Refactored the symbol table code

src/function/UserDefinedFunction.java

@@ -3,15 +3,15 @@ package function;
 import java.util.ArrayList;
 
 import function.builtin.EVAL;
-import function.builtin.special.SETF;
 import sexpression.*;
-import table.SymbolTable;
+import table.*;
 
 public class UserDefinedFunction extends LispFunction {
 
     private String name;
     private Cons body;
     private Cons lambdaExpression;
+    private ExecutionContext executionContext;
     private SymbolTable scope;
     private ArrayList<String> formalParameters;
     private ArgumentValidator argumentValidator;
@@ -20,7 +20,8 @@ public class UserDefinedFunction extends LispFunction {
         this.name = name;
         this.body = body;
         this.lambdaExpression = new Cons(new Symbol(name), new Cons(lambdaList, body));
-        this.scope = SETF.getSymbolTable();
+        this.executionContext = ExecutionContext.getInstance();
+        this.scope = executionContext.getScope();
 
         for (this.formalParameters = new ArrayList<>(); lambdaList.consp(); lambdaList = (Cons) lambdaList.getCdr())
             this.formalParameters.add(lambdaList.getCar().toString());
@@ -37,11 +38,11 @@ public class UserDefinedFunction extends LispFunction {
 
         // store the environment of the S-expression that called this function
         // (the current environment)
-        SymbolTable callingScope = SETF.getSymbolTable();
+        SymbolTable callingScope = executionContext.getScope();
 
         // replace the current environment with the environment of this
         // function
-        SETF.setSymbolTable(scope);
+        executionContext.setScope(scope);
 
         Cons currentSExpression = body;
         SExpression lastValue = null;
@@ -54,7 +55,7 @@ public class UserDefinedFunction extends LispFunction {
 
         // replace the environment of the S-expression that called this
         // function
-        SETF.setSymbolTable(callingScope);
+        executionContext.setScope(callingScope);
 
         // remove the bindings of the arguments to the formal parameter names
         // in the environment of this function

src/function/builtin/EVAL.java

@@ -5,9 +5,9 @@ import java.text.MessageFormat;
 import error.LispException;
 import function.*;
 import function.builtin.cons.LIST;
-import function.builtin.special.*;
+import function.builtin.special.LAMBDA;
 import sexpression.*;
-import table.FunctionTable;
+import table.*;
 
 public class EVAL extends LispFunction {
 
@@ -37,7 +37,7 @@ public class EVAL extends LispFunction {
         else if (symbolName.startsWith(":"))
             return new Symbol(symbolName);
 
-        return SETF.lookupSymbolValue(symbolName);
+        return ExecutionContext.getInstance().lookupSymbolValue(symbolName);
     }
 
     public static SExpression eval(SExpression sExpression) {

src/function/builtin/special/LET.java

@@ -3,9 +3,15 @@ package function.builtin.special;
 import function.*;
 import function.builtin.EVAL;
 import sexpression.*;
-import table.SymbolTable;
+import table.*;
 
 public class LET extends LispFunction {
+    
+    private ExecutionContext executionContext;
+    
+    public LET() {
+        this.executionContext = ExecutionContext.getInstance();
+    }
 
     public SExpression call(Cons argList) {
         // make sure we have received at least one argument
@@ -15,13 +21,13 @@ public class LET extends LispFunction {
 
         // create a new symbol table on top of the current environment to add
         // all the local variables to
-        SymbolTable environment = new SymbolTable(SETF.getSymbolTable());
+        SymbolTable environment = new SymbolTable(executionContext.getScope());
 
         SExpression car = argList.getCar();
         Cons cdr = (Cons) argList.getCdr();
 
         addVariablesToTable(environment, car);
-        SETF.setSymbolTable(environment);
+        executionContext.setScope(environment);
 
         SExpression retval = Nil.getInstance();
 
@@ -32,7 +38,7 @@ public class LET extends LispFunction {
         }
 
         // restore the environment to its original value
-        SETF.setSymbolTable(environment.getParent());
+        executionContext.setScope(environment.getParent());
 
         return retval;
     }

src/function/builtin/special/SETF.java

@@ -3,34 +3,18 @@ package function.builtin.special;
 import function.*;
 import function.builtin.EVAL;
 import sexpression.*;
-import table.SymbolTable;
+import table.*;
 
 public class SETF extends LispFunction {
 
-    private static SymbolTable symbolTable = new SymbolTable();
-
-    public static SExpression lookupSymbolValue(String symbolName) {
-        for (SymbolTable t = symbolTable; t != null; t = t.getParent())
-            if (t.contains(symbolName))
-                return t.get(symbolName);
-
-        return null;
-    }
-
-    public static void setSymbolTable(SymbolTable newSymbolTable) {
-        symbolTable = newSymbolTable;
-    }
-
-    public static SymbolTable getSymbolTable() {
-        return symbolTable;
-    }
-
     private ArgumentValidator argumentValidator;
+    private ExecutionContext executionContext;
 
     public SETF() {
         this.argumentValidator = new ArgumentValidator("SETF");
         this.argumentValidator.setExactNumberOfArguments(2);
         this.argumentValidator.setFirstArgumentExpectedType(Symbol.class);
+        this.executionContext = ExecutionContext.getInstance();
     }
 
     public SExpression call(Cons argumentList) {
@@ -40,14 +24,14 @@ public class SETF extends LispFunction {
         SExpression symbol = argumentList.getCar();
         SExpression value = EVAL.eval(cdr.getCar());
 
-        SymbolTable table = findTableForSymbol(symbol);
+        SymbolTable table = findScopeOfSymbol(symbol);
         table.put(symbol.toString(), value);
 
         return value;
     }
 
-    private SymbolTable findTableForSymbol(SExpression symbol) {
-        SymbolTable table = symbolTable;
+    private SymbolTable findScopeOfSymbol(SExpression symbol) {
+        SymbolTable table = executionContext.getScope();
 
         while (!isSymbolInTable(symbol, table) && !isGlobalTable(table))
             table = table.getParent();

src/table/ExecutionContext.java

@@ -0,0 +1,38 @@
+package table;
+
+import sexpression.SExpression;
+
+public class ExecutionContext {
+
+    private static ExecutionContext uniqueInstance = new ExecutionContext();
+    
+    public static ExecutionContext getInstance() {
+        return uniqueInstance;
+    }
+    
+    private SymbolTable scope;
+    
+    private ExecutionContext() {
+        this.scope = new SymbolTable();
+    }
+
+    public SymbolTable getScope() {
+        return scope;
+    }
+    
+    public void setScope(SymbolTable scope) {
+        this.scope = scope;
+    }
+    
+    public void clearContext() {
+        this.scope = new SymbolTable();
+    }
+
+    public SExpression lookupSymbolValue(String symbolName) {
+        for (SymbolTable t = scope; t != null; t = t.getParent())
+            if (t.contains(symbolName))
+                return t.get(symbolName);
+
+        return null;
+    }
+}

test/function/builtin/special/SETFTester.java

@@ -8,13 +8,19 @@ import org.junit.*;
 import function.ArgumentValidator.*;
 import function.builtin.EVAL.UndefinedSymbolException;
 import sexpression.LispNumber;
-import table.SymbolTable;
+import table.*;
 
 public class SETFTester {
+    
+    private ExecutionContext executionContext;
+    
+    public SETFTester() {
+        this.executionContext = ExecutionContext.getInstance();
+    }
 
     @Before
     public void setUp() {
-        SETF.setSymbolTable(new SymbolTable());
+        executionContext.clearContext();
     }
 
     @Test
@@ -26,45 +32,45 @@ public class SETFTester {
     @Test
     public void lookupDefinedSymbol() {
         evaluateString("(setf a 23)");
-        assertSExpressionsMatch(new LispNumber("23"), SETF.lookupSymbolValue("A"));
+        assertSExpressionsMatch(new LispNumber("23"), executionContext.lookupSymbolValue("A"));
     }
 
     @Test
     public void lookupUndefinedSymbol() {
-        assertNull(SETF.lookupSymbolValue("A"));
+        assertNull(executionContext.lookupSymbolValue("A"));
     }
 
     @Test
     public void setfGlobalVariable() {
         evaluateString("(setf a 23)");
-        SymbolTable global = SETF.getSymbolTable();
-        SETF.setSymbolTable(new SymbolTable(global));
+        SymbolTable global = executionContext.getScope();
+        executionContext.setScope(new SymbolTable(global));
 
         evaluateString("(setf a 94)");
-        SETF.setSymbolTable(global);
+        executionContext.setScope(global);
         assertSExpressionsMatch(new LispNumber("94"), evaluateString("a"));
     }
 
     @Test(expected = UndefinedSymbolException.class)
     public void setfLocalVariableDefined_DoesNotSetGlobal() {
-        SymbolTable global = SETF.getSymbolTable();
+        SymbolTable global = executionContext.getScope();
         SymbolTable local = new SymbolTable(global);
         local.put("A", new LispNumber("99"));
-        SETF.setSymbolTable(local);
+        executionContext.setScope(local);
 
         evaluateString("(setf a 94)");
-        SETF.setSymbolTable(global);
+        executionContext.setScope(global);
         evaluateString("a");
     }
 
     @Test
     public void setfLocalVariableUndefined_SetsGlobal() {
-        SymbolTable global = SETF.getSymbolTable();
+        SymbolTable global = executionContext.getScope();
         SymbolTable local = new SymbolTable(global);
-        SETF.setSymbolTable(local);
+        executionContext.setScope(local);
 
         evaluateString("(setf a 94)");
-        SETF.setSymbolTable(global);
+        executionContext.setScope(global);
         assertSExpressionsMatch(new LispNumber("94"), evaluateString("a"));
     }