Idea: partially pure functions

[Jason House]

Bruno Medeiros Wrote:

> Jason House wrote:
> > Rather than thinking of functions than can mix with pure functions as partially pure, I'd prefer to think of them as having no side effect.  For arguments sake, I'll just use the keyword noside (analogous to nothrow).
> > 
> > class A{}
> > class C{}
> > 
> > void f(); // impure
> > void f() noside; // pure
> > void f(A c) noside; // partially pure
> > void f(invariant(A) c) noside; // pure
> > C f() noside; // illegal
> 
> Why is this last case illegal? It's fine for a pure function to return a 
> mutable value. (see my other discussion with Steven)

That's a trickier case.  I should have explained it.  How does the function get a mutable C to return?  It can't use mutable global data and it can't allocate a new C.  Because of that, it becomes impossible to give a body to f that satisfies the contract.


> > class B{
> >   C cc;
> >   void f(); // impure
> >   void f() noside; // pure
> >   void f(A c) noside; // partially pure
> >   void f(invariant(A) a) noside; // pure
> >   C f() noside; // partially pure
> >   invariant(C) f() noside; // pure
> > }
> 
> All the examples here that you mention as pure, are not pure but in fact 
> partial pure, because the methods have a mutable this parameter.

You're right.  True purity requires the class member functions to be marked as invariant.