[Issue 4458] Static typing for format strings, when possible

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4458



--- Comment #7 from bearophile_hugs at eml.cc 2011-02-28 10:36:27 PST ---
> 1. fixing calls to printf falls outside of the scope of D.

I use printf often in D because writeln is template-heavy (see the size of the
compilation or the asm code produced by a hello world with writeln), printf is
necessary if you want to debug writeln or you want to see if it's buggy, and
because (as I have shown in past) printf is usually much faster than writeln
when I have to print lot.


> 2. the big problems printf has are simply not there with writef. The remaining
issue (compile time vs runtime error) is small enough to be not worth a
language feature customized for it

I don't agree it's small enough. The problem is that languages like Python are
dynamic, when you run them you have the source code, etc. In D you may have
just an already compiled binary. Being format strings dynamically typed you
have the worst of both worlds: the downsides of dynamic typing (unexpected
errors at run-time) with the need to compile and maybe even unavailability of
source code. Most format strings are known at compile-time, so the compiler is
supposed to catch such errors at compile time. GCC and Clang do it, in most
cases.


> 2. such a feature would lockstep the compiler to a specific library
implementation of writef, which is outside of the scope of the core language.

A way to solve this is to introduce pluggable static systems, that means some
functions written in D in Phobos that perform the compile-time tests on the
first argument of certain write/printf functions, if such value is known at
compile-time.

A temporary work-around (that doesn't work for printf is to rename functions
like writefln into longer names, and replace writef/writefln with two template
functions that takes the format string as first template argument, perform the
compile-time tests and then (to reduce template bloat) call the normal writefln
that now have longer names. But this causes some template bloat that's absent
with the pluggable type systems solution.

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