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@@ -14,6 +14,7 @@ I prefer the name *typeclass*, but there has been an egg named typeclass already
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(define-trait Eq
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(==)
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+ ;; with a default implementation
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(/= (lambda (a b) (not (== a b)))))
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(define-trait-impl (Eq number?)
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@@ -78,9 +79,81 @@ I prefer the name *typeclass*, but there has been an egg named typeclass already
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(display (nullable-value (>>= (make-some 3) *2)))
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(newline)
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- (display ((with-type nullable? return) 42))
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- (newline)
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-
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+ ;; For a function in trait, the implementation is selected by
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+ ;; applying the predicates to the first argument.
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+ ;; When the varaible of related type is the return value or
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+ ;; the 2nd/3rd/... argument, use (with-type-of var trait function)
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+ ;; to get the right function.
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(define x (make-some 42))
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(let ((return (with-type-of x Monad return)))
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(display (nullable-value (return 99))))
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+
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+### With Multiple Modules
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+
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+ (module define-macro (define-macro)
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+ ;; import
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+ (import scheme
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+ (chicken syntax))
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+ ;; body
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+ (define-syntax define-macro
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+ (er-macro-transformer
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+ (lambda (exp r c)
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+ (let ((def (cadr exp))
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+ (body (cddr exp)))
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+ `(define-syntax ,(car def)
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+ (er-macro-transformer
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+ (lambda (e2 r2 c2)
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+ (define (transform-func ,@(cdr def))
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+ ,@body)
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+ (apply transform-func (cdr e2))))))))))
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+
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+ (module Eq (Eq == /=)
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+ (import scheme
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+ (chicken base)
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+ trait)
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+
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+ (define-trait Eq
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+ (==)
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+ (/= (lambda (a b)
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+ (display ==) (newline)
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+ (not (== a b))))))
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+
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+ (module point (point?
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+ make-point
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+ point==?
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+ impl-Eq-point)
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+ (import scheme
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+ (chicken base)
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+ Eq
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+ trait
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+ define-macro)
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+
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+ (define-record point x y)
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+
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+ (define (point==? a b)
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+ (and (point? a)
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+ (point? b)
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+ (= (point-x a) (point-x b))
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+ (= (point-y a) (point-y b))))
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+
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+ ;; trait implementations should be defined as macros
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+ ;; to export to other modules
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+ (define-macro (impl-Eq-point)
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+ '(define-trait-impl (Eq point?)
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+ (== point==?))))
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+
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+ (module mymodule ()
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+ (import scheme
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+ (chicken base)
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+ trait
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+ Eq
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+ point)
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+
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+ ;; import trait implementation
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+ (impl-Eq-point)
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+
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+ (display (== (make-point 1 2) (make-point 1 2)))
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+ (newline)
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+ (display (/= (make-point 1 2) (make-point 1 2)))
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+ (newline))
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+
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