for S'Image use Func??
in [ADA]
|
Prev: Question about default discriminants and mutable objects.
Next: Why is not Stream_Access defined Ada.Streams ?
From: Warren on 6 May 2010 13:10 I have tried to google for this and have not yet found a suitable answer, so I'll troll, er, poll for an answer here.. Is there the ability to substitute your own S'Image function? For example, a basic interpreter might define: type LNumber_Type is range 0..99_999; Can I declare.. for LNumber_Type'Image use LNumber_To_String; If so, then the question is what the signature of the S'Image function looks like-- is it: function LNumber_To_String(LNO : LNumber_Type) return String; Finally, there is actually a third question- more along the lines of "Should this language feature be used in this manner?", or is it preferable to just code your own along the lines of (which is what I presently use): function To_String(LNO : LNumber_Type) return String; Inquiring minds need to know, Warren
From: Dmitry A. Kazakov on 6 May 2010 13:23 On Thu, 6 May 2010 17:10:20 +0000 (UTC), Warren wrote: > I have tried to google for this and have not yet found > a suitable answer, so I'll troll, er, poll for an > answer here.. > > Is there the ability to substitute your own S'Image > function? For example, a basic interpreter might > define: > > type LNumber_Type is range 0..99_999; > > Can I declare.. > > for LNumber_Type'Image use LNumber_To_String; > > If so, then the question is what the signature of > the S'Image function looks like-- is it: > > function LNumber_To_String(LNO : LNumber_Type) return String; I am not sure what do you mean, but the following is legal Ada: generic type T is private; with function Image (X : T) return String; package P is ... end P; type LNumber_Type is range 0..99_999; package PI is new P (LNumber_Type, LNumber_Type'Image); So attribute is a "plain" function with a clumsy name. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de
From: Adam Beneschan on 6 May 2010 13:58 On May 6, 10:10 am, Warren <ve3...(a)gmail.com> wrote: > I have tried to google for this and have not yet found > a suitable answer, so I'll troll, er, poll for an > answer here.. > > Is there the ability to substitute your own S'Image > function? For example, a basic interpreter might > define: > > type LNumber_Type is range 0..99_999; > > Can I declare.. > > for LNumber_Type'Image use LNumber_To_String; No. Broadly, you can't use a FOR attribute specification on every attribute, just a few select ones that the language specifically says you can. 'Image isn't one of those. I think it's been mentioned a few times on this newsgroup that it might be nice to have this ability, but I don't see that anyone has submitted an actual language change proposal. -- Adam
From: Yannick Duchêne (Hibou57) on 6 May 2010 14:14 Le Thu, 06 May 2010 19:10:20 +0200, Warren <ve3wwg(a)gmail.com> a écrit: > I have tried to google for this and have not yet found > a suitable answer, so I'll troll, er, poll for an > answer here.. > > Is there the ability to substitute your own S'Image > function? For example, a basic interpreter might > define: > > type LNumber_Type is range 0..99_999; > > Can I declare.. > > for LNumber_Type'Image use LNumber_To_String; > > If so, then the question is what the signature of > the S'Image function looks like-- is it: As far as I could tell, your intuition is good, as there is indeed such things in Ada, called either representation clauses or operational clauses. See [ARM 2005 13.3] for more about it. An excerpt to be kind: [ARM 2005 13.3 (2)] attribute_definition_clause ::= for local_name'attribute_designator use expression; | for local_name'attribute_designator use name; [ARM 2005 13.3 (4)] For an attribute_definition_clause that specifies an attribute that denotes a subprogram, the expected profile for the name is the profile required for the attribute. However, it later says: [ARM 2005 13.3 (5/1)] An attribute_designator is allowed in an attribute_definition_clause only if this International Standard explicitly allows it Comes with a tiny example: [AARM 2005 13.3 (6a)] Ramification: This implies, for example, that if one writes: for T'Read use R; R has to be a procedure with two parameters with the appropriate subtypes and modes as shown in Finally, it should be checked if Image is explicitely allowed as an attribute designator for an operational clause (I may check later to give you a more formal answer to this one question). I use representation clauses from time to time, but I've never used this kind of one, for the reason I give you right after now. > function LNumber_To_String(LNO : LNumber_Type) return String; > > Finally, there is actually a third question- more along > the lines of "Should this language feature be used > in this manner?", or is it preferable to just code your > own along the lines of (which is what I presently use): > > function To_String(LNO : LNumber_Type) return String; > > Inquiring minds need to know, > > Warren I would say, it is more handy to define a function, because a function would be able to hold the exact formatting parameter your application requires, it would be able to hold the exact optional defaults for those parameters and it would better integrates with the overall general design of an Ada application, that, âwithingâ package and using renames clause. Using attribute, you are require to always use thye type name as a prefix for such things as âImage. With function, you may âwithedâ the package defining this function, and make it part of the local scope using something like âfunction F (...) ... renames My_Package.F ...â. More handy IMHO. All of this providing I've understood what you were requesting for. -- No-no, this isn't an oops ...or I hope (TM) - Don't blame me... I'm just not lucky
From: Jeffrey R. Carter on 6 May 2010 14:50
Warren wrote: > > type LNumber_Type is range 0..99_999; > > Can I declare.. > > for LNumber_Type'Image use LNumber_To_String; No. Nor can I see why you'd want to. Once you declare function Image (Value : Lnumber_Type) return String; why would you want to write Lnumber_Type'Image (I) rather than Image (I) ? -- Jeff Carter "I soiled my armor, I was so scared." Monty Python & the Holy Grail 71 |