From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Mon, 2 Jan 2012 15:51:03 +0000 (+0530)
Subject: named let
X-Git-Url: https://git.rkrishnan.org/vdrive/%22news.html/about.html?a=commitdiff_plain;h=3787066c193ec0ad77c9490448c0f73f067d59e2;p=sicp.git

named let
---

diff --git a/src/sicp/metacircular2.rkt b/src/sicp/metacircular2.rkt
index 26f9c12..c45a5e7 100644
--- 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
@@ -223,7 +230,8 @@
     [`(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))]