Re: Get_Line

["Chad R. Meiners" <crmeiners@hotmail.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccy98d2plj.fsf@shell01.TheWorld.com...
> Well, maybe your students didn't have trouble.  Maybe that means you're
> a good teacher.  But I know I've seen *many* questions about
> Text_IO.Get_Line and related stuff on c.l.a. over the years.
> And this stuff *ought* to be simple -- I'm not surprised when
> a beginner is confused about discriminated tasks or something,
> but when folks are confused about how to read in a line of text,
> I think that indicates something wrong with the language.

I see.  I think the problem is that (simple) problems rarely actually call
for a programs to just get a line of text.  Often people think they need to
get a line of text and parse it, but they could just parse the line directly
with Ada.Text_IO's generic data type facilities.  I agree though that
reading an arbitrarily size string takes some work in Ada, but I would
expect this since Ada doesn't natively support resizable arrays.  This
doesn't mean we should sit back and be content with Ada.Text_IO when we
could extend it to make reading in a line a little more friendly, but we
should leave the side-effects out of the functions within Ada.Text_IO.
Would you be happy with the following instead?

type Line is private;

procedure Get_Line (File : File_Type; Item : out Line);

function  To_String (Item : Line) return String;


> Sorry, I didn't mean to defame your character.  I was just making an
> analogy between side-effects in character-stream-reading functions,
> and allocators, where for the latter we happily use function-call-like
> notation.

Okay, I didn't think you meant to defame; I was just making sure of your
intent.


> OK, that convinces me that my analogy was poor.
>
> Still, the reason "Do_Something (Get_Line, Get_Line, Get_Line);" is
> wrong is that Ada does not define evaluation order.  The mere
> *existence* of a Get_Line function should not cause people to write
> that.

My concern is that when students are learning a new language they sometimes
adopt a favorite part of the language and then proceed to try to use that
part whenever possible.  A Get_Line function has that certain appeal to it
that would make it a good candidate for such abuse especially if the student
knows c++.

> I actually think that Ada *should* define the evaluation order,
> precisely because the above example can cause bugs.  Alternatively,
> the language should forbid such examples at compile time, rather
> than just giving wrong answers.  I realize that Ada has no mechanism for
> having such compile-time checking, because the compiler doesn't know
> which global variables a given function reads and writes.

I would definitively like to see warnings generated when possible! ;-)

-CRM