++ as string concatenation operator

[Hans W Borchers]

I was interested to define "++" as operator for concatenating strings. I can define "+" for this purpose, but for "++" I get

    julia> function ++(x::String, y::String)
               x * y
           end
    ERROR: syntax: expected "(" in "function" definition

I looked up "//" in Rational.jl, but did not see the difference in how to define this operation.

[Steve Kelly]

I think the issue is that ++ is not an operator in julia.

[Jake Bolewski]

It is not, look at julia-parser.scm for the list of operators you are able to define.

[Hans W Borchers]

> It is not, look at julia-parser.scm for the list of operators you are able to define.

And I thought Julia allows to define operators as users like.
Is there a special reason (inconsistency) why ++ would not not be allowed while // is?
Could this be changed by an appropriate entry in julia-parser.scm (or somewhere else)?

[Stefan Karpinski]

Julia does not allow user-defined operators. There are just a lot of pre-defined operators. We could add ++ as an operator, but we'd have to decide how it parses and what it's precedence is. In Haskell it's an infix operator used for concatenation. In C it's a prefix and postfix operator that increments things. So there's no single obvious way to parse it.

[Ben Arthur]

relatedly, is there a thread somewhere that explains why * is used for string concatenation and ^ for string repetition?

[Stefan Karpinski]

many threads: search "string concatenation"

[Spencer Russell]

"Why is string concatenation done with * not +"

https://groups.google.com/d/topic/julia-users/nQg_d_n0t1Q/discussion

peace,

s

[Stefan Karpinski]

Thanks for the link, Spencer. This obviously belongs in the FAQ for now.