V2 string

[Frits van Bommel]

Bruno Medeiros wrote:
> Regan Heath wrote:
>> tolower is an interesting case.  As a caller I expect it to modify the 
>> string, or perhaps give a modified copy back (both options are valid 
>> and should perhaps be supported?).
>>
>> So, the 'string tolower(string)' version has 2 cases, the first case 
>> where it doesn't need to modify the input and can simply return it, no 
>> problem. 
>> But case 2, where it does modify it should dup and return char[].  My 
>> reasoning being that after it has completed and returned the copy, the 
>> caller now 'owns' the string (as it's the only copy in existance and 
>> no-one else has a reference to it).
>>
> 
> Indeed, I think this illustrates that some standard library functions 
> may not have the correct signature, and I tolower is likely one of them.
> The most general case for tolower is:
>   char[] tolower(const(char)[] s);
> Since tolower creates a new array, but does not keep it, it can give 
> away it's ownership of the the array (ie, return a mutable).

Sorry, but you seem to have missed a bit above: if the string doesn't 
contain any uppercase characters tolower returns the input without 
.dup-ing it (aka copy-on-write).

> The second case, more specific, is simply syntactic sugar for making 
> that array invariant:
> 
>   invariant(char)[] tolowerinv(const(char)[] str) {
>     return cast(invariant) tolower(str);
>   }

Yes, but only if it actually needs to modify the string.

You seem to have missed that the two cases can't (in general) be 
distinguished at compile time; it's only at run time when a choice is 
made between a copy and no copy.

> The current signature:
>   const(char)[] tolower(const(char)[] str)
> is kinda incorrect, because it returns a const reference for an array 
> that has no mutable references, and that is the same as an invariant 
> reference, so tolower might as well return invariant(char)[].

Again, that only holds if a copy was actually made at run time. If no 
copy was made the original input is returned, to which there may be 
mutable references.