Evaluate conditionals

src/evaluator.js

       case AST.Definition:
         this.env.set(node.symbol.name, node.value)
         return false
+      case AST.Conditional:
+        let result = this.evalNode(node.condition, env)
+
+        if (result.constructor == AST.Boolean && result.value === true) {
+          return this.evalNode(node.ifCase, env)
+        }
+
+        return this.evalNode(node.elseCase, env)
       case AST.Application:
         switch (node.function.constructor) {
           case AST.Identifier:

test/evaluatorTest.js

   tree = helpers.evaluate('(define plusOne (lambda (x) (+ x 1))) (plusOne 5)')
   t.deepEqual(tree[0], new AST.Number({ value: 6 }))
 })
+
+test('conditionals', t => {
+  t.plan(3)
+
+  let tree = helpers.evaluate('(if #t 1 0)')
+  t.deepEqual(tree[0], new AST.Number({ value: 1 }))
+
+  tree = helpers.evaluate('(if #f 1 0)')
+  t.deepEqual(tree[0], new AST.Number({ value: 0 }))
+
+  tree = helpers.evaluate('(if (= (+ 4 9) (- 20 7)) (+ 10 9) (- 4 2))')
+  t.deepEqual(tree[0], new AST.Number({ value: 19 }))
+})