[mercury-users] Event handling in mercury

[Holger Krug]

> If you really need to access the io__state to initialize it, you
> can always use a maybe(T) type or equivalent, e.g.
> 
> 	:- type initialized(T) ---> initialized(T) ; uninitialized.
> 
> define the variable as `foo := no' / `foo := uninitialized',
> and then initialize it in main/2.  That way it is up to
> the programmer to deal with the order of evaluation issues.

OK.

> 
> > So one would have 2 different forms of `:=', one only used to
> > initialize a static mutvar, the other to set it's value in io_state
> > related operations.
> 
> Yes, in my example I was assuming that `:=' would just be used for
> static initialization, and that if dynamic initialization based on the
> io__state is required then you'd use `io_store__set_mutvar' to override
> the static initializer.

`:=/2' is used for initialization.
Why not to use `:=/4' in DCG-rules as follows:

        :- interface.
	% static variable foo accessible only in IO-related operations
        :- static foo = int.
	
	:- import_module io.

	:- pred main(io__state, io__state).
	:- mode main(di, uo) is cc_multi.

        :- implementation 
	%%% initializer `:=/2' %%%
        foo := 42.
 
	main -->
	%%% update `:=/4' %%%
		foo := 43.


-- 
Holger Krug
hkrug at rationalizer.com
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------