On Mon, 2007-09-03 at 10:34 +1000, skaller wrote:
> I am changing the operator . rules: in
OK, I have changed the semantics so that in
LHS . RHS
If LHS is a nominal type and RHS = name it tries Koenig lookup
on get_name first, then tries to do ordinary function lookup.
This should preserve the semantics of all programs using get_* methods.
You should note:
module X {
struct Y { int x; };
fun get_y(Y a)=> x * x; // y is defined as x squared
}
var a = X::Y 2;
fun y(a: X::Y)=>a.x - 1;
println a.y; // prints 4
Even though a local y function is defined, the get_y from
module X is preferred. This may seem counter-intuitive BUT
there is a reason:
module X {
struct Y { int x; };
fun get_y(Y a)=> x * x; // y is defined as x squared
}
var a = X::Y 2;
var y = a.y; // OK!
Now you see, if the local y were used, we'd get a type error.
WELLLLLLlllll .... actually we wouldn't. Because already the
code is equivalent to:
try get_y a in X
with _ -> try y a
in other words, an error is simply ignored and the next lookup
tried.. so actually if we swapped the order around, the type
error would be ignored and we'd proceed to method lookup next.
Anyhow, note that get_* names ONLY work with nominal types
and Koenig lookup (lookup in the module in which the type
is defined).
And remember the reason for that lookup:
var a = X::Y 2;
the module X isn't open.. so in
a.y
a normal lookup wouldn't find the projection function for y.
We have now the guideline:
LIBRARY WRITER:
IF your type is nominal, and you want a user defined
projection function, put a get_* method in the same module
as the type is defined (non-nominal types like tuples
don't have definitions, hence the restriction to nominal types).
LIBRARY USER:
OTHERWISE just use the name you want in a scope visible
from the access expression (a.y above).
And remember: if you write
px . f // px a pointer
it means
(*px) . f
You cannot write projections for pointers.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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