vm: add string concatenation

- compiler: fix string creation

src/compiler.zig

@@ -230,9 +230,10 @@ pub const Compiler = struct {
     }
 
     fn string(self: *Compiler) !void {
+        const lexeme_len = self.parser.previous.lexeme.len;
         try self.emitConstant(values.ObjVal(try objects.copyString(
             self.allocator,
-            self.parser.previous.lexeme,
+            self.parser.previous.lexeme[1 .. lexeme_len - 1],
         )));
     }
 

src/object.zig

@@ -15,15 +15,23 @@ pub const Object = struct {
     value: ObjValue,
 };
 
+fn createString(allocator: *Allocator, data: []u8) !*Object {
+    var obj = try allocator.create(Object);
+    obj.otype = ObjType.String;
+    obj.value = ObjValue{ .String = data };
+    return obj;
+}
+
 pub fn copyString(allocator: *Allocator, data: []const u8) !*Object {
     var str = try allocator.alloc(u8, data.len);
     std.mem.copy(u8, str, data);
 
-    var obj = try allocator.create(Object);
+    return try createString(allocator, str);
-    obj.otype = ObjType.String;
+}
-    obj.value = ObjValue{ .String = str };
 
-    return obj;
+/// Assumes it can take ownership of the given data.
+pub fn takeString(allocator: *Allocator, data: []u8) !*Object {
+    return try createString(allocator, data);
 }
 
 pub fn printObject(stdout: var, obj: Object) !void {

src/vm.zig

@@ -3,6 +3,7 @@ const chunk = @import("chunk.zig");
 const value = @import("value.zig");
 const values = value;
 const compiler = @import("compiler.zig");
+const objects = @import("object.zig");
 
 const Chunk = chunk.Chunk;
 const Value = value.Value;
@@ -124,7 +125,27 @@ pub const VM = struct {
         }
     }
 
+    fn concatenateStrings(self: *VM) !void {
+        var b = self.pop().as.Object.value.String;
+        var a = self.pop().as.Object.value.String;
+
+        var res_str = try std.mem.join(
+            self.allocator,
+            "",
+            [][]u8{ a, b },
+        );
+
+        var val = values.ObjVal(try objects.takeString(self.allocator, res_str));
+        try self.push(val);
+    }
+
     fn doAdd(self: *VM) !void {
+        if (values.isObjType(self.peek(0), .String) and
+            values.isObjType(self.peek(1), .String))
+        {
+            return try self.concatenateStrings();
+        }
+
         var b = try self.popNum();
         var a = try self.popNum();
         try self.push(values.NumberVal(a + b));