Implicit cast to immutable

[Steven Schveighoffer]

On Tue, 18 Oct 2011 02:40:12 -0400, Daniel Murphy
<yebblies at nospamgmail.com> wrote:

> "Steven Schveighoffer" <schveiguy at yahoo.com> wrote in message
> news:op.v3h06olweav7ka at localhost.localdomain...
>>
>> That sounds like an incorrect restriction.  The implicit cast to  
>> immutable
>> should depend on whether the function being *called* qualifies, not if  
>> the
>> function you are calling *from* qualifies.
>>
>
> I think you've misunderstood what I'm saying.  The patch I made  
> implemented
> two ways to implicly convert to immutable: the result of a pure function
> returning immutable and a return statement inside a pure function.
>
> 1)
> int[] fun() { return new int[]; }
> immutable x = fun(); // conversion happens here
>
> 2)
> immutable(int[]) fun() { return new int[]; } // conversion happens here
> immutable x  = fun();

Neither of those is relevant, since they do not contain parameters (I'm  
assuming you meant them both to be pure).

>> Qualifying means the return type should be mutable, and cannot be  
>> derived
>> from the parameters without requiring casts.  The easiest way to do this
>> is to ensure the parameters are all const, immutable, or implicitly cast
>> to immutable.  You could do funky things like assume for instance an  
>> int[]
>> cannot possibly be implicit-casted to a char[], so therefore int[]
>> foo(char[] data) pure can be implicitly casted to immutable, but that
>> might be flirting with dangerous situations.
>>
>> The one exception should be allocating memory, which should always
>> qualify, even though it's not a pure function.
>>
>
> This is valid, but becomes very complicated with complex return types.  I
> doubt this will ever make it into the language.

No, it's not complicated, at least in my view.  The rules are:

1. if the pure function parameters are only immutable or
implicitly-convertible-to-immutable, the result is safe to cast to
immutable, regardless of what happens inside the function, or the type of
the result.
2. if the pure function parameters contain any mutable data that is
mutable and *not* implicitly convertible to mutable, and the result is
only safe to cast to immutable if it can already implicitly cast to
immutable.
3. if the pure function parameters are implicitly castible to immutable,
immutable, or are const, then:
       a) if the return type is implicitly castable to immutable, obviously
it can be cast.
       b) if the return type is not implicitly castable to immutable, but
contains only mutable references, then it's implicitly castable to
immutable.
       c) otherwise, it cannot be implicitly cast.

You do not need the function body to determine this, and the decision
should be made by the compiler at the call site.

Your case 2 where the conversion happens at the return statement is
already covered.

> I've got the beginnings of a patch to enable a sort of 'pure expression',
> such as new, array.dup and array concatenation expressions.  The result  
> of a
> call to a const-pure function using immutable arguments can be converted  
> to
> immutable, while calling it with mutable or const arguments cannot,  
> without
> searching the return type for anything the arguments can implicitly  
> convert
> to (or create).
>
> Eg. I can't see a great way to detect situations like this:
>
> struct S { const void* p; }

This struct could only be cast to immutable under my rule 1 above.  It
does not implicitly cast to immutable.

> S[] fun(int[] arr)
> {
>     return [ S(arr.ptr) ];
> }
> immutable x = fun([1, 2, 3]);

Invalid.  arr is not implicitly castable to immutable and is mutable, so
the result must already be implicitly castable (which it is not).  This
falls under rule 2 above, and fails the test.

-Steve