Julia: Bring back string literal juxtaposition = concatenation?

[IainNZ]

In particular, parse-time concatenation.

Current behaviour on Julia 0.6:

julia> x = "foo""bar"
"foobar"

julia> x = ("foo"
       "bar")
ERROR: syntax: missing comma or ) in argument list

21787 killed off the first case "foo""bar". That was to fix #20575 which was talking about juxtaposition of "foo"x, not two literals. A big past discussion on related topics happened in #2301, which mostly doesn't talk about this issue except in two posts at the end.

It works in C, and Python copied it. In this post Guido asks whether he should deprecate it, and there is useful summary of where the conversation went.

My main interest is in this is for long error messages, which came up a lot when writing library code in Python in the last major codebase I worked on. Triple-quoted strings aren't really a good solution, and the chaff of * is kind of annoying (especially in more nested code and an 80 char limit).

Given this is completely removed from 0.7 and is a parse-time error, I guess adding this in later is possible, so not urgent. But was curious what people thought.

[JeffBezanson]

I read the link. And I quote:

Van Rossum's lament should be a helpful reminder for language designers. It is very difficult to get rid of a feature once it has been added.

Why are triple-quoted strings not a good solution?

[ararslan]

Why are triple-quoted strings not a good solution?

Well, they force line breaks into the text, whereas "a" * "b" * ... does not.

[IainNZ]

Yeah the link isn't exactly strong backing for what I'm asking for...

Triple-quoted newlines make for bad exceptions due to the newlines at an arbitrary position based on how deeply the exception message was indented when it started (given a fixed number of columns).

Here is some fake Julia code that demonstrates the utility:

module Example

function solve_ai(data::Data{Big})
    if weather_looks_good()
        if !valid(data)
            throw(NotIntelligentError("Could not solve AI due to invalid data "
                                      "(size $(size(data)), please reinstall "
                                      "universe and try again (or give up).")))
        else
            return AI()
        end
    end
end

end  # module Example.

With concatenation, its just more visual noise:

            throw(NotIntelligentError("Could not solve AI due to invalid " *
                                      "data (size $(size(data)), please " *
                                      "reinstall universe and try again " *
                                      "(or give up).")))

[RalphAS]

I haven't seen it mentioned that the proposal would be breaking (without strange precedence rules):

julia-0.6> ["abc" "def"]
1×2 Array{String,2}:
 "abc"  "def"

[IainNZ]

Yeah that seems pretty fatal. Ah well.

[IainNZ]

Playing around with string macros could make triple-quotes viable maybe, by stripping newlines.

[omus]

If we go the string macro route we should should make it general and support options with flags:

julia> macro ex_str(str, flags)
           str, flags
       end
@ex_str (macro with 1 method)

julia> ex"foo"abc
("foo", "abc")

[IainNZ]

Thats not quite what I meant, at least. What I meant was:

macro nn_str(foo)
    return replace(foo, "\n", " ")
end

x = nn"""Multiline
         string
         testing"""

@show x

x = "Multiline string testing"

[IainNZ]

(I don't know how to make that work with interpolation but thats the rough idea)

[omus]

I didn't explain myself very well in my previous post. Somewhat similar to your use case I also can also see having string macro for removing the trailing newline (chomp). I find myself formatting longer multi-line strings with the trailing """ on its own line which introduces a trailing newline. For example:

julia> x = """
           Bacon ipsum dolor amet biltong alcatra short loin, meatball jerky corned
           beef porchetta shoulder kevin t-bone ham flank. Strip steak biltong flank bacon.
           """
"Bacon ipsum dolor amet biltong alcatra short loin, meatball jerky corned\nbeef porchetta shoulder kevin t-bone ham flank. Strip steak biltong flank bacon.\n"

Potentially this could be included in the same macro using flags.

[IainNZ]

Yeah. Although - what I'm doing above doesn't actually require a macro at all. It just is the tersest possible thing with a macro.