ADD now concats strings, too

SPEC.md

@@ -179,7 +179,16 @@ All arithmetic operations pop two values, perform operation, push result as numb
 
 #### ADD
 **Stack**: [a, b] → [a + b]
-**Note**: Only for numbers (use separate string concat if needed)
+
+Performs addition or string concatenation depending on operand types:
+- If either operand is a string, converts both to strings and concatenates
+- Otherwise, converts both to numbers and performs numeric addition
+
+**Examples**:
+- `5 + 3` → `8` (numeric addition)
+- `"hello" + " world"` → `"hello world"` (string concatenation)
+- `"count: " + 42` → `"count: 42"` (string concatenation)
+- `100 + " items"` → `"100 items"` (string concatenation)
 
 #### SUB
 **Stack**: [a, b] → [a - b]

examples/add-with-strings.ts

@@ -0,0 +1,73 @@
+/**
+ * Demonstrates the ADD opcode working with both numbers and strings
+ *
+ * ADD now behaves like JavaScript's + operator:
+ * - If either operand is a string, it does string concatenation
+ * - Otherwise, it does numeric addition
+ */
+
+import { toBytecode, run } from "#reef"
+
+// Numeric addition
+const numericAdd = toBytecode(`
+  PUSH 10
+  PUSH 5
+  ADD
+  HALT
+`)
+
+console.log('Numeric addition (10 + 5):')
+console.log(await run(numericAdd))
+// Output: { type: 'number', value: 15 }
+
+// String concatenation
+const stringConcat = toBytecode(`
+  PUSH "hello"
+  PUSH " world"
+  ADD
+  HALT
+`)
+
+console.log('\nString concatenation ("hello" + " world"):')
+console.log(await run(stringConcat))
+// Output: { type: 'string', value: 'hello world' }
+
+// Mixed: string + number
+const mixedConcat = toBytecode(`
+  PUSH "count: "
+  PUSH 42
+  ADD
+  HALT
+`)
+
+console.log('\nMixed concatenation ("count: " + 42):')
+console.log(await run(mixedConcat))
+// Output: { type: 'string', value: 'count: 42' }
+
+// Building a message
+const buildMessage = toBytecode(`
+  PUSH "You have "
+  PUSH 3
+  ADD
+  PUSH " new messages"
+  ADD
+  HALT
+`)
+
+console.log('\nBuilding a message:')
+console.log(await run(buildMessage))
+// Output: { type: 'string', value: 'You have 3 new messages' }
+
+// Computing then concatenating
+const computeAndConcat = toBytecode(`
+  PUSH "Result: "
+  PUSH 10
+  PUSH 5
+  ADD
+  ADD
+  HALT
+`)
+
+console.log('\nComputing then concatenating ("Result: " + (10 + 5)):')
+console.log(await run(computeAndConcat))
+// Output: { type: 'string', value: 'Result: 15' }

src/vm.ts

@@ -145,7 +145,16 @@ export class VM {
         break
 
       case OpCode.ADD:
-        this.binaryOp((a, b) => toNumber(a) + toNumber(b))
+        const b = this.stack.pop()!
+        const a = this.stack.pop()!
+
+        // If either operand is a string, do string concatenation
+        if (a.type === 'string' || b.type === 'string') {
+          this.stack.push(toValue(toString(a) + toString(b)))
+        } else {
+          // Otherwise do numeric addition
+          this.stack.push(toValue(toNumber(a) + toNumber(b)))
+        }
         break
 
       case OpCode.SUB:

tests/opcodes.test.ts

@@ -18,6 +18,87 @@ describe("ADD", () => {
   `
     expect(await run(toBytecode(str2))).toEqual({ type: 'number', value: 600 })
   })
+
+  test("concatenate two strings", async () => {
+    const str = `
+    PUSH "hello"
+    PUSH " world"
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'hello world' })
+
+    const str2 = `
+    PUSH "foo"
+    PUSH "bar"
+    ADD
+  `
+    expect(await run(toBytecode(str2))).toEqual({ type: 'string', value: 'foobar' })
+  })
+
+  test("concatenate string with number", async () => {
+    const str = `
+    PUSH "count: "
+    PUSH 42
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'count: 42' })
+
+    const str2 = `
+    PUSH 100
+    PUSH " items"
+    ADD
+  `
+    expect(await run(toBytecode(str2))).toEqual({ type: 'string', value: '100 items' })
+  })
+
+  test("concatenate string with boolean", async () => {
+    const str = `
+    PUSH "result: "
+    PUSH true
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'result: true' })
+
+    const str2 = `
+    PUSH false
+    PUSH " value"
+    ADD
+  `
+    expect(await run(toBytecode(str2))).toEqual({ type: 'string', value: 'false value' })
+  })
+
+  test("concatenate string with null", async () => {
+    const str = `
+    PUSH "value: "
+    PUSH null
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'value: null' })
+  })
+
+  test("concatenate multiple strings in sequence", async () => {
+    const str = `
+    PUSH "hello"
+    PUSH " "
+    ADD
+    PUSH "world"
+    ADD
+    PUSH "!"
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'hello world!' })
+  })
+
+  test("mixed arithmetic and string concatenation", async () => {
+    const str = `
+    PUSH "Result: "
+    PUSH 10
+    PUSH 5
+    ADD
+    ADD
+  `
+    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'Result: 15' })
+  })
 })
 
 describe("SUB", () => {