From 964c4bc0df95ea59bd79d9739ddb0fdb83ebeafc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stanis=C5=82aw=20Barzowski?= <sbarzowski@google.com>
Date: Fri, 28 Jul 2017 17:23:27 -0400
Subject: [PATCH 1/2] Allow adding strings

---
 interpreter.go | 25 +++++++++++++++++++++----
 main_test.go   |  1 +
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/interpreter.go b/interpreter.go
index 8cb948c..5cbbe83 100644
--- a/interpreter.go
+++ b/interpreter.go
@@ -360,6 +360,21 @@ func (i *interpreter) objectIndex(loc *LocationRange, obj value, f string, offse
 	}
 }
 
+func (i *interpreter) evalAdd(leftVal, rightVal value) (value, error) {
+	switch leftVal := leftVal.(type) {
+	case *valueNumber:
+		left := leftVal.value
+		right := rightVal.(*valueNumber).value
+		return makeValueNumber(left + right), nil
+	case *valueString:
+		left := leftVal.value
+		right := rightVal.(*valueString).value
+		return makeValueString(left + right), nil
+	}
+	// TODO(sbarzowski) More types and more graceful error handling
+	panic("unknown type")
+}
+
 func (i *interpreter) evaluate(a astNode) (value, error) {
 	// TODO(dcunnin): All the other cases...
 	switch ast := a.(type) {
@@ -374,19 +389,21 @@ func (i *interpreter) evaluate(a astNode) (value, error) {
 		return &valueArray{elements}, nil
 
 	case *astBinary:
-		// TODO(dcunnin): Assume it's + on numbers for now
+		// TODO(dcunnin): Assume it's + for now
 		leftVal, err := i.evaluate(ast.left)
 		if err != nil {
 			return nil, err
 		}
 		// TODO(dcunnin): Check the type properly.  The following code just panics.
-		leftNum := leftVal.(*valueNumber).value
 		rightVal, err := i.evaluate(ast.right)
 		if err != nil {
 			return nil, err
 		}
-		rightNum := rightVal.(*valueNumber).value
-		return makeValueNumber(leftNum + rightNum), nil
+		result, err := i.evalAdd(leftVal, rightVal)
+		if err != nil {
+			return nil, err
+		}
+		return result, nil
 
 	case *astDesugaredObject:
 		// Evaluate all the field names.  Check for null, dups, etc.
diff --git a/main_test.go b/main_test.go
index 074e9a2..a13f9d1 100644
--- a/main_test.go
+++ b/main_test.go
@@ -36,6 +36,7 @@ var mainTests = []mainTest{
 	{"simple_arith1", "3 + 3", "6", ""},
 	{"simple_arith2", "3 + 3 + 3", "9", ""},
 	{"simple_arith3", "(3 + 3) + (3 + 3)", "12", ""},
+	{"simple_arith_string", "\"aaa\" + \"bbb\"", "\"aaabbb\"", ""},
 	{"empty_array", "[]", "[ ]", ""},
 	{"array", "[1, 2, 1 + 2]", "[\n   1,\n   2,\n   3\n]", ""},
 	{"empty_object", "{}", "{ }", ""},

From badeb833bbe0a824ab294a9e51961bdc4f14437c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stanis=C5=82aw=20Barzowski?= <sbarzowski@google.com>
Date: Mon, 31 Jul 2017 11:36:52 -0400
Subject: [PATCH 2/2] Tests + default + better panic message

---
 interpreter.go | 5 +++--
 main_test.go   | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/interpreter.go b/interpreter.go
index 5cbbe83..6f39f5e 100644
--- a/interpreter.go
+++ b/interpreter.go
@@ -361,6 +361,7 @@ func (i *interpreter) objectIndex(loc *LocationRange, obj value, f string, offse
 }
 
 func (i *interpreter) evalAdd(leftVal, rightVal value) (value, error) {
+	// TODO(sbarzowski) More types and more graceful error handling
 	switch leftVal := leftVal.(type) {
 	case *valueNumber:
 		left := leftVal.value
@@ -370,9 +371,9 @@ func (i *interpreter) evalAdd(leftVal, rightVal value) (value, error) {
 		left := leftVal.value
 		right := rightVal.(*valueString).value
 		return makeValueString(left + right), nil
+	default:
+		panic(fmt.Sprintf("INTERNAL ERROR: Unrecognised value type: %T", leftVal))
 	}
-	// TODO(sbarzowski) More types and more graceful error handling
-	panic("unknown type")
 }
 
 func (i *interpreter) evaluate(a astNode) (value, error) {
diff --git a/main_test.go b/main_test.go
index a13f9d1..8b77ade 100644
--- a/main_test.go
+++ b/main_test.go
@@ -37,6 +37,9 @@ var mainTests = []mainTest{
 	{"simple_arith2", "3 + 3 + 3", "9", ""},
 	{"simple_arith3", "(3 + 3) + (3 + 3)", "12", ""},
 	{"simple_arith_string", "\"aaa\" + \"bbb\"", "\"aaabbb\"", ""},
+	{"simple_arith_string2", "\"aaa\" + \"\"", "\"aaa\"", ""},
+	{"simple_arith_string3", "\"\" + \"bbb\"", "\"bbb\"", ""},
+	{"simple_arith_string_empty", "\"\" + \"\"", "\"\"", ""},
 	{"empty_array", "[]", "[ ]", ""},
 	{"array", "[1, 2, 1 + 2]", "[\n   1,\n   2,\n   3\n]", ""},
 	{"empty_object", "{}", "{ }", ""},