Macros in Scheme

Andy Balaam

Contents

• The problem
• Hiding complexity
• First attempt
• A solution
• How else could we do this?
• Discussion

The problem

(define num 3)

(cond
    ((zero? num)     (display "Z"))
    ((positive? num) (display "P"))
    (else            (display "N")))

Hiding complexity

• How can we avoid repeating ourselves?
• How can we speak at the right level?

Hiding complexity (2)

• Functions
• Classes
• ???

Hiding complexity (3)

• Functions
• Classes
• Extending the language
• Code generation
• Macros

First attempt

(define (3-state
        value
        positive-body
        zero-body
        negative-body)
    (cond
        ((zero? value)     zero-body)
        ((positive? value) positive-body)
        (else              negative-body)))

> (3-state
    100 (display "P") (display "Z") (display "N"))
PZN

A solution

(define-syntax
    3-state
    (syntax-rules ()
        ((3-state
            value positive-body zero-body negative-body)
        (cond
            ((zero? value)     zero-body)
            ((positive? value) positive-body)
            (else              negative-body)))))

A solution (2)

> (3-state
    100 (display "P") (display "Z") (display "N"))
P

> (3-state
    0 (display "P") (display "Z") (display "N"))
Z

> (3-state
    -100 (display "P") (display "Z") (display "N"))
N

How else could we do this?

/* MACROS */

#define THREESTATE(VALUE,PBODY,ZBODY,NBODY) \
    { \
        int v = (VALUE); \
        if( v == 0 )     { (ZBODY); } \
        else if( v > 0 ) { (PBODY); } \
        else             { (NBODY); } \
    }

int main()
{
    THREESTATE( 3, printf("P\n"), printf("Z\n"), printf("N\n") )
    return 0;
}

How else could we do this?

// Quoting

function threeState( value, pbody, zbody, nbody )
{
    if( value === 0 )     { eval( zbody ); }
    else if ( value > 0 ) { eval( pbody ); }
    else                  { eval( nbody ); }
}

threeState(
    3, "print('P')", "print('Z')", "print('N')" );

How else could we do this?

# Function pointers

def three_state( 
        value, p_body_fn, z_body_fn, n_body_fn ):
    if value == 0:  z_body_fn()
    elif value > 0: p_body_fn()
    else:           n_body_fn()

def pr( x ):
    def ret():
        print x
    return ret

three_state(  3, pr("P"), pr("N"), pr("Z") )

How else could we do this?

class TestJava {

    interface IThreeStateBodies {
        void positiveBody();
        void zeroBody();
        void negativeBody();
    }

    static void threeState(
            int value, IThreeStateBodies bodies ) {
        if ( value == 0 )     bodies.zeroBody();
        else if ( value > 0 ) bodies.positiveBody();
        else                  bodies.negativeBody();
    }
    ...

How else could we do this?

...
    public static void main( String[] args ) {
        threeState( 3,
            new IThreeStateBodies() {
                public void positiveBody() {
                    System.out.println( "P" );
                }
                public void zeroBody() {
                    System.out.println( "Z" );
                }
                public void negativeBody() {
                    System.out.println( "N" );
                }
            }
        );
    }

Discussion