trusted purity?

[Jonathan M Davis]

On Monday, April 29, 2013 12:58:44 monarch_dodra wrote:
> Is there *any* way to make a call to a non-pure function in a
> pure context, if you know you won't violate your own purity?
> 
> This is something you can do with @safe (@trusted), but what
> about pure?
> 
> For example, "free" is not pure, because you can't call it twice
> on the same pointer. But if you manage the pointer yourself
> inside a struct, you can guarantee the purity of your own
> functions. But the language won't allow you to do that.

You can cast a pointer to the function. std.datetime does that for the 
LocalTime and UTC singletons. Take a look at the semi-recently added 
std.traits.SetFunctionAttributes.

> Related question:
> Can a function that "sometimes throws" be considered as pure?

Throwing has nothing to do with purity. pure is purely a question of whether 
the function accesses module-level or static variables which can possibly be 
mutated after they're initialized. Strong purity (which is required for 
most/all optimizations) is then places additional requirements on the function 
parameters, but purity itself is simply a question of whether the function 
accesses module-level or static variables. So, throwing has nothing to do with 
it.

- Jonathan M Davis