Proposal to add 'Elements of Programming' Concepts to std.traits

[Guillaume Chatelet]

On 06/17/12 14:23, Philippe Sigaud wrote:
> An obvious generalization is to template the struct on a predicate:
> 
> struct Constrained(T, alias pred) {
>    private T _value;
> 
>    invariant() { assert(pred(_value)); }
> 
>    this(T x) { _value = x; }
>    this(Constrained c) { _value = c._value; }
> 
>    Constrained opBinary(string op)(const ref Constrained rhs) {
>        return Constrained(mixin("this._value"~op~"rhs._value"));
>    }
>    Constrained opBinary(string op)(T rhs) {
>        return Constrained(mixin("this._value"~op~"rhs"));
>    }
>    ref Constrained opUnary(string op)() if(op=="++"||op=="--"){
>        mixin(op~"this._value;");
>        return this;
>    }
>    ref Constrained opAssign(T rhs) {
>        value = rhs;
>        return this;
>    }
>    ref Constrained opAssign(Constrained rhs) {
>        value = rhs._value;
>        return this;
>    }
> 
>    @property T value() const  { return _value; }
>    @property void value(T newValue) { _value = newValue; }
> 
>    alias this value;
> }
> 
> Constrained!(T,pred) constrained(alias pred, T)(T _value){
>     return typeof(return)(_value);
> }
> 
> void main(){
>    auto a = constrained!(x => x > 0)(1);
>    auto b = a;
>    auto c = b+1;
>    auto d = c+a;
>    a = b-d;
> }

Very nice :)