OpenEuphoria

Re: request for change of boolean

• Posted by CChris <christian.cuvier at agric?lture.gouv.fr> Dec 06, 2007
• 847 views

Kat wrote:
> 
> 
> Is it possible to change how boolean expressions are evaluated such that only
> values greater than 0 pass this test? :
> 
> }}}
<eucode>
> readfile = open(flagfilename,"r")
> if readfile then
> -- it's there, process it
> end if
> </eucode>
{{{

> 
> Currently, if readfile = -1 , it evaluates to TRUE, when in fact the readfile
> was not opened.
> 
> Kat

1/ I routinely use things like

if readfile+1 then
-- readfile is a valid handle
end if

Granted, the "+1" is excess typing, but if only it were the only problem with
Eu...

2/ If the nil logical value was in the language, then open() could return nil on
failure and your test would work. Much cleaner, I wish it were here. But it means
changing two state logic to three state. Some code is expected to break. This has
been discussed already a few months ago.

3/ Another approach, more object oriened (donning firefighter gear fast), and
much easier to use as a result is:
- in the standard library:
<eiffel>
class FILE
-- whatever should go in here
feature {NONE} -- they are private
    last_open_file: INTEGER -- last handle retrieved

    open(file_name,file_mode:STRING) is
        local
            success: BOOLEAN -- some internal flag
        do
            -- whatever it takes
            if success then
                last_open_succeeded = TRUE
                last_open_file = the_handle
            else
                last_open_succeeded = FALSE
            end
        ensure
            success = last_open_succeeded
        end

feature -- public
    last_open_succeeded: BOOLEAN -- set by open on success
    get_last_open_file(): INTEGER is
        require
            last_open_succeeded 
        do
            Result = last_open_file
        end
</eiffel>    

- in your code:
<eiffel>
open(file,mode)
if last_open_succeeded then 
    my_file = get_last_open_file() 
    process(my_file)
else 
    file_not_opened() -- must do something about it
end
</eiffel>

If you try to access a file that couldn't be opened without cheking, you get an
exception.

Again, while the above two scenarios would make coding safer and clearer, they
just don't fit in what Eu is. If I ever can get a corret type system to work in Æ
- not for the near future -, then the nil solution will be used.

CChris