Resolves #1 - Added the IF Special Form

src/function/builtin/special/IF.java

@@ -0,0 +1,52 @@
+package function.builtin.special;
+
+import static function.builtin.EVAL.eval;
+
+import function.*;
+import sexpression.*;
+
+public class IF extends LispFunction {
+
+    private ArgumentValidator argumentValidator;
+
+    public IF() {
+        this.argumentValidator = new ArgumentValidator("IF");
+        this.argumentValidator.setMinimumNumberOfArguments(2);
+        this.argumentValidator.setMaximumNumberOfArguments(3);
+    }
+
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
+
+        SExpression test = eval(argumentList.getCar());
+        SExpression thenForm = getThenForm(argumentList);
+        SExpression elseForm = getElseForm(argumentList);
+
+        return isNil(test) ? eval(elseForm) : eval(thenForm);
+    }
+
+    private boolean isNil(SExpression test) {
+        return test.nullp();
+    }
+
+    private SExpression getThenForm(Cons argumentList) {
+        Cons expressions = getRestOfList(argumentList);
+
+        return expressions.getCar();
+    }
+
+    private Cons getRestOfList(Cons argumentList) {
+        return (Cons) argumentList.getCdr();
+    }
+
+    private SExpression getElseForm(Cons argumentList) {
+        Cons expressions = getRestOfList(argumentList);
+
+        return getRestOfList(expressions).getCar();
+    }
+
+    public boolean evaluateArguments() {
+        return false;
+    }
+
+}

src/table/FunctionTable.java

@@ -60,6 +60,7 @@ public class FunctionTable {
         functionTable.put("FIRST", new CAR());
         functionTable.put("FUNCALL", new FUNCALL());
         functionTable.put("GREATERP", new GREATERP());
+        functionTable.put("IF", new IF());
         functionTable.put("LAMBDA", new LAMBDA());
         functionTable.put("LENGTH", new LENGTH());
         functionTable.put("LET", new LET());

test/function/builtin/special/CONDTester.java

@@ -22,6 +22,13 @@ public class CONDTester {
         assertSExpressionsMatch(parseString("T"), evaluateString(input));
     }
 
+    @Test
+    public void testCondWithNumber() {
+        String input = "(cond ((+ 1 2)))";
+
+        assertSExpressionsMatch(parseString("3"), evaluateString(input));
+    }
+
     @Test
     public void testCondWithSingleExpression() {
         String input = "(cond (T \"true\"))";

test/function/builtin/special/IFTester.java

@@ -0,0 +1,91 @@
+package function.builtin.special;
+
+import static testutil.TestUtilities.evaluateString;
+import static testutil.TypeAssertions.*;
+
+import org.junit.*;
+
+import function.ArgumentValidator.*;
+import function.builtin.EVAL.UndefinedSymbolException;
+import table.ExecutionContext;
+
+public class IFTester {
+
+    private ExecutionContext executionContext;
+
+    public IFTester() {
+        this.executionContext = ExecutionContext.getInstance();
+    }
+
+    @Before
+    public void setUp() {
+        executionContext.clearContext();
+    }
+
+    @After
+    public void tearDown() {
+        executionContext.clearContext();
+    }
+
+    @Test
+    public void ifWithOneExpression_ReturnsExpression() {
+        String input = "(if t t)";
+
+        assertT(evaluateString(input));
+    }
+
+    @Test
+    public void ifWithOneExpression_ReturnsNil() {
+        String input = "(if nil t)";
+
+        assertNil(evaluateString(input));
+    }
+
+    @Test
+    public void ifWithTwoExpressions_ReturnsFirst() {
+        String input = "(if t t nil)";
+
+        assertT(evaluateString(input));
+    }
+
+    @Test
+    public void ifWithTwoExpressions_ReturnsSecond() {
+        String input = "(if nil nil t)";
+
+        assertT(evaluateString(input));
+    }
+
+    @Test
+    public void ifWithNumericConditional() {
+        String input = "(if 23 t nil)";
+
+        assertT(evaluateString(input));
+    }
+
+    @Test(expected = UndefinedSymbolException.class)
+    public void ifWithNilCondition_DoesNotEvaluateThenForm() {
+        String input = "(if nil (setf x 22))";
+
+        assertNil(evaluateString(input));
+        evaluateString("x");
+    }
+
+    @Test(expected = UndefinedSymbolException.class)
+    public void ifWithTrueCondition_DoesNotEvaluateElseForm() {
+        String input = "(if t nil (setf x 22))";
+
+        assertNil(evaluateString(input));
+        evaluateString("x");
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void ifWithTooFewArguments() {
+        evaluateString("(if t)");
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void ifWithTooManyArguments() {
+        evaluateString("(if t t t t)");
+    }
+
+}