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

The values an expression reads by name.

`Feel.run/3` takes a map from name to value. A `:var` node of the IR is looked up
there by its canonical name (`Feel.Name`). It is a function of the runtime, and not
a private part of a backend, because generated code may call only `Feel.Runtime`
and both backends have to find the same value.

A binding whose key is the same name written differently (`"a - b"` for `"a-b"`) is
found too, after the exact key. A name that has no binding is `null`, with a warning
(`:unknown_name`) that does not end a strict run.

# `bind_input`

```elixir
@spec bind_input(Feel.bindings(), Feel.Value.t()) :: Feel.bindings()
```

Binds the input `?` of unary tests to a value, in a copy of the bindings.

The tests of allowed values (D-138) run with the value that is verified as `?`. It is a
function of the runtime for the same reason `lookup/4` is: the interpreter and the generated
code of the BEAM backend have to bind it alike.

    iex> Feel.Runtime.Bindings.bind_input(%{"x" => 1}, 2)
    %{"x" => 1, "?" => 2}

# `fetch`

```elixir
@spec fetch(Feel.bindings(), Feel.Name.t()) :: {:ok, Feel.Value.t()} | :error
```

Looks a name up in the bindings, as `lookup/4` does, but reports nothing when it has no
binding.

    iex> Feel.Runtime.Bindings.fetch(%{"a - b" => 1}, "a-b")
    {:ok, 1}
    iex> Feel.Runtime.Bindings.fetch(%{}, "a")
    :error

# `lookup`

```elixir
@spec lookup(
  Feel.Runtime.Env.t(),
  Feel.bindings(),
  Feel.Name.t(),
  Feel.Diagnostic.span()
) ::
  {Feel.Value.t(), Feel.Runtime.Env.t()}
```

Looks a name up in the bindings.

Returns the value and the environment. `span` is where the name is in the source;
it goes on the warning for a name that is not bound.

    iex> env = Feel.Runtime.Env.new()
    iex> {"x", ^env} = Feel.Runtime.Bindings.lookup(env, %{"x" => "x"}, "x", {0, 1})
    iex> {nil, env} = Feel.Runtime.Bindings.lookup(env, %{}, "y", {4, 5})
    iex> [warning] = Feel.Runtime.Env.diagnostics(env)
    iex> {warning.severity, warning.code, warning.span}
    {:warning, :unknown_name, {4, 5}}

---

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