Re: How does Ada.Text_IO.Enumeration_IO work?

[Adam Beneschan <adam@irvine.com>]

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