From e59af856b025d82e63cd9f4e0242b88c63da8c36 Mon Sep 17 00:00:00 2001
From: Rahul Arya <rahularya@berkeley.edu>
Date: Mon, 15 Apr 2019 04:28:19 -0700
Subject: [PATCH] Made some recursive procedures iterative. (#156)

---
 editor/datamodel.py | 41 ++++++++++++-----------------------------
 editor/helper.py    |  7 ++++---
 editor/lists.py     | 19 +++++++------------
 3 files changed, 23 insertions(+), 44 deletions(-)

diff --git a/editor/datamodel.py b/editor/datamodel.py
index d5549b10..f9f70541 100644
--- a/editor/datamodel.py
+++ b/editor/datamodel.py
@@ -48,36 +48,19 @@ def __init__(self, first: Expression, rest: Expression):
                 f"Unable to construct a Pair with a cdr of {rest}, expected a Pair, Nil, or Promise.")
         self.rest = rest
 
-    # def __str__(self):
-    #     if isinstance(self.first, Symbol):
-    #         if isinstance(self.rest, Pair) and self.rest.rest == Nil:
-    #             if self.first.value == "quote":
-    #                 return f"'{str(self.rest.first)}"
-    #             elif self.first.value == "unquote":
-    #                 return f",{str(self.rest.first)}"
-    #             elif self.first.value == "unquote-splicing":
-    #                 return f",@{str(self.rest.first)}"
-    #             elif self.first.value == "quasiquote":
-    #                 return f"`{str(self.rest.first)}"
-    #
-    #     if isinstance(self.rest, Pair):
-    #         rest_str = str(self.rest)
-    #         if rest_str[0] == "(" and rest_str[-1] == ")":
-    #             return f"({self.first} {rest_str[1:-1]})"
-    #         else:
-    #             return f"({self.first} . {rest_str})"
-    #     elif self.rest is Nil:
-    #         return f"({self.first})"
-    #
-    #     return f"({str(self.first)} . {str(self.rest)})"
-
     def __repr__(self):
-        if isinstance(self.rest, Pair):
-            return f"({repr(self.first)} {repr(self.rest)[1:-1]})"
-        elif self.rest is Nil:
-            return f"({repr(self.first)})"
-        else:
-            return f"({repr(self.first)} . {repr(self.rest)})"
+        pos = self
+        out = []
+        while True:
+            if isinstance(pos, Pair):
+                out.append(repr(pos.first))
+                pos = pos.rest
+            elif isinstance(pos, NilType):
+                break
+            else:
+                out.append(f". {repr(pos)}")
+                break
+        return "(" + " ".join(out) + ")"
 
 
 class NilType(Expression):
diff --git a/editor/helper.py b/editor/helper.py
index 6c96b639..6b6ad455 100644
--- a/editor/helper.py
+++ b/editor/helper.py
@@ -43,6 +43,7 @@ def verify_min_callable_length(operator: Expression, expected: int, actual: int)
 
 
 def make_list(exprs: List[Expression], last: Expression = Nil) -> Union[Pair, NilType]:
-    if len(exprs) == 0:
-        return last
-    return Pair(exprs[0], make_list(exprs[1:], last))
+    out = last
+    for expr in reversed(exprs):
+        out = Pair(expr, out)
+    return out
diff --git a/editor/lists.py b/editor/lists.py
index d1815e46..d98c06ba 100644
--- a/editor/lists.py
+++ b/editor/lists.py
@@ -14,21 +14,16 @@ class Append(BuiltIn):
     def execute_evaluated(self, operands: List[Expression], frame: Frame) -> Expression:
         if len(operands) == 0:
             return Nil
-        out = []
+        exprs = []
         for operand in operands[:-1]:
             if not isinstance(operand, Pair) and operand is not Nil:
                 raise OperandDeduceError(f"Expected operand to be valid list, not {operand}")
-            out.extend(pair_to_list(operand))
-        try:
-            last = operands[-1]
-            if not isinstance(last, Pair):
-                raise OperandDeduceError()
-            last = pair_to_list(last)
-        except OperandDeduceError:
-            return make_list(out, operands[-1])
-        else:
-            out.extend(last)
-            return make_list(out)
+            exprs.extend(pair_to_list(operand))
+        out = operands[-1]
+        for expr in reversed(exprs):
+            out = Pair(expr, out)
+        return out
+
 
 
 @global_attr("car")