Implementing Pure Functions

[Don]

Andrei Alexandrescu wrote:
> On 6/17/11 1:56 AM, Kagamin wrote:
>> Andrei Alexandrescu Wrote:
>>
>>> This has sparked an interesting discussion, to which I added my
>>> bit.
>>
>> int fun(int a) pure { if (a>  10) writeln("I'm impure); }
>>
>> As I understand, even if some calls to a function have some
>> repeatability properties, this doesn't mean the function is pure. In
>> this example fun is obviously impure. Here one can talk about
>> allowing to call impure functions from pure ones, but that's a way
>> different task.
> 
> Right. I gave that example within the context of the discussion, which 
> considered purity a path-sensitive property. By that definition, if fun 
> is provably never invoked with a > 10, then it's effectively pure.
> 
> Andrei

And that's an interesting thing about CTFE: although it can never do 
anything impure, it can call impure functions, provided it avoids the 
impure bits of them:

int foo(int n)
{
    static int x = 56;
    if (n<10)
	return n;
    ++x;
    return x;
}

static assert(n(5) == 5);

Likewise, it can never doing anything unsafe, but can call unsafe functions.

bool bar(int n)
{
    if (n < 2) return true;
    corruptTheStack();
    return false;
}

static assert(bar(1));