Added var uniqueness check

pkg/compiler/compiler_let_test.go

@@ -208,4 +208,27 @@ func TestLet(t *testing.T) {
 
 		So(err, ShouldNotBeNil)
 	})
+
+	Convey("Should not compile if a variable not defined", t, func() {
+		c := compiler.New()
+
+		_, err := c.Compile(`
+			RETURN foo
+		`)
+
+		So(err, ShouldNotBeNil)
+	})
+
+	Convey("Should not compile if a variable is not unique", t, func() {
+		c := compiler.New()
+
+		_, err := c.Compile(`
+			LET foo = "bar"
+			LET foo = "baz"
+
+			RETURN foo
+		`)
+
+		So(err, ShouldNotBeNil)
+	})
 }

pkg/compiler/visitor.go

@@ -1013,6 +1013,13 @@ func (v *visitor) doVisitVariableDeclaration(ctx *fql.VariableDeclarationContext
 	var init core.Expression
 	var err error
 
+	name := ctx.Identifier().GetText()
+	err = scope.SetVariable(name)
+
+	if err != nil {
+		return nil, err
+	}
+
 	exp := ctx.Expression()
 
 	if exp != nil {
@@ -1039,10 +1046,6 @@ func (v *visitor) doVisitVariableDeclaration(ctx *fql.VariableDeclarationContext
 		return nil, err
 	}
 
-	name := ctx.Identifier().GetText()
-
-	scope.SetVariable(name)
-
 	return expressions.NewVariableDeclarationExpression(
 		v.getSourceMap(ctx),
 		name,