Ok, so here's the core advantage of
BEXPR_prj (n,d,c),d->c
This term is projection number "n" from type d to type c.
For example given
struct X {
int a;
double b;
double c;
};
we have three projections:
prj0: X -> int
prj1: X -> double
prj2: X -> double
At present, we can write:
var x = X (1, 4.2,1.4);
var xc = x . c; // prj2
var xb = x . b; // prj1
but we cannot write:
var p = &X::b; // prj1
var d = x .p;
which we CAN do in C++. Here p is a projection *variable*, i.e. an expression
yielding a projection of a known type, but it is not known which one it is.
The new BEXPR_prj has a *run time* representation. First, we can do this anyhow:
fun proj1 (x:X) => x.b;
var d = x . proj1;
remember, operator . is just reverse application! This one is a bit tricker
though:
fun proj1 (px: &X) =? px . b;
(&x) . proj1 <- 7.3;
The representation at run time is of course just an offset.
In C you can add offsets but it isn't safe, in C++ offsets
are safer but you can't add them. Addition of offsets is just
functional composition.
Now, there's another trick here: compact linear types.
There is an "offset" to one of them too, but its the divisior
required to shift the value right inside an integer to the
units position (the modulus which masks higher parts out
is given by the size of the type).
What this means is that to compose two projections to
compact linear terms, we divide, then we divide again.
In other words, the projections are just multiplied!
There's a deep lesson here. The whole of your address
space can be considered compact linear. Ordinary
addressing by addition of offsets, but that's a special
case! The general case is just multiplication, and the obvious
relation is a logarithm.
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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