update

Author: raymondworkshop

README.md

@@ -16,8 +16,6 @@
     - **All objects can be treated as data**    
 
 
-#### dynamic program language  
-
 #### tips  
 * ipdb debugger  
     >  import ipdb; ipdb.set_trace()  

sicp/python/link.py

@@ -26,13 +26,51 @@ def rest(s):
     return s[1]
 
 
+# A linked list is a sequence:
+# it has a finite length and supports element selection by index
 # Linked List Class
 class Link:
-    pass
+    """ A linked list with a first element and the rest"""
+    empty = ()
+    def __init__(self, first, rest=empty):
+        # the empty list is a special case of a linked list 
+        # that has no first element or rest
+        assert rest is Link.empty or isinstance(rest, Link)
+        self.first = first
+        self.rest = rest
 
+    def __getitem__(self, i):
+        if i==0:
+            return self.first
+        else:
+            return self.rest[i-1]
+
+    def __len__(self):
+        return 1 + len(self.rest)
+
+    def __repr__(self):
+        if self.rest is Link.empty:
+            rest = ''
+        else:
+            rest = ', ' + repr(self.rest)
+        return 'Link({0}{1})'.format(s.first, rest)
+
+
+def link_expression(s):
+    """return a string that would evaluate to s"""
+    if s.rest is Link.empty:
+        rest = ''
+    else:
+        rest = ', ' + link_expression(s.rest)
+    return 'Link({0}{1})'.format(s.first, rest)
 
 
 if __name__ == "__main__":
     four = link(1, link(2, link (3, link(4, "empty"))))
     print(first(four))
-    print(rest(four))
+    print(rest(four))
+    #
+    s = Link(3, Link(4, Link(5)))
+    print(s[0])
+    #Link.__repr__ = link_expression
+    print(s)