Re: S-expression I/O in Ada

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Natacha Kerensikova <lithiumcat@gmail.com> writes:

> (probably Integer mod 2**16 or something like that).

There's no such thing as "Integer mod 2**16".  "mod ..." is
how you declare an unsigned integer type (called "modular type" in
Ada), and "range ..." is how you declare a signed integer type.

Integer is a predefined signed integer type.  It's range is
not portable, so if you care about that, it should be avoided.

It's a good idea to declare your own integer types -- different
types for different purposes.  For example, if you have an array
of indexes into another array, using different index types for
the two arrays will often make the code readable -- it will be
obvious that (say) X is intended for indexing into the
outer array.

Modular types are evil, and should usually be avoided.
You might want modular types when interfacing with C code,
but don't use them just because you happen to have
no negative numbers.  For example, I would usually
(not always!) prefer this:

    type Octet is range 0..2**8-1; -- signed

over this:

    type Octet is mod 2**8; -- unsigned

The index type of an unconstrained array should (almost) never
be modular.

- Bob