Better explanation of the unless procesure if it is not defined

[sicp.git] / src / sicp / metacircular2.rkt

diff --git a/src/sicp/metacircular2.rkt b/src/sicp/metacircular2.rkt
index 26f9c12b52d4b02fe898b0259dabdb4c9f8d021c..ef387d444483742257056a6c6e12afd5ca60587f 100644 (file)
--- a/src/sicp/metacircular2.rkt
+++ b/src/sicp/metacircular2.rkt
@@ -180,6 +180,13 @@
                [`((,var ,val) ...) bindings])
     `((lambda ,var ,@body) ,@val)))
 
+;; named let
+(define (named-let->combination lexpr)
+  (match-let* ([`(let ,(? symbol? name) ,(? (lambda (x) (or (pair? x) (empty? x))) bindings) ,body ..1) lexpr]
+               [`((,var ,val) ...) bindings])
+    `(begin (define ,name (lambda ,var ,@body))
+            (,name ,@val))))
+
 ;; let*
 (define (let*->nested-lets lexpr)
   (match lexpr
@@ -220,10 +227,12 @@
     [`(define ,(? (lambda (x) (not (pair? x))) var) ,b) (define-variable! var (eval b env) env)]
     [`(define ,(? pair? var) ,b ..1) (define-variable! (car var) (eval (make-lambda (cdr var) b) env) env)]
     [`(if ,pred ,consequent ,alternative) (if (true? (eval pred env)) (eval consequent env) (eval alternative env))]
+    [`(unless ,condition ,consequent ,alternative) (if (true? (eval condition env)) (eval alternative env) (eval consequent env))]
     [`(lambda ,parameters ,body ..1) (make-procedure parameters body env)]
     [`(begin ,exp ...) (eval-sequence exp env)]
     [`(cond ,clauses ...) (eval (cond->if clauses) env)]
-    [`(let ,bindings ,body ..1) (eval (let->combination exp) env)]
+    [`(let ,(? (lambda (x) (or (pair? x) (empty? x))) bindings) ,body ..1) (eval (let->combination exp) env)]
+    [`(let ,(? symbol? name) ,bindings ,body ..1) (eval (named-let->combination exp) env)]
     [`(let* ,bindings ,body ..1) (eval (let*->nested-lets exp) env)]
     [`(letrec ,bindings ,body ..1) (eval (letrec->combination exp) env)]
     [(list f x ...) (apply (eval f env) (list-of-values x env))]