On Aug 13, 2008, at 5:48 AM, Edgar Friendly wrote:
Brighten Godfrey wrote:
Two things come to mind:
(1) The type of get_f1 is handled analogously to the way it is
handled
for objects, something like this:
val get_f1 : < x : 'a; .. > -> 'a = <fun>
I'm guessing that if you did this, you would have to "instantiate"
`get_f1' each time it is applied to a new record type, which I
assume is
inconvenient (or not?).
Yes - this breaks separate compilation.
Makes sense.
(2) Require that all record field accesses refer to a globally-unique
record type, making conversion to a record field index is easy.
So the
example code Edgar gave would result in a compilation error
because the
compiler cannot determine which `.f1' field the access refers to.
But
consider this code:
let return_garlic () =
let x = {M2.f2=5; M2.f1="garlic"} in
x.f1
In line 2, globally unique record field names are given, which allows
the compiler to tag variable `x' with type `M2.t2'. Then in line
3, the
record field access `x.f1' can only mean `x.M2.f1'.
In this situation, the type information influences what code is
generated. The OCaml developers have been as careful as possible to
avoid this. The typing stage of compilation acts as a filter to
eliminate incorrect programs, and that's it.
I've had my share of ideas of how typing information could usefully
connect with code generation, but have been shut down because the
ocaml
developers (probably rightly) don't want to bridge this separation.
(Probably because the quality of the compiler would go down the tubes
right quick.) Although... hmmm.. I guess type information is
used for
some optimization (specializing = for ints and such).
This is a good point. Thanks for the explanation. I'm having a hard
time thinking of any case other than =,<,> etc where type information
would be necessary to determine code generation. On the other hand
if you break the separation for those operators, maybe it's OK to
break it for record names as well.
Also you lose the compositionality as before - you can't break this
function into two parts because the second line "needs" the first line
to work.
It can still work, for example this would work:
let garlic_part_1 () = {M2.f2=5; M2.f1="garlic"}
let garlic_part_2 x = x.f1
let return_garlic () =
garlic_part_2 (garlic_part_1 ())
Using this notation the programmer could of course choose to always
specify the full record name (x.M2.f1) if desired.
~Brighten
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