| name | kida-expression-semantics |
| description | Understand Kida expression behavior: `+` operator, list vs string, export accumulation. Use when debugging unexpected output or expression behavior. |
Kida Expression Semantics
The + Operator
Kida's add_polymorphic handles + with these rules:
- Numeric:
int + int, float + float → numeric addition
- String-like: If either operand is
str → str(left) + str(right)
- Otherwise: Python native
left + right — list + list, tuple + tuple, etc.
Export Accumulation Pattern
Common pattern for building lists in loops:
{% for el in all_members %}
{% let m = el | member_view %}
{% if m.is_private %}
{% export internal_members = internal_members + [m] %}
{% else %}
{% export public_members = public_members + [m] %}
{% end %}
{% end %}
members + [m] must stay list concatenation. If + incorrectly stringifies:
[a, b] + [c] becomes "[a, b]c" (string)- Iterating over that string yields one output per character
- Result: thousands of empty or garbage entries instead of a few list items
Why This Matters
A regression that treated list + list as string concat caused API docs to explode from 33k to 140k lines — one <code> per character instead of per list item.
Correct Behavior (Current)
[a, b] + [c] → [a, b, c] (list)count + " items" → "5 items" (string)"Hello " + name → "Hello Alice" (string)(1, 2) + (3,) → (1, 2, 3) (tuple)
When to Use ~
For explicit string concatenation (URLs, paths, IDs), use ~ — always coerces both sides to strings:
{{ "/path/" ~ id ~ "/action" }}
hx-post="{{ '/chains/' ~ chain_id ~ '/add-step' }}"
Debugging Unexpected Output
If a loop produces far more items than expected, or empty repeated blocks:
- Check for
{% export x = x + [item] %} or similar accumulation
- Verify both operands are lists, not strings
- Ensure no filter or expression is stringifying the list before
+
