immutable bug?

[Peter Alexander]

On Sunday, 12 January 2014 at 02:11:18 UTC, Manu wrote:
> But pure functions can (and do) return their arguments, and 
> it's obviously
> not a 'strongly pure' function. So I just can't see how the 
> assertion that
> it should be unique stands?

That's the bug. Your function isn't strongly pure, so the result 
shouldn't be convertible to immutable and isn't necessarily 
unique. Only strongly pure functions can have results convertible 
to immutable.


> Also, I was under the impression a 'strongly pure' function's 
> arguments
> only need to be const, not necessarily immutable. Purity says 
> something
> about the transformation performed by the function, nothing 
> about the data
> it operates on.
> Why should all arguments need to be immutable?

You don't need immutable arguments for purity, just strong 
purity. It's a stronger guarantee, more than normally guaranteed. 
Think of strong purity as pure + referentially transparent.

Sorry, yes you're right, they only need to be const. And it is 
only if you return a mutable value that the result becomes 
convertible to immutable.

int* f(const(int)* x); // convertible
const(int)* f(const(int)* x); // not-convertible

This is safe in the first instance because the result could not 
have come from x due to x being const. In the second instance, 
the result could have come from x, so it cannot be implicitly 
converted to immutable.