Weird error when trying to use `≔` (\coloneq) as infix operator

General Usage
1

When overloading ≔ like this:

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

the following works just fine:

julia> ≔(3,2)
false

But I get a weird error when trying to use it as an infix-operator:

julia> 2≔3
ERROR: syntax: invalid syntax (≔ 2 3)

Is this even supposed to work? The error message looks more like a bug in Julia’s Lisp parser or maybe I’m missing something?

2

The list of symbols recognized as infix operators (and associated precedence) can be found in the Julia parser source code. For the most part, this list is a subset of unicode category Sm(Symbol, math).

At the moment, it includes for example:

parsed with the same precedence as + (addition):

+ - ⊕ ⊖ ⊞ ⊟ ∪ ∨ ⊔ ± ∓ ∔ ∸ ≏ ⊎ ⊻ ⊽ ⋎ ⋓ ⧺ ⧻ ⨈ ⨢ ⨣ ⨤ ⨥ ⨦
⨧ ⨨ ⨩ ⨪ ⨫ ⨬ ⨭ ⨮ ⨹ ⨺ ⩁ ⩂ ⩅ ⩊ ⩌ ⩏ ⩐ ⩒ ⩔ ⩖ ⩗ ⩛ ⩝ ⩡ ⩢ ⩣

parsed with the same precedence as * (multiplication):

* / ÷ % & ⋅ ∘ × ∩ ∧ ⊗ ⊘ ⊙ ⊚ ⊛ ⊠ ⊡ ⊓ ∗ ∙ ∤ ⅋ ≀ ⊼ ⋄ ⋆ ⋇
⋉ ⋊ ⋋ ⋌ ⋏ ⋒ ⟑ ⦸ ⦼ ⦾ ⦿ ⧶ ⧷ ⨇ ⨰ ⨱ ⨲ ⨳ ⨴ ⨵ ⨶ ⨷ ⨸ ⨻
⨼ ⨽ ⩀ ⩃ ⩄ ⩋ ⩍ ⩎ ⩑ ⩓ ⩕ ⩘ ⩚ ⩜ ⩞ ⩟ ⩠ ⫛ ⊍ ▷ ⨝ ⟕ ⟖ ⟗
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3

I believe you can only use it as an infix operator in a macro. It is parsed as an infix operator:

julia> dump(:(2 ≔ 3))
Expr
  head: Symbol ≔
  args: Array{Any}((2,))
    1: Int64 2
    2: Int64 3

with assignment precedence, but you can’t overload assignment operators. The 1.13 parser gives a more descriptive error:

julia> 3 ≔ 2
ERROR: syntax: unsupported assignment operator "≔"
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

However, since it is parsed, you can write a macro that rewrites it into something else. You can do the same thing with :=.

julia> dump(:(2 := 3))
Expr
  head: Symbol :=
  args: Array{Any}((2,))
    1: Int64 2
    2: Int64 3

julia> Base.operator_precedence(:(=))
1

julia> Base.operator_precedence(:(:=))
1

julia> Base.operator_precedence(:(≔))
1

See also Is it possible to overload `:=`?

PS. Whoops, I didn’t realize that this was an ancient thread.

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