add string comparison support (#294)

* enable relative imports

* update per 1st review

* remove symlink stuffs

* fix cli run in make file

* make resolving import path explicit

* fix importDir

* add string comparison operators

* fix duplicates

docs/operators.md

@@ -86,6 +86,13 @@
 - `(string) + (string) = (string)`: concatenation
 - `(string) + (other types) = (string)`: concatenation (after string-converted)
 
+### Comparison Operators
+
+- `(string) < (string) = (bool)`: less than
+- `(string) > (string) = (bool)`: greater than
+- `(string) <= (string) = (bool)`: less than or equal to
+- `(string) >= (string) = (bool)`: greater than or equal to
+
 ## Char
 
 ### Equality
@@ -184,4 +191,3 @@ Tests whether two _(immutable)_ maps contain the same key-objects.
 - `(immutable-map) != (immutable-map) = (bool)`: inequality
 - `(immutable-map) == (map) = (bool)`: equality
 - `(immutable-map) != (map) = (bool)`: inequality
-

docs/tutorial.md

@@ -291,12 +291,12 @@ _In Tengo, all values can be either
 | `&^`   | bitclear (AND NOT) | int |
 | `<<`   | shift left | int |
 | `>>`   | shift right | int |
-| `<`   | less than | int, float, char, time |
+| `<`   | less than | int, float, char, time, string |
-| `<=`   | less than or equal to | int, float, char, time |
+| `<=`   | less than or equal to | int, float, char, time, string |
-| `>`   | greater than | int, float, char, time |
+| `>`   | greater than | int, float, char, time, string |
-| `>=`   | greater than or equal to | int, float, char, time |
+| `>=`   | greater than or equal to | int, float, char, time, string |
 
-_See [Operators](https://github.com/d5/tengo/blob/d5-patch-1/docs/operators.md)
+_See [Operators](https://github.com/d5/tengo/blob/master/docs/operators.md)
 for more details._
 
 ### Ternary Operators

objects.go

@@ -1342,6 +1342,38 @@ func (o *String) BinaryOp(op token.Token, rhs Object) (Object, error) {
 			}
 			return &String{Value: o.Value + rhsStr}, nil
 		}
+	case token.Less:
+		switch rhs := rhs.(type) {
+		case *String:
+			if o.Value < rhs.Value {
+				return TrueValue, nil
+			}
+			return FalseValue, nil
+		}
+	case token.LessEq:
+		switch rhs := rhs.(type) {
+		case *String:
+			if o.Value <= rhs.Value {
+				return TrueValue, nil
+			}
+			return FalseValue, nil
+		}
+	case token.Greater:
+		switch rhs := rhs.(type) {
+		case *String:
+			if o.Value > rhs.Value {
+				return TrueValue, nil
+			}
+			return FalseValue, nil
+		}
+	case token.GreaterEq:
+		switch rhs := rhs.(type) {
+		case *String:
+			if o.Value >= rhs.Value {
+				return TrueValue, nil
+			}
+			return FalseValue, nil
+		}
 	}
 	return nil, ErrInvalidOperator
 }

vm_test.go

@@ -3273,6 +3273,15 @@ func TestString(t *testing.T) {
 	expectRun(t, `out = "Hello" != "Hello"`, nil, false)
 	expectRun(t, `out = "Hello" != "World"`, nil, true)
 
+	expectRun(t, `out = "Hello" > "World"`, nil, false)
+	expectRun(t, `out = "World" < "Hello"`, nil, false)
+	expectRun(t, `out = "Hello" < "World"`, nil, true)
+	expectRun(t, `out = "World" > "Hello"`, nil, true)
+	expectRun(t, `out = "Hello" >= "World"`, nil, false)
+	expectRun(t, `out = "Hello" <= "World"`, nil, true)
+	expectRun(t, `out = "Hello" >= "Hello"`, nil, true)
+	expectRun(t, `out = "World" <= "World"`, nil, true)
+
 	// index operator
 	str := "abcdef"
 	strStr := `"abcdef"`