Accessing mutable data that isn't

[Spott]

On Thursday, 21 November 2013 at 06:48:40 UTC, qznc wrote:
> On Wednesday, 20 November 2013 at 22:49:42 UTC, Spott wrote:
>> I've been screwing around with templates lately, and I'm
>> attempting to figure out why the following won't compile:
>>
>> struct value
>> {
>>     int a;
>>
>>     const auto
>>         opBinary(string op, T)(in T rhs) const pure {
>>             static if (op == "+")
>>                 return 
>> intermediateValue!(value.plus,this,rhs)();
>>         }
>>
>>     ref value opAssign(T)( in T t ) {
>>         a = t.a;
>>         return this;
>>     }
>>
>>     static
>>     int plus(T1, T2)(in T1 x, in T2 y) pure {
>>         return x.a + y.a;
>>     }
>>
>> }
>>
>> struct intermediateValue(alias Op, alias A, alias B)
>> {
>>
>>     auto opBinary(string op, T)(in T rhs) const pure {
>>         static if (op == "+")
>>             return intermediateValue!(value.plus,this,rhs)();
>>     }
>>
>>     @property auto a() const pure {
>>         return Op(A, B);
>>     }
>>
>> }
>>
>> void main()
>> {
>>     value a = value(2);
>>     value b = value(3);
>>     value c;
>>     c = a + b;
>> }
>>
>> The error is:
>> d_playground.d(34): Error: pure nested function 'a' cannot 
>> access
>> mutable data 'this'
>> d_playground.d(34): Error: pure nested function 'a' cannot 
>> access
>> mutable data 'this'
>> d_playground.d(10): Error: template instance
>> d_playground.value.opBinary!("+",
>> value).opBinary.intermediateValue!(plus, this, rhs) error
>> instantiating
>> d_playground.d(44):        instantiated from here: 
>> opBinary!("+",
>> value)
>> d_playground.d(44): Error: template instance
>> d_playground.value.opBinary!("+", value) error instantiating
>>
>> What is going on?  Why is 'a' not allowed to "access" mutable
>> data (even though it isn't modifying it)? How do I tell the
>> compiler to pass "this" in a const fashion?
>
> No answer, but two notes.
>
> First, use dpaste for such code snippets: 
> http://dpaste.dzfl.pl/f2f39b32
>
> Second, what are you trying to do? intermediateValue is a 
> struct without members. I am not sure what 'this' means in such 
> a case.

intermediateValue is a structure that ideally should only exist
at compile time.

Theoretically, (at least in my head), opAssign will traverse the
expression and reduce it to the addition that it is inside
opAssign.

I'm attempting to create a vector DSL (similar to blaze-lib:
https://code.google.com/p/blaze-lib/), but starting with just a
plain number.  Mostly it is for fun, it is a way of working on my
understanding of the compile time primitives and template system.