
 49

(the first example is more simply collect(1:2:13))

The way I understand this example is that “if you put a Generator inside brackets, you get the same result as collect(…)'ing that generator”. Is my interpretation correct?

Also, if my interpreation is correct, I believe that mentioning the [...] notation/operator would require an additional warning, because the similarity is partial:

  • indeed [i for i in 1:3] and collect(i for i in 1:3) both returns the same 3-element Vector{Int64}
  • however, [1:3] returns a 1-element Vector{UnitRange{Int64}}, while collect(1:3) returns the same 3-element Vector{Int64} as before
1 Like
2

The bracket generator syntax in your example more or less ends up lowering to this:

f(x) = x^2
itr = 1:8
filterfunc(x) = isodd(x)
gen = Base.Generator(f, Base.Filter(filterfunc, itr))
collect(gen)

You can take a look at the exact lowered code with Meta.@lower.

So yes, it is exactly the same as calling collect on a Base.Generator.

Only bracket expressions with a for in them are generator expressions - without them, they are just syntax for array literals.

3

Thanks for the explanation, but my question was more about the clarity of the documentation (I’ve updated the topic title to emphasize this).

I’ve now created the Pull request #50619 with a proposition to expand the docstring of collect.