Re: Ada.Strings.Bounded

On Sat, 05 Apr 2008 11:43:39 +0200, Pascal Obry wrote:

Works fine! But then it becomes a bit messy when converting back and
forth with the string type.

U1 := To_Unbounded_String ("whatever");

Put_Line (To_String (U1));

Not a big deal, but if we can find a nice way to tell that such routine
must be used for conversion between type it will be quite handy.

type Unbounded_String is ...;

function To_String (U : Unbounded_String) return String;

for Unbounded_String'Conversion (String) use To_String;

No, that would be a can of worms. Arbitrary type conversions automatically
applied is a mess. Below you immediately spot the problem of conversions
preferences. Because conversions are arbitrary, there is no any order
defined on the types to make a choice.

What is actually needed is to say that Unbounded_String is a member of
String'Class and you wanted to inherit everything form String.

The implementation of inheritance (by the compiler) will require you to
provide two *private* conversion functions when the types are untagged.
Note that you need two conversions: a forward conversion for in-parameters
and a backward conversion for out-ones. In out parameters will use both
(copy-in, copy-out).

For tagged [actually, by-reference] types the compiler can generate
conversions because they are necessarily view conversions.

Just to get the idea, then one could write:

Put_Line (U1);

Of course this raises some questions:

What to do if we have Put_Line defined for String and
Unbounded_String? Which version gets called?

It will one of Unbounded_String, because that will be an override.

Note though, that Put_Line (File, String) will open further questions. Is
it primitive in File or in String? Clearly it should be in both. This is
how multiple dispatch issue comes in.

Anyway that's just some wild thoughts :)

It is not wild, it is how Ada types system should be completed.

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
.

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