Implement integer? predicate

src/main/java/me/predatorray/bud/lisp/buitin/BuiltinsEnvironment.java

@@ -1,6 +1,7 @@
 package me.predatorray.bud.lisp.buitin;
 
 import me.predatorray.bud.lisp.buitin.arithmetic.AddFunction;
+import me.predatorray.bud.lisp.buitin.arithmetic.IntegerPredicate;
 import me.predatorray.bud.lisp.buitin.arithmetic.MonoDecreasingPredicate;
 import me.predatorray.bud.lisp.buitin.arithmetic.MonoIncreasingPredicate;
 import me.predatorray.bud.lisp.buitin.arithmetic.MonoNonDecreasingPredicate;
@@ -41,6 +42,7 @@ public class BuiltinsEnvironment {
         initial.put(">", new MonoDecreasingPredicate());
         initial.put(">=", new MonoNonIncreasingPredicate());
         initial.put("<=", new MonoNonDecreasingPredicate());
+        initial.put("integer?", new IntegerPredicate());
 
         initial.put("null-environment", new NullEnvironmentFunction());
         initial.put("builtins-environment", new BuiltinsEnvironmentFunction());

src/main/java/me/predatorray/bud/lisp/buitin/arithmetic/IntegerPredicate.java

@@ -0,0 +1,39 @@
+package me.predatorray.bud.lisp.buitin.arithmetic;
+
+import me.predatorray.bud.lisp.buitin.Predicate;
+import me.predatorray.bud.lisp.evaluator.EvaluatingException;
+import me.predatorray.bud.lisp.lang.BudNumber;
+import me.predatorray.bud.lisp.lang.BudObject;
+import me.predatorray.bud.lisp.lang.BudType;
+import me.predatorray.bud.lisp.lang.FunctionType;
+
+import java.math.BigDecimal;
+import java.util.List;
+
+public class IntegerPredicate extends Predicate {
+
+    private final FunctionType thisType = new FunctionType(this);
+
+    @Override
+    protected void checkArgumentTypes(List<BudType> argumentTypes) {
+        if (argumentTypes.size() != 1) {
+            throw new EvaluatingException("requires one argument");
+        }
+    }
+
+    @Override
+    protected boolean predicate(List<BudObject> arguments) {
+        BudObject arg = arguments.get(0);
+        if (!BudType.NUMBER.equals(arg.getType())) {
+            return false;
+        }
+        BudNumber number = (BudNumber) arg;
+        BigDecimal decimal = number.getValue();
+        return decimal.scale() <= 0;
+    }
+
+    @Override
+    public FunctionType getType() {
+        return thisType;
+    }
+}