7.3 Default Arguments

Sometimes a procedure (or function) will quite often be called with a particular (constant) value for one of its parameters, and it might be nice if you didn't have to fill this value in all the time. Luckily, E allows you to define default values for a procedure's parameters when you define the procedure. You can then just leave out that parameter when you call the procedure and it will default to the value you defined for it. Here's a simple example:

PROC play(track=1)
  WriteF('Starting to play track \d\n', track)
  /* Rest of the code... */
ENDPROC

PROC main()
  play(1)  -> Start playing from track 1
  play(6)  -> Start playing from track 6
  play()   -> Start playing from track 1
ENDPROC

This is an outline of a program to control something like a CD player. The play procedure has one parameter, track, which represents the first track that should be played. Often, though, you just tell the CD player to play, and don't specify a particular track. In this case, play starts from the first track. This is exactly what happens in the example above: the track parameter has a default value of 1 defined for it (the =1 in the definition of the play procedure), and the third call to play in main does not specify a value for track, so the default value is used.

There are two constraints on the use of default arguments:

1. Any number of the parameters of a procedure may have default values defined for them, although they may only be the right-most parameters. This means that for a three parameter procedure, the second parameter can have a default value only if the last parameter does as well, and the first can have one only if both the others do. This should not be a big problem because you can always reorder the parameters in the procedure definition (and in all the places it has been called!). The following examples show legal definitions of procedures with default arguments:

   PROC fred(x, y, z) IS x+y+z         -> No defaults
   
   PROC fred(x, y, z=1) IS x+y+z       -> z defaults to 1
   
   PROC fred(x, y=23, z=1) IS x+y+z    -> y and z have defaults
   
   PROC fred(x=9, y=23, z=1) IS x+y+z  -> All have defaults
   

   On the other hand, these definitions are all illegal:

   PROC fred(x, y=23, z) IS x+y+z   -> Illegal: no z default
   
   PROC fred(x=9, y, z=1) IS x+y+z  -> Illegal: no y default
   

2. When you call a procedure which has default arguments you can only leave out the right-most parameters. This means that for a three parameter procedure with all three parameters having default values, you can leave out the second parameter in a call to this procedure only if you also leave out the third parameter. The first parameter may be left out only if both the others are, too. The following example shows which parameters are considered defaults:

   PROC fred(x, y=23, z=1)
     WriteF('x is \d, y is \d, z is \d\n', x, y, z)
   ENDPROC
   
   PROC main()
     fred(2, 3, 4) -> No defaults used
     fred(2, 3)    -> z defaults to 1
     fred(2)       -> y and z default
     fred()        -> Illegal: x has no default
   ENDPROC
   

   In this example, you cannot leave out the y parameter in a call to fred without leaving out the z parameter as well. To make y have its default value and z some value other than its default you need to supply the y value explicitly in the call:

     fred(2, 23, 9) -> Need to supply 23 for y
   

These constraints are necessary in order to make procedure calls unambiguous. Consider a three-parameter procedure with default values for two of the parameters. If it is called with only two parameters then, without these constraints, it would not be clear which two parameters had been supplied and which had not. If, however, the procedure were defined and called according to these constraints, then it must be the third parameter that needs to be defaulted (and the two parameters with default values must be the last two).

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