# F1 — Null Safety Reporting (+7)

**Suite:** `hs-upstream-core/runtimeErrors`  
**Target:** 7 currently-failing tests (decrement, default, increment, put, remove, settle, transition commands)

## 1. Failing tests

The suite has 18 tests total; 11 already pass. The 7 failures all share the pattern:

```
Expected '#doesntExist' is null, got 
```

The `eval-hs-error` helper already exists (landed in null-safety piece 1). It compiles and runs a HS snippet and returns the error string. The problem is that the listed commands don't guard against null targets before operating, so they produce no error (or a cryptic one) instead of `"'#doesntExist' is null"`.

| Test | Command | Null target expression |
|------|---------|----------------------|
| decrement | `decrement #doesntExist's innerHTML` | `#doesntExist` |
| default | `default #doesntExist's innerHTML to 'foo'` | `#doesntExist` |
| increment | `increment #doesntExist's innerHTML` | `#doesntExist` |
| put | `put 'foo' into/before/after/at start of/at end of #doesntExist` | `#doesntExist` |
| remove | `remove .foo/.@foo/#doesntExist from #doesntExist` | `#doesntExist` |
| settle | `settle #doesntExist` | `#doesntExist` |
| transition | `transition #doesntExist's *visibility to 0` | `#doesntExist` |

Note: add, hide, measure, send, sets, show, toggle, trigger already pass — they already guard.

## 2. Required error format

```
'#doesntExist' is null
```

The apostrophe-quoted selector string followed by ` is null`. The selector text is the original source text of the element expression (e.g. `#doesntExist`, not a stringified DOM node).

This is the same format already used by passing commands. The null-safety piece 1 commit added `eval-hs-error` and `hs-null-error` helper — just need to call it at the right point in each missing command.

## 3. Where to add guards

All in `lib/hyperscript/runtime.sx`. Pattern for each command:

```
(when (nil? target)
  (hs-null-error target-source-text))
```

Where `hs-null-error` (or equivalent) raises with the formatted message.

### Per-command location

- **decrement / increment** — after resolving the target element, before reading/writing innerHTML
- **default** — after resolving target element, before reading current value
- **put** — after resolving destination element (covers all put variants: into, before, after, at start, at end)
- **remove** — after resolving the `from` target element
- **settle** — after resolving target element, before starting transition poll
- **transition** — after resolving target element, before reading/setting style

## 4. Implementation checklist

1. Find each failing command's runtime function in `lib/hyperscript/runtime.sx` using `sx_find_all`.
2. For each: `sx_read_subtree` on the function body, locate where target is resolved, insert null guard calling `hs-null-error` (or the equivalent raise form already used by passing commands).
3. After all 7: run `hs_test_run suite="hs-upstream-core/runtimeErrors"` — expect 18/18.
4. Run smoke range 0–195 — expect no regressions.
5. Commit: `HS: null-safety guards on decrement/default/increment/put/remove/settle/transition (+7)`

## 5. Risk

Low. The pattern is established by the 11 already-passing tests. The only risk is finding the correct point in each command where the element is resolved and before it's first used.