Julia: -- parses as an operator but ++ doesn't

[jaakkor2]

I would like to be able to write 1--2 to get 3. Additional spaces or parenthesis are not convenient.

Matlab and Python allow this. JavaScript not, because there ++ and -- are increment/decrement operators, but -+ and +- are ok.

julia> 1++2
3

julia> 1-+2
-1

julia> 1+-2
-1

julia> 1--2
ERROR: -- not defined

julia> 1- -2
3

julia> 1-(-2)
3

[tkelman]

I hope you're not expecting anyone else to read code that has 1--2 in it, I'd almost call that an enforced-readability feature if not for 1++2 currently working.

[ivarne]

I would be surprised to find 1--2 == 3. Adding space makes it much more readable.

[mschauer]

Why is this a bug? -- is a free operator symbol that can be defined by the user, for example even by -- = + if that is really wanted.

[ihnorton]

@mschauer agreed, but it does seem inconsistent that ++ is not parsed as a unique operator:

julia> parse("1++2")
:(1 + 2)

[JeffBezanson]

Here's what we'll do:

1. Decide whether we want -- and ++ as operators.
2. For now keep a++b parsing as a + (+b) to avoid spurious breakage.
3. Make -- a syntax error while we decide on (1). Note ** is already a syntax error to avoid confusion with ^.

[jakebolewski]

-1 on having -- and ++ be valid operators.

[StefanKarpinski]

I once proposed that ++x and --x could default to x += one(x) and x -= one(x), but allow more efficient implementations (e.g. for BigInts), the main advantage being that it avoids the type instability that can be introduced by x += 1, e.g. for UInt32 on a 64-bit system. I'm not sure how important that is, however, since it doesn't seem to come up that often.

[JeffBezanson]

It would be nice to have pred and succ functions. It would be a bit odd to have syntax for those functions in combination with assignment, but not the functions themselves.

I ran into this yesterday, where in the UnitRange constructor pred(start) would be better than start-one(stop-start)).

[jakebolewski]

I don't think there is anything left to do here. Deciding on -- and ++ as operators seems like a separate issue.