I would like to get rid of lvalues, because they pollute the system
severely. It isn't just the type system .. the problem is that
you cannot write *Felix* functions that operate on lvalues, only
define C primitives which do so.
A proposed solution to this is rather interesting: change
the meaning of
var x : int;
so that x is a POINTER to an int (i.e. a reference, as in ML).
Already we can use pointers to X in some places where X is needed,
for example
a . b
works for struct A where a is an A or an &A: the pointer is deref
automatically (since pointers can't have components, this is safe).
For a function
f (x:int)=> x + x;
it isn't quite so clear if we can do this hack. I'm not sure how
overload resolution would cope (matching parameter 'int' with
argument '&int') .. given that some functions will actually
operate on pointers.
However the lvalue->value decay already being done already
has to cope with this kind of crap.. so maybe it will work.
Clearly
x + 1
is ambiguous .. incrementing the pointer? or the value it points to?
There is another issue:
proc pre_incr: lvalue[int] = "++$1";
would now become
proc pre_incr: &int = "++*$1";
which means all functions -- both Felix and C primitives -- would use
pass by value. This will work fine.. but it is potentially inefficient
because it hides the variable being incremented, introducing an
aliasing problem.. it isn't clear C++ compilers such as gcc can
unravel this..
var x = 1;
++x; // generates ++*&x
but it has to unravel immediately to
++x
to allow some optimisations to work.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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