• d-bugmail puremagic.com (40/40) Jan 05 https://issues.dlang.org/show_bug.cgi?id=24320

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=24320

          Issue ID: 24320
           Summary: Change std.writefln() into a regular variadic function
                    not a template
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: bugzilla digitalmars.com

Print formatting goes a long way back. The problem to be solved is to format
and print values of various types, in a way appropriate to the type.

C printf does this with a format string, with embedded format specifiers that
specify the type of the corresponding argument. The flaw in this, of course, is
it's up to the user to get the types right. It is not inherently typesafe.

D1's writefln did it by adding a hidden additional hidden argument that
identified the types of each of the arguments. The flaw here is the cost of the
TypeInfo which has to be created for each type.

D2's writefln did it by making writefln a variadic template. Then, with compile
time introspection, the template generates a custom writefln for each instance.
The flaw here is the instantiation of a large number of templates, consuming a
lot of space in the executable, and slowing down builds. If it's possible to
say, it's been *too* successful!

I'd like to find a way to make a writefln that is typesafe, efficient, and
needs only one instance.

I realized that the answer is the printf format string. (printf is small, fast,
and compact.) It has, embedded in it, the types of each argument. We just have
to eliminate the type unsafe aspect of this.

The way to do it is to mark the first parameter of writefln as a format string.
Use %s as formats. Have the compiler then patch the %s to %d, %g, etc., as
necessary. Formats like %02x will remain untouched, but the corresponding
argument's type will be compiler checked to be a suitable match for it.

We're already nearly there with the pragma(printf) directive to check printf
arguments against the format string. Fixing the format string is the next
logical step.

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