[M3devel] null type?

[mika at async.caltech.edu]

What is lost is some of the mathematical simplicity of the type system.

A type is just a set of values.

There is nothing odd about a type that can contain a single value.

By your logic, we should outlaw declarations of variables of type [1..1]
as well.  Or variables of type { OK } (one-value enumerations).

The upshot would be to require special-case code in programs that
*generate* Modula-3 code to check for types of a single value
and avoid those through some gyrations involving value substitutions.

I can see how that could be a big hassle in some cases---it would require
the aforementioned code generators to be able to perform value substitutions
and possibly dead-code elimination.

     Mika




Darko writes:
>Probably not a good idea to make it illegal for backwards compatibility =
>if you really think it's going to break things, so perhaps issue a =
>warning instead. But besides providing a type for NIL it doesn't serve =
>much purpose and I can't see anything being lost by outlawing it, =
>eliminating the confusion with REFANY might be gained.
>
>Just because it's a valid bit of code doesn't mean it's not a bug. =
>Looking at the code you can reasonably assume the author thought NIL was =
>of type REFANY. It's a useless declaration unless it's trying to make =
>some sort of point by being useless. It's a declaration to pass in one =
>possible value which it's already assigned and which you can't change.=20=
>
>
>
>On Apr 24, 2013, at 7:53 PM, mika at async.caltech.edu wrote:
>
>> Darko writes:
>>>=20
>>> --Apple-Mail-2--196294281
>>> Content-Transfer-Encoding: quoted-printable
>>> Content-Type: text/plain;
>>> 	charset=3Dus-ascii
>>>=20
>>> I think the correct approach is to treat a variable declaration with =
>a =3D
>>> type NULL as an error. The correspondence to C would be void, but =
>just =3D
>>> void, not void*.  What is the representation of the declaration "void =
>=3D
>>> x"? I'm guessing C doesn't allow that.
>>=20
>> It's not an error since there is precisely one legal value for such a =
>variable,
>> namely, NIL.  That value can be inferred from the type without =
>inspecting the
>> variable.
>>=20
>> If you make it illegal to declare variables of type NULL you may have =
>to go
>> back and change various code generators that depend on it.
>>=20
>> There are types that are truly empty in Modula-3, and it is illegal to
>> declare variables of those types (but not illegal to declare those =
>types
>> or to use them to construct other types), e.g.,
>>=20
>> CONST
>>  A =3D 3;
>>  B =3D 1;
>>=20
>> TYPE
>>  T =3D [A..B];
>>=20
>> VAR
>>  t : T; (* static error *)
>>=20
>> TYPE
>>  A =3D ARRAY T OF X; (* not an error *)
>>=20
>> etc.