[MLton] bool and MLton FFI

Wesley W. Terpstra wesley@terpstra.ca
Fri, 23 Jun 2006 00:30:58 +0200

On Jun 23, 2006, at 12:08 AM, Matthew Fluet wrote:
> So, unlike int32_t and friends, there is no required size for a bool.
There is no required size for 'int' or 'short' either.
How is bool different in this respect?

> [fluet@localhost temp]$ gcc -o z z.c
> [fluet@localhost temp]$ ./z
> sizeof(bool) = 1
> sizeof(bool[5]) = 5
kiwiw:~/src terpstra$ gcc -o z z.c
kiwiw:~/src terpstra$ ./z
sizeof(bool) = 4
sizeof(bool[5]) = 20


> Point being that there may not be a universal choice for the  
> representation of 'bool'.  Furthermore, equating ML 'bool' with C  
> 'bool' would seem to rule out a bit array implementation, though it  
> does admit a word8 array implementation.

As long as we forbid bool vector in the FFI, then MLton is free to  
store bools however it wants.
I don't see a problem with MLton converting whatever representation  
it uses for bool to a C bool (of the appropriate type for that  
system) automatically in the FFI. ie: Compare to zero when it's a  
return value, or promote to 8/32 bits on call.