From 107dee15d5422f327ce55625a0a321b98693394e Mon Sep 17 00:00:00 2001
From: Matthias Andreas Benkard <matthias@benkard.de>
Date: Thu, 3 Jul 2008 19:37:31 +0200
Subject: Interpreter: Improve error handling in the face of nonsensical
 expressions.

---
 MLKInterpreter.m | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/MLKInterpreter.m b/MLKInterpreter.m
index e03df1d..247bc8e 100644
--- a/MLKInterpreter.m
+++ b/MLKInterpreter.m
@@ -136,7 +136,7 @@ static MLKSymbol *_LAMBDA;
     {
       id car = [program car];
 
-      if ([car isKindOfClass:[MLKSymbol class]])
+      if ([car isKindOfClass:[MLKSymbol class]] || !car)
         {
           if (car == APPLY)
             {
@@ -145,10 +145,13 @@ static MLKSymbol *_LAMBDA;
                                                withEnvironment:lexenv]
                                           objectAtIndex:0]);
 
-              id <MLKFuncallable> function = denullify([[self eval:[[program cdr] car]
-                                                              inLexicalContext:context
-                                                              withEnvironment:lexenv]
-                                                         objectAtIndex:0]);
+              id function = denullify([[self eval:[[program cdr] car]
+                                             inLexicalContext:context
+                                             withEnvironment:lexenv]
+                                        objectAtIndex:0]);
+
+              if ([function isKindOfClass:[MLKSymbol class]])
+                function = [lexenv functionForSymbol:function];
 
               return [function applyToArray:(rest
                                              ? (id)[rest array]
@@ -530,12 +533,19 @@ static MLKSymbol *_LAMBDA;
                 }
             }
         }
-      else if (![car isKindOfClass:[MLKCons class]] && [car car] == LAMBDA)
+      else if ([car isKindOfClass:[MLKCons class]] && [car car] == LAMBDA)
         {
           return [self eval:[MLKCons cons:FUNCALL with:program]
                        inLexicalContext:context
                        withEnvironment:lexenv];
         }
+      else
+        {
+          [NSException raise:@"MLKInvalidExpressionException"
+                       format:@"%@ is not a valid operator name.",
+                       [car descriptionForLisp]];
+          return nil;  
+        }
     }
 }
 
@@ -554,6 +564,7 @@ static MLKSymbol *_LAMBDA;
                            preserveWhitespace:NO];
 
       //NSLog (@"%@", code);
+      //NSLog (@"%@", [code descriptionForLisp]);
       //NSLog (@"%@", stream);
       //NSLog (@"...");
 
@@ -565,6 +576,7 @@ static MLKSymbol *_LAMBDA;
                  eval:code
                  inLexicalContext:[MLKLexicalContext globalContext]
                  withEnvironment:[MLKLexicalEnvironment globalEnvironment]];
+      //NSLog (@"; LOAD: Top-level form evaluated.");
 
       if (print)
         {
-- 
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