If in === ∈, why does .∈ work but .in fails with a ParseError?

General Usage
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1

Hello everyone,

I’m trying to understand the following behavior regarding in and ∈ (I guess they’re called operators?). While they are functionally identical (in === ∈ returns true), they behave differently when used with broadcasting (.) inside macros like @subset in DataFramesMeta.jl. Here’s the code:

using DataFrames, DataFramesMeta

df = DataFrame(x = [1, 1, 2, 2], y = [1, 2, 101, 102])

# This works as expected:
@subset df :y .∈ Ref([101, 102])

# This fails with a ParseError:
@subset df :y .in Ref([101, 102])

ParseError: whitespace is not allowed here.

Why is that?

2

because parser treats them differently. It may sound like a non-answer but that’s basically the whole reason. The parser COULD special-case .in, but that would be more confusing / inconsistent than helpful

3

This is it. The object bound to the names in and ∈ are the same function object (and this === but that doesn’t have any bearing on how they’re parsed. One is a name, the other is an operator. You can also easily change what they are bound to do that they don’t refer to the same object.

4

Of course it’s implicit in the previous answers - and even in the question, but I think it is worth telling, for other readers at least, that broadcasting does work with in, but it should be coded as for regular functions, i.e. in.(y, Ref([101, 102])) - or in the OP’s example with DataFramesMeta:

@subset df in.(:y, Ref([101, 102]))
5

Another thing implicit in previous answers, but perhaps should be made explicit, is that whether in === ∈ is true depends on the namespace, that is, they’re identical in Main, but not in all other modules:

julia> in === ∈
true

julia> baremodule MyModule
           const in = 3
           const ∈ = 7
           const v = in === ∈
       end
Main.MyModule

julia> MyModule.in === MyModule.:(∈)
false

julia> MyModule.v
false
6

To elaborate on this:

  • in is just a name
  • ∈ is an operator, meaning it’s possible to call it, for example, like so: x ∈ y, and the broadcasting syntax is different