[mercury-users] ok/[01] and file descriptors

[Fergus Henderson]

On 15-Apr-2001, John Eikenberry <jae at zhar.net> wrote:
> 
> 1. Where are ok/1 and ok/0 explained? If they're not, could someone explain
> them. ... They are mentioned in the comments of some declarations
> but never described themselves (that I could find).

ok/1 and ok/0 are alternatives in the io__res, io__result, and io__read_result
types defined in the standard library module `io':

 | 		% Various types used for the result from the access predicates
 | 
 | 	:- type io__res		--->	ok
 | 				;	error(io__error).
 | 
 | 	:- type io__res(T)	--->	ok(T)
 | 				;	error(io__error).
 |
 | 	:- type io__result	--->	ok
 | 				;	eof
 | 				;	error(io__error).
 | 
 | 	:- type io__result(T)	--->	ok(T)
 | 				;	eof
 | 				;	error(io__error).
 | 
 | 	:- type io__read_result(T)	--->	ok(T)
 | 					;	eof
 | 					;	error(string, int).
 | 						% error message, line number

These types are used as return types for certain I/O operations.
If ok/1 or ok/0 is returned, it means that the I/O operation was
carried out successfully; for ok/1, which is used for input operations,
the argument of ok/1 is the value that was input.

For I/O operations which return one of these types, if the I/O operation
could not be carried out successfully, it will return error/1 or eof/0.
If error/1 is returned, the argument gives the reason why the operation
could not be carried out successfully.

> 2. I'm figuring out the posix module because I need to use select over
> stdin and stdout. select expects file descripter pointers, and I've not
> having much luck in figuring out how to get these for stdin/stdout. Any
> tips?

:- func stdin = posix__fd.
:- func stdout = posix__fd.
:- func stderr = posix__fd.
stdin = posix__fd(0).
stdout = posix__fd(1).
stderr = posix__fd(2).

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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