Idea : Expression Type

[Kyle Furlong]

Don Clugston wrote:
> Kyle Furlong wrote:
>> Andrei Alexandrescu (See Website for Email) wrote:
>>> 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
>>
>> This seems to me to be an obvious good extension to the template 
>> system. With the ability to pass aliases into the template, you gain 
>> arbitrary and easy expression composition.
> 
> The name mangling looks tricky. I think an expression alias would have 
> to behave like a typedef: given
> alias (a*b) X;
> alias (a*b) Y;
> X and Y would not be the same.
> A name mangling scheme for an arbitrary expression would be a bit of a 
> nightmare.

I suppose I meant easy use, not easy implementation. <g>