logical const is a subset of transitive const

[Bruce Adams]

Steven Schveighoffer Wrote:

> "Janice Caron" wrote
> > What I /don't/ understand is why D lets you declare a member function
> > (as opposed to an object) as being invariant. To my brain, that ought
> > to mean the function sees "this" as having the type invariant(T) - but
> > that's clearly an invalid assumption, because "this" /can/ be
> > modified. You have a read-only view, that's all.
> >
> 
> Yeah, I totally agree that invariant declared on a function doesn't seem to 
> be right.  I'm guessing the only reason they did that is because it makes no 
> sense to return an invariant type, so you could put invariant before the 
> function name and not be ambiguous.
> 
> i.e.
> 
> const int* f(); // is f const, or is it returning a const int*?
> invariant int* f(); // can't have a pointer to invariant (at least in 
> current spec), so invariant must refer to f
> 
> If that is the reason, then I say use const, and put it after the func 
> declaration, like in C++.  May look weird, but it's unambiguous.
> 
> > "Pure" on the other hand, only makes sense for functions. So far as I
> > know, there's no such thing as a pure object.
> 
> pure implies that:
> 1. you can only call pure functions
> 2. you can only access invariant data (data that cannot be changed anywhere 
> else).
> 
> so I think you are right, pure is more for functions.
> 
> BTW, if we get rid of invariant for functions meaning the same thing as 
> const functions, I think pure should be replaced with invariant.  It makes 
> more sense to me as an intuitive keyword.  An invariant function cannot 
> change anything and can only access invariant data, and call other invariant 
> functions.  Makes a lot of sense to me...
> 
> -Steve 
> 
Pure is a good intuitive name to use for several reasons.
For a start it has exactly the same meaning as its counterpart in Fortran-90. If it exists in any functional languages out there most likely they use the same terminology.