# `Feel.Runtime.Boxed`
[🔗](https://github.com/FeeJai/BDM/blob/feel-v0.1.0/apps/feel/lib/feel/runtime/boxed.ex#L1)

The strict reading of the condition of a boxed expression (DMN 1.6, 10.2.1.8 to 10.2.1.10; D-212).

The `if` of a boxed conditional, the brackets of a boxed filter and the `satisfies` of a boxed `some` or `every` MUST resolve to
a boolean. Written as text, FEEL takes a condition that is not `true` as false (Table 50: `if "abc" then 1 else 2` is `2`), and a
layer above `feel` that builds the node from a box marks it `boxed: true` in its `meta`, which says that a condition that is a
value other than a boolean is `null` and an error, for the whole node, as the DMN TCK has it (1150 #003, 1151 #004 and #005,
1153 #006).

`null` is not this case: it is the result of a comparison with a missing value, and it is taken as false (the `else` branch, the
element is dropped), as it is in the text. Only a value that is neither a boolean nor `null` is a violation. A number in the
brackets of a filter is an index, as it is in the text, and is decided before this module is asked.

Both backends ask this module, so they report the same.

# `part`

```elixir
@type part() :: :if | :match | :satisfies
```

The part of the box that must be a boolean.

# `allowed?`

```elixir
@spec allowed?(Feel.Value.t()) :: boolean()
```

Whether the value of a condition is allowed: a boolean, or `null`.

    iex> Feel.Runtime.Boxed.allowed?(true)
    true
    iex> Feel.Runtime.Boxed.allowed?(nil)
    true
    iex> Feel.Runtime.Boxed.allowed?("abc")
    false

# `diagnostic`

```elixir
@spec diagnostic(Feel.Value.t(), part()) :: Feel.Diagnostic.t()
```

The diagnostic of a condition that is not a boolean: a `:type_mismatch`, with the part of the box in its `details`.

    iex> diagnostic = Feel.Runtime.Boxed.diagnostic("abc", :if)
    iex> {diagnostic.code, diagnostic.severity, diagnostic.details}
    {:type_mismatch, :error, %{boxed: :if}}

# `violation`

```elixir
@spec violation(Feel.Runtime.Env.t(), Feel.Value.t(), part(), Feel.Diagnostic.span()) ::
  {Feel.Value.t(), Feel.Runtime.Env.t()}
```

The result of a boxed node whose condition is not a boolean: `null` and the error, at the span of the node.

    iex> env = Feel.Runtime.Env.new()
    iex> {nil, env} = Feel.Runtime.Boxed.violation(env, "abc", :if, {0, 3})
    iex> Enum.map(Feel.Runtime.Env.diagnostics(env), & &1.code)
    [:type_mismatch]

---

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