Idea : Expression Type

Andrei Alexandrescu (See Website for Email) SeeWebsiteForEmail at erdani.org
Wed Jan 31 23:17:50 PST 2007


Xinok wrote:
> I've held off this idea for a little while now, but I think it could
> really be a nice addition to D.
> 
> Expressions are basically aliases which can accept arguments. Unlike
> aliases though, an expression can accept non-const arguments.
> 
> expression mul(a, b) = a * b; // Syntax is different from aliases,
> for good reason...
> 
> int main(){ int a = 35, b = 62; a = mul(a, b); }
> 
> 'mul' isn't a constant expression. The expression is inlined into the
> code, it doesn't call a function.
> 
> So basically, expressions are inline functions. BUT, because D lacks
> references (which I'll never be able to understand), functions can't
> return l-values. Expressions can be treated as so though.

Walter had the idea of allowing an expression to bind to an alias, and
to allow the following:

template mul(alias a, alias b)
{
   alias (a * b) mul;
}

with the effect that you want. (The parens are there to keep alias
happy, not to prevent breakage of order of operations.)

> One example of a recent post, the max function: expression max(a, b)
> = a > b ? a : b; Can handle any and all types, and you can use it as
> an l-value.

Nah, that won't work. It will doubly evaluate one of its arguments. But
the feature will allow very simple implementation of my varargs_reduce
and varargs_reduce_parallel primitives, e.g.:

template varargs_reduce(alias fun)
{
   template impl(alias args...)
   {
     static if (args.length == 2)
       alias fun(args) impl;
     else
       alias impl!(fun(args[0], args[1]), args[2..$]) impl;
   }
}

This has a similar structure to the varargs_reduce that I posted a while
ago, except that it doesn't need to do any type inference - it just
replaces the expression and lets the compiler take care of the rest.


Andrei



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