Generic code: @autoconst, @autopure, @autonothrow

[Philippe Sigaud]

2010/8/30 Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org>

> On 8/30/10 14:16 PDT, Tomek Sowiński wrote:
>
>> Dnia 30-08-2010 o 21:10:10 Tomek Sowiński <just at ask.me> napisał(a):
>>
>>  How about reduce!fun(range)? It's pure/nothrow when fun is
>>> pure/nothrow. Plenty of std.algorithm would benefit.
>>>
>>
>> Eh, nevermind. popFront() must mutate the range so it can't be pure.
>> Need to get some sleep...
>>
>
> It can still be nothrow depending on input, which makes a solid point.
> Arguments against qualifier/attribute propagation based on sheer semantics
> ("it's abs so it must be pure") break badly in the face of higher-order
> functions.
>
> It's pretty clear we need an attribute propagation mechanism if we want
> e.g. to make Phobos const-aware.


Could something like this be a possible building block for pure?

import std.stdio, std.bigint;

/**
Test whether expression expr is pure for type Type.
*/
template isPure(Type, string expr)
{
    enum isPure = isPureImpl!(Type, expr);
}

bool isPureImpl(Type, string expr)()
{
    mixin("auto pureTester(T)(T a) pure { return " ~ expr ~ ";}");
    return is(typeof( pureTester(Type.init) ));
}

int foo(int i) { return i;}     // not pure
int bar(int i) pure { return i;}// pure

void test(T)(T num) if (isPure!(T, "-1*a"))          // test that with
"-1*a", "-1*foo(a)" and "-1*bar(a)"
{
    writeln("Yes, pure expr for type " ~ T.stringof);
}

void main()
{
    test(1);
    auto b = BigInt("1");
//    test(b); // is -1*b a pure expr for BigInts?
}

test(1) will compile for expression -1*a or just "a"
test(BigInt("1")) will not compile with expression -1*a, because it's not
pure for BigInts. It will compile with just "a" as constraint expression.

I also tried to instantiate test(1)  with -1*bar(a) and -1*foo(a) as
constraints. It passes for -1*bar(a), because bar is pure, but not with
foo(a).

Maybe then it's possible to create a pure version of a function if this
passes the test. I'm not sure, it's late in there.
Also, this could be done also for nothrow. I didn't think it through for
const.

Philippe
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