a/docs/utils.md

# `utils`

The `utils` module is a grab-bag of helpers that the rest of the standard
library leans on: JSON encoding and decoding, a `null` marker for round-tripping
JSON and SQL, error-trapping wrappers, and a value pretty-printer. It is always
available as the global `utils`.

```lua
local data = utils.fromJSON('{"name":"Alice","age":30}')
print(data.name, data.age)            --> Alice   30

print(utils.toJSON({1, 2, 3}))        --> [1,2,3]
print(utils.dump({a = 1, b = {2, 3}})) --> {a=1, b={2, 3}}
```

## JSON

### `utils.toJSON(value [, pretty])`

Serialize a Lua value to a JSON string. Pass `pretty = true` for indented,
multi-line output; the default is compact.

```lua
utils.toJSON({ name = "test", count = 5 })       --> {"count":5,"name":"test"}
utils.toJSON({ name = "test" }, true)            --> pretty-printed over several lines
```

How values are converted:

| Lua                         | JSON              |
|-----------------------------|-------------------|
| `nil`                       | `null`            |
| `utils.NULL`                | `null`            |
| `boolean`                   | `true` / `false`  |
| integer                     | number            |
| float                       | number            |
| `string`                    | string            |
| table (sequence)            | array             |
| table (other)               | object            |

A table is encoded as a **JSON array** when its keys are exactly the integers
`1..#t` with no gaps, and as a **JSON object** otherwise. Object keys must be
strings or integers (an integer key becomes its decimal string); any other key
type is an error.

These cases raise an error rather than producing invalid JSON:

- `NaN` or infinity (JSON has no representation for them),
- a value that has no JSON form (function, coroutine, userdata other than
  `utils.NULL`),
- nesting deeper than 64 levels.

### `utils.fromJSON(string)`

Parse a JSON string into a Lua value. The inverse of `toJSON`:

| JSON              | Lua            |
|-------------------|----------------|
| `null`            | `utils.NULL`   |
| `true` / `false`  | `boolean`      |
| integer number    | integer        |
| fractional number | float          |
| string            | `string`       |
| array             | sequence table |
| object            | table          |

Invalid JSON raises an error. Note that JSON `null` decodes to
[`utils.NULL`](#utilsnull), **not** Lua `nil` — so a `null` inside an array does
not create a gap in the sequence.

```lua
local arr = utils.fromJSON('[1, null, 3]')
print(#arr)                  --> 3
print(utils.isNull(arr[2]))  --> true
```

## `null`

JSON and SQL both have a `null`/`NULL` value distinct from "absent". Lua's `nil`
cannot fill that role: a table cannot *store* `nil` (assigning `nil` deletes the
key), so a literal `nil` can never survive inside a table to reach a JSON array
slot or a SQL parameter. `utils.NULL` is a sentinel that can.

### `utils.NULL`

A unique marker standing for an explicit null. Store it in a table where you mean
"null, not missing":

```lua
http.postJSON(url, { nickname = utils.NULL })  -- sends  {"nickname":null}
con:execute("UPDATE u SET nickname = ?", { utils.NULL })  -- binds SQL NULL
```

It is the value `fromJSON` produces for JSON `null`, the value `toJSON` and the
`http`/`sqlite` modules turn back into `null`/`NULL`. Comparable by identity, so
`value == utils.NULL` works, but prefer `utils.isNull` for clarity.

### `utils.isNull(value)`

Return `true` if `value` is `utils.NULL`. Everything else, including Lua `nil`,
returns `false`.

```lua
utils.isNull(utils.NULL)  --> true
utils.isNull(nil)         --> false
utils.isNull(0)           --> false
```

## Error handling

### `utils.try(fn [, ...])`

Call `fn` and trap any error, returning `(result, nil)` on success or
`(nil, err)` on failure — a more ergonomic `pcall` for the common
single-return-value case. Extra arguments are forwarded to `fn`.

```lua
local data, err = utils.try(function()
    return utils.fromJSON(input)
end)
if not data then
    log.error("bad JSON: " .. tostring(err))
    return
end
-- use data
```

If `fn` errors with a `nil` value, `err` is the string `"unspecified error"`, so
a falsy first return reliably means failure.

### `utils.tryn(n, fn [, ...])`

Like `try`, but for a function that returns several values. `n` is how many
values `fn` returns on success. On success it returns those `n` values followed
by a trailing `nil` (the error slot); on failure it returns `n` `nil`s followed
by the error.

```lua
-- fn returns two values
local x, y, err = utils.tryn(2, function()
    return parsePoint(s)   -- returns x, y
end)
if err then
    log.error("parse failed: " .. tostring(err))
else
    print(x, y)
end
```

`n` must be an integer between 0 and 64. Use `try` when `fn` returns at most one
value; reach for `tryn` only when you genuinely need to capture several.

## Debugging

### `utils.dump(value)`

Render any Lua value as a readable string — handy for logging and quick
inspection. Strings are quoted, sequences print as `{ a, b, c }`, other tables as
`{ key = value, ... }`, and `utils.NULL` prints as `null`. Cycles are shown as
`<circular>` rather than looping forever, and nesting is capped at 20 levels.

```lua
print(utils.dump({ id = 1, tags = {"a", "b"}, parent = utils.NULL }))
--> { id = 1, parent = null, tags = { "a", "b" } }
```

This is for human eyes, not machine parsing — use `toJSON` when you need output
you can read back.

## Notes

- **`utils` is always loaded.** No `require`; the global is present in every
  script.
- **`NULL` bridges three worlds.** The same marker represents null for `toJSON`,
  `fromJSON`, the `http` JSON helpers, and SQL `NULL` binding in `sqlite`, so a
  null value can round-trip through any of them.
- **`toJSON`/`fromJSON` are not symmetric for `nil`.** `nil` *encodes* to `null`,
  but `null` *decodes* to `utils.NULL`. This is deliberate, so decoding never
  silently drops array elements.