fix: array.pop() returns 0 instead of proper none for empty arrays

[mjm918]

Summary

The current option representation in naml uses 0 for none and the value directly for some(value). This makes it impossible to distinguish between:

• A valid value of 0 (e.g., some(0))
• The absence of a value (none)

Current Behavior

array.pop()

var arr: [int] = [0, 1, 2];
var val1: option<int> = arr.pop();  // Returns 2 ✓
var val2: option<int> = arr.pop();  // Returns 1 ✓
var val3: option<int> = arr.pop();  // Returns 0 ✓
var val4: option<int> = arr.pop();  // Returns 0 ✗ (should be none!)

// Cannot distinguish val3 from val4!
println(val3 ?? -1);  // Prints 0
println(val4 ?? -1);  // Prints 0 (should print -1)

as? fallible cast

var valid: option<int> = "0" as? int;      // Returns 0 (some(0))
var invalid: option<int> = "hello" as? int; // Returns 0 (none)

// Cannot distinguish valid from invalid!
println(valid ?? -1);    // Prints 0
println(invalid ?? -1);  // Prints 0 (should print -1)

Expected Behavior

// pop()
var arr: [int] = [0];
var val1: option<int> = arr.pop();  // Returns some(0)
var val2: option<int> = arr.pop();  // Returns none
println(val1 ?? -1);  // Should print 0
println(val2 ?? -1);  // Should print -1

// as?
var valid: option<int> = "0" as? int;       // Returns some(0)
var invalid: option<int> = "hello" as? int; // Returns none
println(valid ?? -1);    // Should print 0
println(invalid ?? -1);  // Should print -1

Root Cause

Options are represented as simple i64 values where 0 means none. This conflicts with 0 being a valid value.

Example from namlc/src/runtime/array.rs:

pub unsafe extern "C" fn naml_array_pop(arr: *mut NamlArray) -> i64 {
    if (*arr).len == 0 {
        return 0;  // Bug: 0 is indistinguishable from popping the value 0
    }
    // ...
}

Proposed Solution

Use a consistent tagged option representation across the codebase:

• Allocate 16-byte struct: { tag: i32, value: i64 }
• tag = 0 means none
• tag = 1 means some(value)

This requires updating:

1. Runtime functions that return option<T>
2. The ?? operator to check tag field instead of null check
3. The some() expression (already uses this format)
4. Methods like .is_some(), .is_none(), .unwrap()
5. The as? fallible cast operator

Affected Functions

• naml_array_pop() - returns 0 for empty array
• naml_array_get() - may have similar issue for out-of-bounds
• naml_map_get() - may have similar issue for missing keys
• as? fallible cast - returns 0 for parse failure
• Any other function returning option<T>

Impact

Until this is fixed, option<T> cannot reliably hold the value 0 (or 0.0 for floats, false for bools, empty string for strings, etc.).