# Trait

A trait/typeclass system for Chicken Scheme.

## Example

### Eq

    (import trait)
    
    (define-record point x y)
    
    (define-trait Eq
      (==)
      (/= (lambda (a b) (not (== a b)))))
    
    (define-trait-impl (Eq number?)
      (== =))
    
    (define-trait-impl (Eq symbol?)
      (== eq?))
    
    (define-trait-impl (Eq list?)
      (== equal?))
    
    (define-trait-impl (Eq point?)
      (== (lambda (a b) (and (point? b)
                             (= (point-x a) (point-x b))
                             (= (point-y a) (point-y b))))))
    
    (display (list (== 'a 'a)
                   (/= 'a 'a)
    
                   (== 1 1)
                   (/= 1 1)
    
                   (== 1 2)
                   (/= 1 2)
    
                   (== (list 1 2) (list 1 2))
                   (/= (list 1 2) (list 1 2))
    
                   (== (make-point 3 4) (make-point 3 4))
                   (/= (make-point 3 4) (make-point 3 4))))

### Monad

    (import trait)
    
    (define-trait Monad
      (>>=)
      (return))
    
    (define-record nullable is-null value)
    (define (make-some value)
      (make-nullable #f value))
    (define (make-null)
      (make-nullable #t '()))
    (define (nullable-type x)
      (if (nullable-is-null x)
          'null
          'some))
    
    (define-trait-impl (Monad nullable?)
      (>>= (lambda (m f)
             (let ((type (nullable-type m)))
               (cond ((eq? type 'null)
                      (make-null))
                     ((eq? type 'some)
                      (f (nullable-value m)))))))
      (return make-some))
    
    (define (*2 a)
      (make-some (* 2 a)))
    
    (display (nullable-value (>>= (make-some 3) *2)))
    (newline)
    
    (display ((with-type nullable? return) 42))
    (newline)
    
    (define x (make-some 42))
    (display ((with-type-of x return) 81))
    (newline)