.. (two dots) as a operator name

General Usage
1

IntervalSets.jl defines .. as a interval operator. I tried using ... as an operator but Julia complains that it is an invalid identifier name. This means that .. is treated somewhat special. Where can I find the list of legal operators and are there any other special ones, like the ..?

2

How did you try to define it? It works for me:

julia> ..(a,b) = a:b
.. (generic function with 1 method)

julia> 3..4
3:4

What error exactly do you get for your code?

3

Two dots work. Try doing that with ... (three dots).

4

Yes. It’s not .. that’s special, but ..., since it is handled specially during lowering:

julia> Meta.@lower add(xs...) = reduce(+, xs)
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      $(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─     return $(Expr(:method, :add))
)))
│        $(Expr(:method, :add))
│   %3 = Core.Typeof(add)
│   %4 = Core.apply_type(Vararg, Core.Any)
│   %5 = Core.svec(%3, %4)
│   %6 = Core.svec()
│   %7 = Core.svec(%5, %6, $(QuoteNode(:(#= REPL[2]:1 =#))))
│        $(Expr(:method, :add, :(%7), CodeInfo(
    @ REPL[2]:1 within `none`
1 ─ %1 = reduce(+, xs)
└──      return %1
)))
└──      return add
))))

You can’t override the splatting/slurping operator.

5

True, I forgot about slurping. It seems that it parses three or more dots as a slurping operator.

Is there a way then of defining .... as an operator or does it need to be hard coded somewhere in Julia’s parser?

6

I like to survey the parser source code when looking for candidate characters for my own obscure operators. Take note of built-in operator precedences (increasing as you go down) and make sure it won’t surprise you later on.

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