# `Feel.Builtin.List`
[🔗](https://github.com/FeeJai/BDM/blob/feel-v0.1.0/apps/feel/lib/feel/builtin/list.ex#L1)

The list functions of FEEL (DMN 1.6, section 10.3.4.4, Table 76), and `sort`
(section 10.3.4.9).

## What is written (D-79, D-87)

All of them. `Feel.Builtin.List.Aggregate` has the functions that make one value of a list
(`count`, `sum`, `mean`, `median`, `mode`, `stddev`, `product`, `min`, `max`, `all`, `any`), and
`Feel.Builtin.List.Sequence` the others (`append`, `concatenate`, `distinct values`, `flatten`,
`index of`, `insert before`, `list contains`, `list replace`, `remove`, `reverse`, `sort`, `sublist`,
`union`). Each says what it does with an argument that is not a list, a position that is out of range, the empty
list, and an item of the wrong kind.

What they share:

  * **A list is wanted, a value that is not one is the list of it** (section 10.3.2.9.4): `count(1)` is
    `count([1])`. `null` is no list, and the call is `null` with an `:argument_out_of_domain` error. The TCK
    flags every `null` argument as an error (0059, 1155), with one exception, `null` among the items
    of a list of booleans.
  * **The aggregates have two forms**, `sum(list)` and `sum(n1, ..., nN)`, which is what a variadic
    parameter (`Feel.Builtin.Function.parameter/3`) is for. `append`, `concatenate` and `union` take any
    number of arguments after the first, and none.
  * **Equality is that of `=`** (`Feel.Runtime.Compare`), and what cannot be compared is not equal.
  * **A position is a whole number**, a fraction being cut off (section 10.3.2.9.4), that counts from the
    end when it is negative.

## The functions that call a function

`list replace` (with a `match`) and `sort` take a function value. A built-in can call one
(`Feel.Runtime.Invoke.call/4`, D-82) and its outcome carries the environment (`t:Feel.Runtime.Env.outcome/0`), so
what the function reports is reported. `sort` is a stable merge sort that threads the environment through its
comparisons (D-87).

# `functions`

```elixir
@spec functions() :: [Feel.Builtin.Function.t()]
```

The functions of this module that are written, as `Feel.Builtin.Function`s: all 24.

    iex> length(Feel.Builtin.List.functions())
    24

# `names`

```elixir
@spec names() :: [Feel.Name.t()]
```

Returns the canonical names of the functions of this module, sorted.

    iex> "sort" in Feel.Builtin.List.names()
    true

---

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