# `Predicator.FunctionProvider`
[🔗](https://github.com/riddler/predicator-ex/blob/v9.4.1/lib/predicator/functions/provider.ex#L1)

Behaviour for a module that supplies callable functions to the evaluator.

A provider names its functions by binding each name to an `{arity, atom}`
pair - the atom is the name of a `def` on the same module, with signature
`(args :: [Types.value()], context) :: {:ok, Types.value()} | {:error, binary()}`.
Naming the implementation by atom, rather than exposing a closure directly,
is what lets a caller resolve `module` and `atom` separately - `apply/3`
against the pair - instead of carrying a `function()` value around.

`Predicator.Context.new/2`'s `:providers` option resolves a list of provider
modules into the evaluator's dispatch map; the four builtin modules
(`SystemFunctions`, `DateFunctions`, `JSONFunctions`, `MathFunctions`) are
the default provider list, named by `builtin_providers/0`.

# `entry`

```elixir
@type entry() :: {Predicator.Evaluator.function_arity(), atom()}
```

A function's binding: its accepted arity/arities, and the atom naming its implementation.

# `name`

```elixir
@type name() :: binary()
```

A function name, as it appears in a predicator expression.

# `functions`

```elixir
@callback functions() :: %{required(name()) => entry()}
```

Returns the functions this provider exposes, keyed by name.

Each value is `{arity, atom}`, where `atom` names a public function on the
same module accepting `(args, context)` and returning
`{:ok, Types.value()} | {:error, binary()}`.

# `builtin_providers`

```elixir
@spec builtin_providers() :: [module()]
```

The four builtin provider modules, in shadowing order.

`Predicator.Context.new/2` resolves this list first when `builtins: true`
(the default), so a later entry - a `:providers` module, then the inline
`:functions` map - overrides a same-named builtin. This is the one place
the default list is written down; conformance tooling that needs "every
registered name" calls `Predicator.Context.new().functions` rather than
duplicating it.

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