Using pure to create immutable

[Steven Schveighoffer]

On Thu, 22 Sep 2011 17:03:08 -0400, Jonathan M Davis <jmdavisProg at gmx.com>  
wrote:

>
> In any case, in order for a function to be able to have its return value
> implicitly value implicitly cast to immutable, it must pure and all of  
> its
> arguments must be immutable or implicitly convertible to immutable (or -  
> if
> the compiler is ever improved to treat pure functions with const  
> parameters
> and immutable arguments the same way - the requirement would be that the
> function must be pure and all of its _arguments_ must be immutable or
> implicitly convertible to immutable).

No, the parameter types can be const, and can accept mutable arguments.   
The main point is, the return value has to be proven to be *unique*.  The  
only way to do this with pure functions is to prove that the result is  
*not* a subset of the parameters.  That's all.

Observe:

char[] foo(const(char)[] x) pure {...}

There is no way to write the body of this function such that the return  
value is a substring of x.  So you are guaranteed that the result is *new  
memory*, and since it cannot be stored globally anywhere (per pure rules),  
it's guaranteed to be unique, and should be implicitly castable to  
immutable.

Even if you pass a char[] into foo.

-Steve