ka@cs.washington.edu (Kenneth Almquist) (03/06/90)
Here's a question about programming style for you to debate. I have a package which accesses command line arguments. My current implementa- tion is for systems without the concept of a command line arguments; it reads the command arguments from the standard input. The interface to the package consists of two functions: nargs - returns the number of command line arguments. arg(i) - returns the i'th command line argument. Being a former C programmer, I naturally made arg(i) return a pointer to a string, using a separate package named mytypes to contain the definition of a pointer to a string: package mytypes is type string_ptr is access string; end mytypes; with mytypes; use mytypes; package args is function nargs return integer; function arg(index: integer) return string_ptr; end args; Since I haven't been exposed to a lot of Ada programming styles, I'd be curious to hear from people who would specify the interface differ- ently, for example making arg(i) return a string rather than a pointer to a string, or making "arg" a procedure with an "out" argument like "get_line". Kenneth Almquist
defaria@hpclapd.HP.COM (Andy DeFaria) (03/07/90)
>/ hpclapd:comp.lang.ada / ka@cs.washington.edu (Kenneth Almquist) / 3:19 pm Mar 5, 1990 / >Being a former C programmer, I naturally made arg(i) return a pointer >to a string, using a separate package named mytypes to contain the >definition of a pointer to a string: > > package mytypes is > type string_ptr is access string; > end mytypes; > > with mytypes; use mytypes; > package args is > function nargs return integer; > function arg(index: integer) return string_ptr; > end args; My question would be why the package mytypes? Why not: package ARGS is type ARG_PTR is access STRING; function NARGS return integer; function ARG (index: integer) return ARG_PTR; end ARGS; It is a tendency for C programmers to use packages as #include files. In my opinion this causes confusion. This example creates a string pointer called ARG_PTR but by its usage it can only point to an command line arguement.
eachus@aries.mitre.org (Robert I. Eachus) (03/08/90)
In article <920019@hpclapd.HP.COM> Andy DeFaria writes: > My question would be why the package mytypes? Why not: > package ARGS is > type ARG_PTR is access STRING; > function NARGS return integer; > function ARG (index: integer) return ARG_PTR; > end ARGS; Why not return a string? This is not the I/O case where successive calls (for example to GET_LINE) return different values so: package COMMAND_LINE is function NARGS return Integer; function ARG (Index: Integer) return String; end COMMAND_LINE; This is conceptually much cleaner, and if it is necessary to assign an argument string to a slice of a fixed length string it can be easily done: Path: String(1..80); ... begin Path(1..Command_Line.Arg(1)'LENGTH) := Command_Line.Arg(1); ... This looks a little messy, but only because it is fighting the language, which would perfer that you write: Path: constant String := Command_Line.Arg(1); this is one of those features/tricks in the language which makes perfect sense, but only to a compiler writer. A compiler can easily allocate a dynamically sized object on the stack, but only if its size never changes. And compilers have to be able to handle Ada functions which return values whose size cannot be determined at compile time, because certain language primitives such as "&" work that way. So in Ada objects (other than records with default descriminant values) must be constrained, but values and constants need not be. If the user needs to use strings designated by pointers, he can now do it himself: type Pointer is access String; Path: Pointer; ... begin Path := new String'(Command_Line.Arg(1)); ... This way the package need not export a new type, and need not depend on a particular library package to provide a pointer type. The other alternative would be to make the string pointer type a generic formal (and the package a generic package, but that would be overkill in this case. -- Robert I. Eachus with STANDARD_DISCLAIMER; use STANDARD_DISCLAIMER; function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...
westley@corsair.uucp (Terry J. Westley) (03/08/90)
In article <EACHUS.90Mar7172539@aries.aries.mitre.org> eachus@aries.mitre.org (Robert I. Eachus) writes: > > package COMMAND_LINE is > > function NARGS return Integer; > function ARG (Index: Integer) return String; > > end COMMAND_LINE; > This is precisely what we have built during our evaluation of Telesoft and Verdix compilers on Sun 4s. Both of these compilers have Unix command line interface packages in Ada. But, they are different! I love the fact that I can build one spec (as above) with two different bodies which perform the necessary calls to the vendor-specific packages. Besides, neither Verdix or Telesoft came up with something as clean and clear as Mr. Eachus shows us. Terry J. Westley Arvin/Calspan Advanced Technology Center P.O. Box 400, Buffalo, NY 14225 acsu.buffalo.edu!planck!westley@hercules
bbard@atr-15.hac.com (Bryce Bardin) (03/09/90)
Come on, guys! It should be:
package COMMAND_LINE is
function NARGS return Natural;
function ARG (Index : in Positive) return String;
end COMMAND_LINE;