# `Localize.Exception`
[🔗](https://github.com/elixir-localize/localize/blob/v0.39.0/lib/localize/exception.ex#L1)

Conventions and a small behaviour shared by structured Localize
exceptions.

## The `:reason` field

Exceptions that distinguish between multiple failure categories
carry a `:reason` field whose value is a **documented atom** from
a closed set. Callers can pattern-match on `:reason` to branch on
category without parsing the rendered message; `message/1` is the
single place a user-facing sentence is assembled.

Modules that adopt this convention declare `@behaviour
Localize.Exception` and implement `reason_atoms/0`, returning the
exhaustive list of valid `:reason` values for that struct. This
lets tooling — including the structural-exception test suite —
iterate the documented reasons and verify that `message/1` has a
clause for each.

## The `:cause` field

When a higher layer must report a lower layer's failure, the outer
exception carries the inner one in a `:cause` field of type
`Exception.t() | nil`. The convention is:

* `:cause` is set when, and only when, the outer exception is a
  wrapper. A direct error sets `:cause` to `nil`.

* The outer `message/1` may delegate to the inner via
  `Exception.message(cause)`, possibly with a leading context
  phrase.

* Programmatic callers can pattern-match on the outer struct for
  the operation context, and call `Exception.message/1` on
  `:cause` for the original detail.

This convention is used by `Localize.FormatError`,
`Localize.LocaleDownloadError`, and `Localize.ParseError`.

## Why no prose in structural fields

Fields like `:reason`, `:expected`, `:path`, and `:detail` must
hold structured values — atoms, paths, short labels, struct
references — not free-form sentences with interpolated values.
Putting a sentence in a structural field defeats pattern matching,
duplicates content into `message/1`, and prevents translation.

# `reason_atoms`

```elixir
@callback reason_atoms() :: [atom()]
```

Returns the closed set of atoms permitted in this exception's
`:reason` field.

Used by tests to verify that `message/1` has a rendering clause
for each documented reason atom. The list MUST be exhaustive —
any atom assigned to `:reason` at runtime must appear here.

# `safe_message`

```elixir
@spec safe_message(binary(), binary(), keyword()) :: binary()
```

Render an MF2-format Gettext message safely for use inside an
exception's `message/1` callback.

`Exception.message/1` is called from `raise`, `Exception.format/2`,
log handlers, and inspect paths — none of which can tolerate a
raise. This helper wraps `Gettext.dpgettext/5` so that any failure
in the message formatter (a NIF crash, a hostile `String.Chars` or
`Inspect` impl on a bound value, a future regression) falls back
to the raw msgid rather than propagating.

Use this only in `defexception` `message/1` callbacks. General MF2
formatting should call `Gettext.dpgettext/5` (or the higher-level
macros) directly so callers see the underlying formatter error.

### Arguments

* `msgctxt` is the gettext context string (e.g. `"locale"`).

* `msgid` is the source-language MF2 message string.

* `bindings` is a keyword list of variable bindings for the
  message. The default is `[]`.

### Returns

* The interpolated message string, or `msgid` unchanged if
  interpolation failed.

---

*Consult [api-reference.md](api-reference.md) for complete listing*
