Re: How does Ada.Text_IO.Enumeration_IO work?

[Jerry <lanceboyle@qwest.net>]

On Nov 3, 7:52 am, Adam Beneschan <a...@irvine.com> wrote:
> On Nov 3, 1:52 am, Jerry <lancebo...@qwest.net> wrote:
>
>
>
>
>
> > I don't understand how the following program works, specifically, how
> > instances of Ada.Text_IO.Enumeration_IO read input. I understand that
> > it will skip leading white space, leave anything after finding a
> > proper element including line terminator, that apostophe ' is a valid
> > character, and that it can raise an exception (quoting ARM):
>
> > "The exception Data_Error is propagated if the sequence input does not
> > have the required syntax, or if the identifier or character literal
> > does not correspond to a value of the subtype Enum."
>
> > I want the program to read input until a valid element is found, then
> > quit.
>
> > with Ada.Text_IO; use Ada.Text_IO;
> > with Ada.IO_Exceptions; use Ada.IO_Exceptions;
> > procedure Day_Proc is
> >     type Day_Type is (Sunday, Monday, Tuesday);
> >     Day : Day_Type;
> >     package Day_Type_IO is new Ada.Text_IO.Enumeration_IO(Day_Type);
> >     Have_Good_Value : Boolean;
> > begin
> >     Put("Enter a day: ");
> >     loop
> >         Have_Good_Value := True;
> >         begin
> >             Day_Type_IO.Get(Day);
> >             --Skip_Line; -- Doesn't matter if present or not.
> >         exception
> >         when Ada.IO_Exceptions.Data_Error =>
> >             Have_Good_Value := False;
> >             Put_Line("Exception raised");
> >         end;
> >         Put_Line(Boolean'image(Have_Good_Value));
> >         exit when Have_Good_Value;
> >     end loop;
> >     Put_Line("Your day is " & Day_Type'image(Day));
> > end Day_Proc;
>
> > This works as I expect for inputs such as
>
> >    Monday
> > Monday Friday
> > Friday Monday
> > Fri7day Monday
> > Friday ' Monday
> > Monday.
> >  etc.
>
> > but anytime the input contains a non-apostrophe punctuation mark
> > before a valid element, or an otherwise inproperly syntaxed element,
> > it loops endlessly, outputting Exception raised and FALSE for each
> > passage through the loop. For instance, these lines cause infinite
> > looping:
>
> > Friday. Monday
> > Friday.Monday
> > Friday ? Monday
> > Fri?day Monday
> > Friday 7Thursday Monday
> > This is (or is not) a comment
> >  etc.
>
> > It is correctly raising the exception upon encountering the bad input,
> > but why does it keep looping and not proceed past the bad input to
> > find the following correct element?
>
> 'Cuz the rules say so.  A.10.6(5) is the important one here: "Next,
> characters are input only so long as the sequence input is an initial
> sequence of an identifier or of a character literal (in particular,
> input ceases when a line terminator is encountered). The character or
> line terminator that causes input to cease remains available for
> subsequent input."  A.10.6(10): "The exception Data_Error is
> propagated by a Get procedure if the sequence finally input is not a
> lexical element corresponding to the type, in particular if no
> characters were input ...".  That's the case when your input (skipping
> leading blanks) starts with an invalid character like a comma.  Since
> the comma can't be the first character of an enumeration literal, the
> comma "remains available for subsequent input", and thus "no
> characters are input" and Data_Error is raised.
>
> By the way, apostrophes are "allowed" but they have to be in the
> correct syntax.  So if your line begins with
>
> 'ABCDE'
>
> I believe the characters 'A will be input, but since 'AB cannot be the
> start of an enumeration literal, input stops at that point, and B will
> be the next character available for input.
>
>                          -- Adam

[Not sure why my earlier reply didn't make it but here is a
reconstruction of it.]

Thanks, Adam. I believe that the clue for me is that "remains
available for subsequent input" means that it is available only to non-
enumeration input, characters excepted.

Jerry