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Sather - A Language Manual - Kapitel 9
Closures


Routine and iter closures are similar to the 'function pointer' and 'closure' constructs of other languages. They bind a reference to a method together with zero or more argument values (possibly including self). The type of a closure begins with the keywords ROUT or ITER and followed by the modes and types of the underscore arguments, if any, enclosed in braces (e.g. 'ROUT{A, out B, inout C}', 'ITER{once A, out B, C}'). These are followed by a colon and the return type, if there is one (e.g. 'ROUT{INT}:INT', 'ITER{once INT}:FLT').


9.1 Creating and Calling Closures


9.1.1 Creating a closure

A closure is created by an expression that binds a routine or an iterator, along with some of its arguments. The outer part of the expression is 'bind(...)'. This surrounds a routine or iterator call in which any of the arguments or self may have been replaced by the underscore character '_'. Such unspecified arguments are unbound. Unbound arguments are specified when the closure is eventually called.

a:ROUT{INT}:INT := bind(3.plus(_))
b:ITER:INT := bind(3.times!);

Out and inout arguments must be specified in the closure type. If the routine has inout or out arguments

swap(inout x, inout y:INT) is
   tmp ::= x;
   x := y;
   y := tmp;
end;

as show below, they are mentioned in the type of the closure:

The routine 'swap' swaps the values of the two arguments, 'x' and 'y'. 'r' is a closure for binding the 'swap' routine.

r:ROUT{inout INT, inout INT} := bind(swap(_,_));


9.1.2 Calling a closure

Each routine closure defines a routine named 'call' and each iterator closure defines an iterator named 'call!'. These have argument and return types that correspond to the closure type specifiers. Invocations of these features behave like a call on the original routine or iterator with the arguments specified by a combination of the bound values and those provided to call or call!. The arguments to call and call! match the underscores positionally from left to right .

The previously defined closures are invoked as shown

#OUT + a.call(4);  -- Prints out 7, where a is bind(3.plus(_)
sum:INT := 0;
loop
   sum := sum + b.call!;
end;
#OUT + sum;        -- Prints out 3 (0+1+2)

In the following example, we define a bound routine that takes an INT as an argument and returns an INT.

br:ROUT{INT}:INT := bind(1.plus(_));
#OUT + br.call(9);      -- Prints out '10'

The variable br is typed as a bound routine which takes an integer as argument and returns an integer. The routine 1.plus, which is of the appropriate type, is then assigned to br. The routine associated with br may then be invoked by the built in function call. Just as we would when calling the routine INT::plus(INT), we must supply the integer argument to the bound routine.


9.1.3 Binding overloaded routines

When binding a routine which is overloaded, there might be some ambiguity about which routine is meant to be bound

class FLT is
   plus(f:FLT):FLT  -- add self and 'i' and return the result
   plus(i:INT):FLT; -- add self and 'f' (after converting 'i' to FLT)
end;

When binding the plus routine, it might not be obvious which routine is intended

b ::= bind(_.plus(_));

In case of ambiguity, the right method must be determined by the context in which the binding takes place.

Binding in an assignment

If there is ambiguity about which method is to be bound, the type of the variable must be explicitly specified

b:ROUT{FLT,FLT}:FLT := bind(_.plus(_)); -- Selects the first 'plus'

Binding in a call

A method may also be bound at the time a call is made. The type of the closure is determined by the type of the argument in the call.

reduce(a:ARRAY{FLT}, br:ROUT{FLT,FLT}:FLT):FLT is
   res:FLT := 0.0;
   loop
      el:FLT := a.elt!;
      res := br.call(res,el);
   end;
   return res;
end;

We can call the reduction function as follows:

a:ARRAY{FLT} := |1.0,7.0,3.0|;
#OUT + reduce(a,bind(_.plus(_)));
-- Prints '11.0', the sum of the elements of 'a'

The second argument to the function reduce expects a ROUT{FLT,FLT}:FLT and this type was used to select which plus routine should be bound. When there could be doubt about which routine is actually being bound, it is very good practice to specify the type explicitly

r:ROUT{FLT,FLT}:FLT := bind(_.plus(_));
#OUT + reduce(a,r);


9.1.4 Points to note


9.1.5 Binding some arguments

When a routine closure is created, it can preset some of the values of the arguments.

class MAIN is
   foo(a:INT, b:INT):INT is
      return(a+b+10);
   end;

   main is
      br1:ROUT{INT,INT}:INT := bind(foo(_,_));
      br2:ROUT{INT}:INT := bind(foo(10,_));
      #OUT + br1.call(4,3) + ',' + br2.call(9);
            -- Should print 17 and  29
   end;
end;

In the example above, br2 binds the first argument of foo to 10 and the second argument is left unbound. This second argument will have to be supplied by the caller of the bound routine. br1 binds neither argument and hence when it is called, it must supply both arguments.

Here we double every element of an array by applying a routine closure r to each element of an array.

r :ROUT{INT}:INT := bind(2.times(_));
loop
   a.set!(r.call(a.elt!))
end


9.1.6 Leaving self unbound

bound routines are often used to apply a function to arbitrary objects of a particular class. For this usage, we need the self argument to be unbound. This illustrates how self may be left unbound. The type of self must be inferred from the type context (ROUT{INT}).

r:ROUT{INT} := bind(_.plus(3));
#OUT + r.call(5);                        -- prints '8'

In the following example we will make use of the plus routine from the INT class.

... from the INT class
plus(arg:INT):INT is
... definition of plus

main is
   plusbr1:ROUT{INT,INT}:INT := bind(_.plus(_)); -- self and arg unbound
   br1res:INT := plusbr1.call(9,10);             -- Returns 19
   plusbr2:ROUT{INT}:INT := bind(3.plus(_));     -- Binding self only
   br2res:INT := plusbr2.call(15);               -- Returns 18
   plusbr3:ROUT{INT}:INT := bind(_.plus(9));     -- Binding arg only
   br3res:INT := plusbr3.call(11);               -- Returns 20
   #OUT + br1res + ',' + br2res + ',' + br3res;
              -- 19,18,20
end;

In the above example, plusbr1 leaves both self and the argument to plus unbound. Note that we must specify the type of self when creating the bound routine, otherwise the compiler cannot know which class the routine belongs to (the type could also be an abstract type that defines that feature in its interface). plusbr2 binds self to 3, so that the only argument that need be supplied at call time is the argument to the plus. plusbr3 binds the argument of plus to 15, so that the only argument that need be supplied at call time is self for the routine.


9.2 Further Examples of Closures

Just as is the case with C function pointers, there will be programmers who find closures indispensible and others who will hardly ever touch them. Since Sather's closures are strongly typed, much of the insecurity associated with function pointers in C disappears.


9.2.1 Closures for Applicative Programming

Closures are useful when you want to write Lisp-like 'apply" routines in a class which contains other data . Routines that use routine closures in this way may be found in the class ARRAY{T}. Some examples of which are shown below.

every(test:ROUT{T}:BOOL):BOOL is
   -- True if every element of self satisfies 'test'.
   loop
      e ::= elt!;   -- Iterate through the array elements
      if ~test.call(e) then
         return false;
      end
      -- If e fails the test, return false immediately
   end;
   return true
end;

The following routine which takes a routine closure as an argument and uses it to select an element from a list

select(e:ARRAY{INT}, r:ROUT{INT}:BOOL):INT is
   -- Return the index of the first element in the array 'e' that
   -- satisfies the predicate 'r'.
   -- Return -1 if no element of 'e' satisfies the predicate.
   loop i:INT := e.ind!;
      if r.call(e[i]) then
         return i;
      end;
   end;
   return -1;
end;

The selection routine may be used as shown below:

a:ARRAY{INT} := |1,2,3,7|;
br:ROUT{INT}:BOOL := bind(_.is_eq(3));
#OUT + select(a,br);  -- Prints the index of the first element of 'a'
                      -- that is equal to '3'. The index printed is '2'


9.2.2 Menu Structures

Another common use of function pointers is in the construction of an abstraction for a set of choices. The MENU class shown below maintains a mapping between strings and routine closures associated with the strings.

class MENU is
   private attr menu_actions:MAP{STR,ROUT};
        -- Hash table from strings to closures
   private attr default_action:ROUT{STR};

   create(default_act:ROUT{STR}):SAME is
      res:SAME := new;
      res.menu_actions := #MAP{STR,ROUT};
      res.default_action := default_act;
      return(res)
   end;

   add_item(name:STR, func:ROUT) is menu_actions[name] := func end;
     -- Add a menu item to the hash table, indexed by 'name'

   run is
      loop
         #OUT + '>';
         command: STR := IN::get_str;  -- Gets the next line of  input
         if command = 'done' then
            break!;
         elsif menu_actions.has_ind(command) then
            menu_actions[command].call;
         else
            default_action.call(command);
         end;
      end;
   end;
end;

We use this opportunity to create a textual interface for the calculator described earlier (See A Stack Calculator, subsection 5.1.2):

class CALCULATOR is
   private attr stack:A_STACK{INT};
   private attr menu:MENU;

   create:SAME is
      res ::= new;
      res.init;
      return res;
   end;

   private init is   -- Initialize the calculator attributes
      stack := #;
      menu := #MENU(bind(push(_)));
      menu.add_menu_item('add',bind(add));
      menu.add_menu_item('times',bind(times));
   end;

   run is menu.run; end;

...

--- Now, the main routines of the calculator computation are:

   push(s:STR) is
      -- Convert the value 's' into an INT and push it onto the stack
      -- Do nothing if the string is not a valid integer
      c: STR_CURSOR := s.cursor;
      i: INT := c.int;
      if c.has_error then
         #ERR + 'Bad integer value:' + s;
      else
         stack.push(i);
      end;
   end;

   add is  -- Add the two top stack values and push/print the result
      sum:INT := stack.pop + stack.pop;
      #OUT + sum+'\n';
      stack.push(sum);
   end;

   times is  -- Multiply the top stack values and push/print the result
      product:INT := stack.pop * stack.pop;
      #OUT + product + '\n';
      stack.push(product);
   end;
end;  -- class CALCULATOR

This calculator can be started by a simple main routine

class MAIN is
   main is
      c:CALCULATOR := #;
      c.run;
   end;
end;

:

After compiling the program, we can then run the resulting executable

prompt> a.out
>3
>4
>add
7
>10
>11
>times
110
>done
prompt>


9.2.3 Iterator closures


[zurück] [Zusammenfassung] [Copyright] [Inhaltsverzeichnis] [nächstes]
Sather - A Language Manual
12 Oktober 1999
B. Gomes, D. Stoutamire, B. Vaysman and H. Klawitter
Norbert Nemec nobbi@gnu.org