Function Clauses

Imported functions can be easily invoked, in almost every case, without any additional declarations. However, by exerting explicit control over argument handling, the interfaces to some functions may be made cleaner. This control is exerted via function clauses. The primary purpose of these clauses is to specify additional type information for specific parameters or to specify alternative argument passing conventions. For example, if we had two alternate "read-integers" functions with the following declarations:

int ReadInts1(int **VectorPtr);  /* result is a count of integers */
int *ReadInts2(int *Count);      /* result is a vector of  integers */

we might use the following interface definition:

define interface
   #include "readints.h",
      rename: {"int *" => int-vector};
   function "ReadInts1",
      output-argument: 1;
   function "ReadInts2" => Read-Integers-Vector,
   output-argument: Count;
end interface;

This would produce two functions, both of which take 0 arguments but return two values. The first would return an <integer> following by an "<int-vector>", while the second would return the <int-vector> first and the <integer> second.

let (count :: <integer>, values :: <int-vector>)
   = Read-Ints1();
let (values :: <int-vector>, count :: <integer>) 
   = Read-Integers-Vector();

The function clause consists of a function name (which is a string), an optional renaming (as illustrated above), and an optional sequence of "options". The options include the following:

seal:

specifies whether the resulting method should be sealed. Possible values are sealed or open, and the default is taken from the value specified in the initial file clause. (The "default default" is sealed.)

equate-result:

overrides the default interpretation of the result type. The named type is assumed to be fully defined.

map-result:

specifies that "import-value" should be called to map the result value to the named type.

ignore-result:

specifies that the functions result value should be ignored, just as if the function had been declared "void". Although you may specify any boolean literal, the only meaningful value is #t.

equate-argument:

overrides the default interpretation of some argument's type. The argument may be specified by name or by position.

map-argument:

specifies that "export-value" should be called to map the given argument into the named type. Again, the argument may be specified by position or by name.

input-argument:

indicates that the specified argument should be passed by value. This is the default.

output-argument:

indicates that the specified argument should be be treated as a return value rather than a "parameter". The effect is to declare that the C parameter will be passed by reference and that the reference variable need not be initialized to any object. This option assumes that the C parameter will have been declared as a "pointer" type, and will strip one "*" off of the argument type. Thus, if the parameter declaration specifies "int **", the actual value returned will have the Dylan type corresponding to "int *".

input-output-argument:

indicates that the specified argument should be considered both an input argument and that its (potentially modified) value should be returned as an additional result value. The effect is similar to that of "output-argument" except that the reference variable will be initialized with the argument value.

The following (nonsensical) example demonstrates all of the options, as they might be applied to the functions:

extern struct object *bar(int first, int *second,struct object **third);
extern baz(char first, struct object *second);

define interface
   #include "demo.h";
   function "bar",
      seal: open,
      equate-result: <object>,
      map-result: <bar-object>,
      input-argument: first,   // passed normally
      output-argument: 2,      // nothing passed in, second result value
            // will be <integer>
      input-output-argument: third;   // passed in as second argument, 
            // returned as third result
   function "baz" => arbitrary-function-name,
      seal: sealed,      // default
      ignore-result: #t,
      equate-argument: {second => <object>},
      map-argument: {2 => <baz-object>};
end interface;