Re: ADA.STRINGS.INDEX_ERROR : a-strunb.adb:782

["G.B." <bauhaus@notmyhomepage.invalid>]

On 18.01.18 15:17, Mehdi Saada wrote:
> Since universal_integer is meant to be compatible with everything, and since Ada83 is long gone,
> could "function Length return Integer renames Object_Array'Length" be accepted now ? I don't think it could break any existing code to accept something that wasn't.
> 

There is more to 'Image, which is a type attribute like 'Size.
Handling types is the realm of compilers.

Consider 'Value, which is paired with 'Image. Suppose there is
an enumeration subtype like that of

    type T is ('$', '¥', '£');

We now have

    X = T'Value (T'Image (X))

and, usually,

    S = T'Image (T'Value (S))


However, given the history of String design in programming
languages, things have become tricky.

Not because of a lack of symmetry between 'Image and 'Value
as written above.
Rather, I think, because of that tendency in language design
which bows before elementary school training. Hence,
Ada has removed symmetry: for example, in place of making
"+" a required part of numeric literals such as are input
to 'Image, it can be omitted. So, these values are now all
equal in Ada:

  Positive'Value ("42")
  Positive'Value (" 42")
  Positive'Value ("+42")
  Positive'Value ("   42  ")
  Positive'Value ("+42        ")

Very flexible and convenient, at first sight, I think. And done
for good reasons, at the time, perhaps: because, for output,
everyone would use an instance of a package from Text_IO,
and not 'Image. Right? No problem could have been anticipated.

This design decision has led to an endless discussion about
leading spaces in the result of 'Image for non-negatives.
At the root, elementary school habits in engineering make
programmers proudly dismiss the idea of writing numbers
symmetrically, with signs for both negatives and positives,
and maybe some technical prefix like " " attached to (non-FPT) 0.

OTOH, if programmers are smart enough to handle the compiler's
job of defining 'Image and 'Value, they are able to perform
well as substitutes: by providing the compiler with all correct
implementations for parsing and printing all user-defined literals
of any user-defined type.