TRY_CALL

GUIDE.md

@@ -219,6 +219,7 @@ CALL
 - `CALL` - Call function (see calling convention above)
 - `TAIL_CALL` - Tail-recursive call (no stack growth)
 - `RETURN` - Return from function (pops return value)
+- `TRY_CALL <name>` - Call function (if found), push value (if exists), or push name as string (if not found)
 - `BREAK` - Exit iterator/loop (unwinds to break target)
 
 ### Arrays
@@ -404,6 +405,32 @@ STORE factorial
   TAIL_CALL             ; Reuses stack frame
 ```
 
+### Optional Function Calls (TRY_CALL)
+Call function if defined, otherwise use value or name as string:
+```
+; Define optional hook
+MAKE_FUNCTION () .onInit
+STORE onInit
+
+; Later: call if defined, skip if not
+TRY_CALL onInit       ; Calls onInit() if it's a function
+                       ; Pushes value if it exists but isn't a function
+                       ; Pushes "onInit" as string if undefined
+
+; Use with values
+PUSH 42
+STORE answer
+TRY_CALL answer        ; Pushes 42 (not a function)
+
+; Use with undefined
+TRY_CALL unknown       ; Pushes "unknown" as string
+```
+
+**Use Cases**:
+- Optional hooks/callbacks in DSLs
+- Shell-like languages where unknown identifiers become strings
+- Templating systems with optional transformers
+
 ## Key Concepts
 
 ### Truthiness

SPEC.md

@@ -404,6 +404,43 @@ The created function captures `currentScope` as its `parentScope`.
 
 **Errors**: Throws if no call frame to return from
 
+#### TRY_CALL
+**Operand**: Variable name (string)
+**Effect**: Conditionally call function or push value/string onto stack
+**Stack**: [] → [returnValue | value | name]
+**Errors**: Never throws (unlike CALL)
+
+**Behavior**:
+1. Look up variable by name in scope chain
+2. **If variable is a function**: Call it with 0 arguments (no positional, no named) and push the returned value onto the stack.
+3. **If variable exists but is not a function**: Push the variable's value onto stack
+4. **If variable doesn't exist**: Push the variable name as a string onto stack
+
+**Use Cases**:
+- DSL/templating languages with "call if callable, otherwise use as literal" semantics
+- Shell-like behavior where unknown identifiers become strings
+- Optional function hooks (call if defined, silently skip if not)
+
+**Implementation Note**:
+- Uses intentional fall-through in VM switch statement from TRY_CALL to CALL case
+- When function is found, stacks are set up to match CALL's expectations exactly
+- No break target marking or frame pushing occurs when non-function value is found
+
+**Example**:
+```
+MAKE_FUNCTION () .body
+STORE greet
+PUSH 42
+STORE answer
+TRY_CALL greet     ; Calls function greet(), returns its value
+TRY_CALL answer    ; Pushes 42 (number value)
+TRY_CALL unknown   ; Pushes "unknown" (string)
+
+.body:
+  PUSH "Hello!"
+  RETURN
+```
+
 ### Array Operations
 
 #### MAKE_ARRAY

src/bytecode.ts

@@ -27,6 +27,7 @@ type InstructionTuple =
   // Variables
   | ["LOAD", string]
   | ["STORE", string]
+  | ["TRY_LOAD", string]
 
   // Arithmetic
   | ["ADD"] | ["SUB"] | ["MUL"] | ["DIV"] | ["MOD"]
@@ -54,6 +55,7 @@ type InstructionTuple =
   | ["CALL"]
   | ["TAIL_CALL"]
   | ["RETURN"]
+  | ["TRY_CALL", string]
 
   // Arrays
   | ["MAKE_ARRAY", number]
@@ -329,6 +331,8 @@ function toBytecodeFromArray(program: ProgramItem[]): Bytecode /* throws */ {
 
         case "LOAD":
         case "STORE":
+        case "TRY_LOAD":
+        case "TRY_CALL":
         case "CALL_NATIVE":
           operandValue = operand as string
           break

src/opcode.ts

@@ -44,6 +44,7 @@ export enum OpCode {
   CALL,          // operand: none | stack: [fn, ...args, posCount, namedCount] → [result] | marks break target
   TAIL_CALL,     // operand: none | stack: [fn, ...args, posCount, namedCount] → [result] | reuses frame
   RETURN,        // operand: none | stack: [value] → (restored with value) | return from function
+  TRY_CALL,      /// operand: variable name (identifier) | stack: [] → [value] | call a function, load a variable, or load a string
 
   // arrays
   MAKE_ARRAY,  // operand: item count (number) | stack: [item1, ..., itemN] → [array]

src/validator.ts

@@ -34,6 +34,8 @@ const OPCODES_WITH_OPERANDS = new Set([
   OpCode.PUSH,
   OpCode.LOAD,
   OpCode.STORE,
+  OpCode.TRY_LOAD,
+  OpCode.TRY_CALL,
   OpCode.JUMP,
   OpCode.JUMP_IF_FALSE,
   OpCode.JUMP_IF_TRUE,
@@ -327,8 +329,9 @@ export function validateBytecode(source: string): ValidationResult {
         }
       }
 
-      // Validate variable names for LOAD/STORE
-      if ((opCode === OpCode.LOAD || opCode === OpCode.STORE) &&
+      // Validate variable names for LOAD/STORE/TRY_LOAD/TRY_CALL
+      if ((opCode === OpCode.LOAD || opCode === OpCode.STORE ||
+           opCode === OpCode.TRY_LOAD || opCode === OpCode.TRY_CALL) &&
           !isValidIdentifier(operand)) {
         errors.push({
           line: lineNum,

src/vm.ts

@@ -353,6 +353,27 @@ export class VM {
         })
         break
 
+      // @ts-ignore
+      case OpCode.TRY_CALL: {
+        const varName = instruction.operand as string
+        const value = this.scope.get(varName)
+
+        if (value?.type === 'function') {
+          this.stack.push(value)
+          this.stack.push(toValue(0))
+          this.stack.push(toValue(0))
+          // No `break` here -- we want to fall through to OpCode.CALL!
+        } else if (value) {
+          this.stack.push(value)
+          break
+        } else {
+          this.stack.push(toValue(varName))
+          break
+        }
+      }
+
+      // don't put any `case` statement here - `TRY_CALL` MUST go before `CALL!`
+
       case OpCode.CALL: {
         // Pop named count from stack (top)
         const namedCount = toNumber(this.stack.pop()!)

tests/functions.test.ts

@@ -375,3 +375,124 @@ test("CALL - named args with defaults on fixed params", async () => {
   // x should use default value 5
   expect(result).toEqual({ type: 'number', value: 5 })
 })
+
+test("TRY_CALL - calls function if found", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", [], ".body"],
+    ["STORE", "myFunc"],
+    ["TRY_CALL", "myFunc"],
+    ["HALT"],
+    [".body:"],
+    ["PUSH", 42],
+    ["RETURN"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result).toEqual({ type: 'number', value: 42 })
+})
+
+test("TRY_CALL - pushes value if variable exists but is not a function", async () => {
+  const bytecode = toBytecode([
+    ["PUSH", 99],
+    ["STORE", "myVar"],
+    ["TRY_CALL", "myVar"],
+    ["HALT"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result).toEqual({ type: 'number', value: 99 })
+})
+
+test("TRY_CALL - pushes string if variable not found", async () => {
+  const bytecode = toBytecode([
+    ["TRY_CALL", "unknownVar"],
+    ["HALT"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result).toEqual({ type: 'string', value: 'unknownVar' })
+})
+
+test("TRY_CALL - handles arrays", async () => {
+  const bytecode = toBytecode([
+    ["PUSH", 1],
+    ["PUSH", 2],
+    ["MAKE_ARRAY", 2],
+    ["STORE", "myArray"],
+    ["TRY_CALL", "myArray"],
+    ["HALT"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result.type).toBe('array')
+  if (result.type === 'array') {
+    expect(result.value).toEqual([
+      { type: 'number', value: 1 },
+      { type: 'number', value: 2 }
+    ])
+  }
+})
+
+test("TRY_CALL - handles dicts", async () => {
+  const bytecode = toBytecode([
+    ["PUSH", "key"],
+    ["PUSH", "value"],
+    ["MAKE_DICT", 1],
+    ["STORE", "myDict"],
+    ["TRY_CALL", "myDict"],
+    ["HALT"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result.type).toBe('dict')
+  if (result.type === 'dict') {
+    expect(result.value.get('key')).toEqual({ type: 'string', value: 'value' })
+  }
+})
+
+test("TRY_CALL - handles null values", async () => {
+  const bytecode = toBytecode([
+    ["PUSH", null],
+    ["STORE", "myNull"],
+    ["TRY_CALL", "myNull"],
+    ["HALT"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  expect(result).toEqual({ type: 'null', value: null })
+})
+
+test("TRY_CALL - function can access its parameters", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["STORE", "addFive"],
+    ["PUSH", 10],
+    ["STORE", "x"],
+    ["TRY_CALL", "addFive"],
+    ["HALT"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["PUSH", 5],
+    ["ADD"],
+    ["RETURN"]
+  ])
+
+  const result = await new VM(bytecode).run()
+  // Function is called with 0 args, so x inside function should be null
+  // Then we add 5 to null (which coerces to 0)
+  expect(result).toEqual({ type: 'number', value: 5 })
+})
+
+test("TRY_CALL - with string format", async () => {
+  const bytecode = toBytecode(`
+    MAKE_FUNCTION () #4
+    STORE myFunc
+    TRY_CALL myFunc
+    HALT
+    PUSH 100
+    RETURN
+  `)
+
+  const result = await new VM(bytecode).run()
+  expect(result).toEqual({ type: 'number', value: 100 })
+})