Re: Unusual "use" semantics

[jm21@prism.gatech.EDU (Jim Marks)]

In article <20@polymnia.tsd.arlut.utexas.edu> fred@polymnia.tsd.arlut.utexas.edu (Fred Hosch) writes:
>My Ada compiler complains about the relation on the fourth line of the
>following
>
>	with TEXT_IO;
>	procedure TEST is
>	begin
>	    if TEXT_IO.COL = 1 then
>...
>
>reporting: no operator visible for positive_count "=" universal integer.
>
>If I add a context clause "use TEXT_IO;" it compiles successfully.
>

This is a result of the strong typing and operator overloading of Ada.  
The "=" operator you have visible can take two integer type arguments.  
However, the return value of TEXT_IO.COL is of type POSITIVE_COUNT, which 
is a subtype of type COUNT, declared in the Text_IO package (LRM 14.3.10).  
This type has its own "=" operator implicitly declared for it in Text_IO.
When you include the "use" clause for Text_IO, then the "=" operator for 
this type (COUNT) is then visible (in this case, the "1" is considered 
of this type so that it matches).  You could also do the following:

if integer(Text_IO.Col) = 1 then ...   which converts the return value
				to integer, OR

if Text_IO."=" ( Text_IO.Col , 1 ) then ...   which is UGLY.

I've had this problem many times when I don't think about it in advance.
This is a good application of the "use" clause, even though some coding
standards discourage (or prohibit) the use of "use" clauses.

-- 
Jim Marks   		   | Georgia Tech Research Institute
Compuserve: 72310,2410     | Concepts Analysis Laboratory
Internet:   jm21@prism.gatech.edu -or- jmarks@gtri01.gatech.edu