Against enforce()

[Jonathan M Davis]

On Wednesday, March 16, 2011 18:53:06 bearophile wrote:
> Andrei:
> > Yum, love the enforce.
> 
> You are silly :-)
> 
> > So perhaps the language could be improved as enforce does not break
> > purity.
> 
> Currently enforce() uses a lazy argument. A lazy argument is a delegate,
> and generally such delegate can't be pure, because the expressions you
> give to enforce() usually refer to variables in the scope where you call
> enforce(). So what kind of language improvements are you thinking about?
> 
> This is an example:
> http://d.puremagic.com/issues/show_bug.cgi?id=5746
> 
> > > - It kills inlining (with the current DMD, and I don't think this
> > > problem will be fixed soon);
> > 
> > Not a problem of enforce.
> 
> In my opinion it's not wise use widely in the standard library something
> that requires an optimization that the DMD compiler is not going to have
> soon, and that makes the code significantly slower. In some cases this
> almost forces me to keep a patched version of Phobos, or to add more
> nearly duplicated functions to my dlibs2.
> 
> > enforce helps such idioms, does not prevent them. From the docs:
> > 
> > ===============
> > T enforce(T)(T value, lazy Throwable ex);
> > 
> > If value is nonzero, returns it. Otherwise, throws ex.
> > ===============
> 
> Then why is iota() using a nude enforce() instead of the enforce() with a
> more meaningful exception like WrongArgumentException? I have seen the
> nude enforce used in other parts of Phobos. So maybe enforce() makes even
> the standard library writers lazy.
> 
> > enforce's specification specifies it throws. It would therefore be
> > difficult for it to not throw. This complaint is non sequitur.<
> 
> I know, and I agree in some situations you want an assert (to test
> arguments coming from "outside"), so in some situations an enforce is
> acceptable. The problem is that currently enforce is used as a patch for a
> DMD/Phobos problem that I hope will be fixed in a short time, see below.
> 
> >> I see enforce() just as a temporary workaround for a problem of Phobos
> >> (that it's compiled in release mode, so its asserts are vanished) that
> >> risks to become a permanent part of Phobos.<<
> > 
> > enforce is a simple abstraction of the idiom "if (!condition) throw new
> > Exception(args)". If that idiom were rare, then occurrences of enforce
> > would be rare and therefore there would be little need to have enforce
> > at all.<
> >
> >There may be some confusion somewhere. enforce is not supposed to be a
> >sort of assert. It is a different tool with a different charter. Use
> >assert for assertions.<
> 
> Right. But saying just that is not enough. You have to add that such "if
> (!condition) throw new Exception(args)" idiom is common in Phobos because
> Phobos is present only in release mode. If the zip distribution of DMD
> contains two Phobos and dmd becomes able to use the right one according to
> the compilation switches, then I think that "if (!condition) throw new
> Exception(args)" will become more rare, and the enforce() too will be less
> commonly needed.

For the most part, I don't think that this is true. There was discussion of 
using enforce instead of assertions because of assertions being compiled out 
with -release, but I'm not sure that that ever actually happened. Every case 
that I can think of at the moment where enforce is used, an exception _should_ 
be used. The only question is whether enforce should be used due to the current 
problems with inlining it and whatnot.

- Jonathan M Davis