[MLton] C codegen and world* regressions
Wesley W. Terpstra
Sun, 18 Jun 2006 01:19:20 +0200
On Jun 17, 2006, at 10:49 PM, Matthew Fluet wrote:
> I don't know if that quite works, because I'm not sure that the
> natural C representation of bool is necessarily equal to MLton's
> representation of bool array. In particular, we could turn it into
> a bit-array, while I think C will make it a 8-bit char array. So
> you still have pin things down to what the C-side does.
C actually makes it a 32 bit array (absolutely horrible!).
I would absolutely love it if a future MLton represented 'bool
vector' as a bit vector! :-)
> Would removing bool from the FFI types be a horrible idea?
If you promise to make it a bit vector, I wouldn't be devastated to
see it disappear from the FFI.
OTOH, as long as it's documented to be a bit vector, then I don't see
how that would be a problem..? It just wouldn't be the case that a
MLton bool is a C bool. However, I think that's going to be the case
with 'int' now too, right?
>> I wonder if the right thing to do for situations where we are unsure
>> or have C functions that return int instead of bool, is to expose
>> as int in the FFI and convert the int type to bool in SML code with a
>> comparison to zero.
> If we disallowed bool in indirect FFI types (i.e., bool ref, bool
> array, bool vector), I could imagine implementing this as a type
> directed elaboration of FFI functions.
I feel pretty uncomfortable with the prospect that FFI functions
returning bool need to carefully return either 0 or 1. While I agree
with Stephen that we can define it however we want, I think that the
average C programmer expects to be able to return the result of a C
test as a bool. So, however it's implemented, I think that comparing
to zero is definitely a good idea.