String concatination.

examples/strings_test.fun

@@ -0,0 +1,34 @@
+#!/usr/bin/env fun
+// Strings test: concatenation with variables and literals, functions returning strings
+
+print("=== strings test start ===")
+
+string a = "Hello"
+string b = "World"
+string sep = ", "
+string excl = "!"
+
+// Basic concatenations
+print(a + sep + b + excl)         // expect: Hello, World!
+print("Hi" + " " + "there")       // expect: Hi there
+
+// Empty strings
+string empty = ""
+print(empty + "x")                 // expect: x
+print("x" + empty)                 // expect: x
+
+// Function returning concatenated string
+fun greet(name)
+  return "Hello, " + name
+
+print(greet("Fun"))                // expect: Hello, Fun
+
+// Using a variable prefix with a function result
+string prefix = "Hi, "
+print(prefix + greet("You"))       // expect: Hi, Hello, You
+
+// Non-empty check (string comparison)
+if (a != "")
+  print("a is non-empty")
+
+print("=== strings test end ===")

src/vm.c

@@ -153,15 +153,35 @@ void vm_run(VM *vm, Bytecode *entry) {
             case OP_ADD: {
                 Value b = pop_value(vm);
                 Value a = pop_value(vm);
-                if (a.type != VAL_INT || b.type != VAL_INT) {
+                if (a.type == VAL_INT && b.type == VAL_INT) {
-                    fprintf(stderr, "Runtime type error: ADD expects ints, got %s and %s\n",
+                    Value res = make_int(a.i + b.i);
+                    free_value(a);
+                    free_value(b);
+                    push_value(vm, res);
+                } else if (a.type == VAL_STRING && b.type == VAL_STRING) {
+                    const char *sa = a.s ? a.s : "";
+                    const char *sb = b.s ? b.s : "";
+                    size_t la = strlen(sa);
+                    size_t lb = strlen(sb);
+                    char *buf = (char*)malloc(la + lb + 1);
+                    if (!buf) {
+                        fprintf(stderr, "Runtime error: out of memory during string concatenation\n");
+                        exit(1);
+                    }
+                    memcpy(buf, sa, la);
+                    memcpy(buf + la, sb, lb);
+                    buf[la + lb] = '\0';
+                    Value res;
+                    res.type = VAL_STRING;
+                    res.s = buf;
+                    free_value(a);
+                    free_value(b);
+                    push_value(vm, res);
+                } else {
+                    fprintf(stderr, "Runtime type error: ADD expects both ints or both strings, got %s and %s\n",
                             value_type_name(a.type), value_type_name(b.type));
                     exit(1);
                 }
-                Value res = make_int(a.i + b.i);
-                free_value(a);
-                free_value(b);
-                push_value(vm, res);
                 break;
             }