Dynamic Format Strings in Julia's @printf macro

[JimDavidson]

julia> using Printf julia> fmt = "%10d" "%10d" julia> @printf(fmt, 123) ERROR: LoadError: ArgumentError: @printf: first or second argument must be a format string

It would be nice to have dynamic format strings for the @printf macro.

[carstenbauer]

See also this old issue, #4248

[JimDavidson]

Yes, I did see that, but hoped something changed in the last 5 years or so. It looks like this behavior is going to be baked into the language.

[lobingera]

It would be nice to have dynamic format strings for the @printf macro.

Macros are evaluated at compile time, once a printf macro is compiled, the format string is fixed (and yes, that's baked into the language). If you need dynamic formatting goto https://github.com/JuliaIO/Formatting.jl or similar.

[stevengj]

@lobingera, in principle, the @printf macro could look at whether the first argument is a literal string (this is known at macro-expansion time since literal strings appear as String nodes in the AST), and if not fall back to slow dynamic code.

This would be backwards-compatible, so in principle it could be supported in 1.x. But it might mean folding something like Formatting.jl into the standard library.

[o314]

What is a bit puzzling is that Printf @ stdlib does the macro impl exporting @printf and @sprintf, but Formatting.jl does its function part by exporting printfmt instead of printf, sprintf that have not been reserved by stdlib.
Not a good pattern there

[quinnj]

This is now implemented in stdlib Printf: you can create Printf.Format objects by calling Printf.format"..." or Printf.Format(...), then pass a formatting object to Printf.format(::Format, args...) to format arguments according to the format.