parse command interpolations at parse time

[malmaud]

It would be nice to have a way to "double interpolate" a variable with respects to nested possible interpolation contexts to make it easier to write quoted code blocks that wants to interpolate a variable inside a string. e.g., so the below code echos "hi". Not sure what the best way to go about that is.

macro example()
 f="hi"
 quote
  run(`echo $$f`)
 end
end

[vtjnash]

the following already works without ambiguity, and doesn't require changing string parsing based on context:

macro example()
 f="hi"
 quote
  run($(`echo $f`))
 end
end

Is there an example where this isn't quite so straightforward?

[malmaud]

What about something like

macro example()
 f="a"
 esc(quote
  g="b"
  run(`echo $f $g`)
 end)
end

You could do instead

macro example()
 f="a"
 esc(quote
  g="b"
  h=$(f)
  run(`echo $h $g`)
 end)
end

but it's a little weird to be forced to create a temporary variable, since I'm not sure there is any other situation in Julia in which replacing a reference to a variable with its known value will change the semantics of the program. While here, changing $h to $(f) in the echo line changes the meaning of the program.

(edit: fix the variable in the second example)

[vtjnash]

That's tricky, but you don't exactly see it until you try to expand the macro. In the first example, f only exists at macro expansion time and g only exists at run time. Assigning the value of f to h seems to be necessary to convert the value into a variable. For strings, it is easy to use string(...) instead of interpolation to avoid the distinction.

julia> macro example()                             
               f="a"
               quote
                g="b"
                run(Cmd(ByteString["echo",$(f),g]))
               end                                       
              end

julia> @example
a b

julia> macroexpand(quote @example end)
quote  # none, line 1:
    begin  # none, line 4:
        g#4 = "b" # line 5:
        run(Cmd(ByteString["echo","a",g#4]))
    end
end

(edit: I forgot Cmd() is the analog to string(), so I fixed the example)

[JeffBezanson]

$($f) would work if we moved command expression expansion to the parse stage like we did for string interpolation.

The variable-to-value substitution you mention would be:

  h=$(f)
  run(`echo $h $g`)

to

  run(`echo $($f) $g`)

You can't drop the extra $. This doesn't work now, but would with the change I described.

[vtjnash]

this would be a new feature and thus not appropriate for the v0.4.x bugfix series. i'm going to tentatively move this to v0.5, please comment if you think that is not appropriate.