# `Feel.Runtime.Path` [🔗](https://github.com/FeeJai/BDM/blob/feel-v0.1.0/apps/feel/lib/feel/runtime/path.ex#L1) Selecting from a value by a name: `e.name` (DMN 1.6, Tables 65 to 68 and section 10.3.2.5, D-65). `select/2` takes the value of `e` and the canonical name (`Feel.Name`) and returns an `t:Feel.Runtime.Env.outcome/0`. What it selects depends on the kind of the value: | The value | `e.name` | | ------------------------- | ------------------------------------------------------------------------ | | a context | the entry of that key; `null` if the entry is `null`; **an error** if there is none | | a list | a list of the same length: the selection from each element (see below) | | a date | `year`, `month`, `day`, `weekday`, `value` | | a time | `hour`, `minute`, `second`, `time offset`, `timezone`, `value` | | a date and time | all of those | | a years and months duration | `years`, `months`, `value` | | a days and time duration | `days`, `hours`, `minutes`, `seconds`, `value` | | a range | `start`, `end`, `start included`, `end included` | | `null` | **an error** (`null.b` is `null`, flagged as an error: TCK `0057#010`) | | anything else | **an error** | A name that the kind does not have is an error with the code `:invalid_property`, and so is a key that a context does not have (`{a: 1}.b`, TCK `0057#009`). So is a component of the other kind of duration (`duration("P1Y").days`): Table 67 says `null` for it, and the TCK flags an error (`0074#ym_duration_003` to `#006`, `dt_duration_001`, `#002`), which is what the run reports (D-65). What Table 67 says is `null` and the TCK does not flag is not an error: `time offset` and `timezone` of a value that has none. The components of a duration carry its sign, and are the normalized ones (`duration("P1DT25H").days` is 2, and `duration("-P1Y2M").months` is -2, notes 05, E36). ## A list The selection from a list is the list of the selections from its elements (section 10.3.2.5): `[{x: 1, y: 2}, {x: 2}].y` is `[2, null]`. Inside a list a missing entry is `null` and not an error (TCK `0069#033`, `#034`), and so is an element that has no such entry because it is no context, a date with no such property, `null`, or a list: there is no rule for a list of lists, and it is not looked into (notes 02, A-29). An empty list gives the empty list. ## The descendant `e...name` is the values that a context has under the key `name`, at any depth (section 10.3.2.5, Table 71): the context's own entry first, then those of the contexts nested in its entries, each in turn (`{a: {b: {b: 1}}}...b` is `[{b: 1}, 1]`). It is a list always, and `[]` when there is none (notes 05, E38). `descend/3` reads it as the notes do, where the text is silent (D-65); with the environment of the run it also holds the list of a context to `max_list_length` as it makes it: * **The order** is a pre-order walk, with the entries of a context taken in the order they have (D-135). * **Contexts nested in a list are not looked into**; the text speaks of nested contexts. An entry whose value is a list is a value like any other, and is in the result if it is the key. * **A list** on the left is the list of the descendants of each element, as a path is (`[c1, c2]...b` is a list of two lists), with `null` for an element that is no context. * **`null`**, and a value that is not a context, are an error, as for a path. ## `value` The `value` of a duration is its months or its seconds. That of a date, a time and a date and time is the number of seconds that `Feel.Runtime.Temporal.value/1` gives (D-68): from the epoch `1970-01-01T00:00:00Z`, for a date and for a date and time with an offset or a zone id, and the seconds of the day less the offset for a time. A date and time with no zone has none, and is `null` with an error (`:invalid_property`). iex> Feel.Runtime.Path.select(%{"a" => Decimal.new(1)}, "a") {:ok, Decimal.new(1)} iex> {:error, %Feel.Diagnostic{code: :invalid_property}} = Feel.Runtime.Path.select(%{"a" => 1}, "b") iex> Feel.Runtime.Path.select([%{"y" => Decimal.new(2)}, %{"x" => Decimal.new(1)}], "y") {:ok, [Decimal.new(2), nil]} iex> Feel.Runtime.Path.select(~D[2018-12-10], "weekday") {:ok, Decimal.new(1)} iex> Feel.Runtime.Path.select(Feel.Value.DaysAndTimeDuration.new(93_784), "minutes") {:ok, Decimal.new(3)} # `descend` ```elixir @spec descend(Feel.Value.t(), Feel.Name.t(), Feel.Runtime.Env.t() | nil) :: Feel.Runtime.Env.outcome() ``` Selects the values under a key from a context and from the contexts nested in it. See the module documentation. With the environment of the run, the descendants of a context are counted as they are found, and the walk ends at the first one over `max_list_length`, which is the error that a finished list gets (`Feel.Runtime.Env.check_list_length/2`). A context that is in a context twice is walked twice, so `n` levels of it are 2ⁿ descendants, and the list is made whole before the limit looks at it. The descendants of the contexts in a list are lists of their own, and are not looked at: no list but the one that a run gives is checked. iex> inner = %{"b" => Decimal.new(1)} iex> Feel.Runtime.Path.descend(%{"a" => %{"b" => inner}}, "b") {:ok, [inner, Decimal.new(1)]} iex> Feel.Runtime.Path.descend(%{"a" => 1}, "b") {:ok, []} iex> {:error, %Feel.Diagnostic{code: :invalid_property}} = Feel.Runtime.Path.descend(nil, "b") iex> shared = %{"a" => %{"b" => 1}} iex> env = Env.new(limits: Limits.new(max_list_length: 2)) iex> {:ok, _} = Feel.Runtime.Path.descend(%{"x" => shared, "y" => shared}, "b", env) iex> {:error, %Error{kind: :limit}} = Feel.Runtime.Env.run(env, fn env -> ...> {Feel.Runtime.Path.descend(%{"x" => shared, "y" => shared, "z" => shared}, "b", env), env} ...> end) # `select` ```elixir @spec select(Feel.Value.t(), Feel.Name.t()) :: Feel.Runtime.Env.outcome() ``` Selects the entry or the property of a name from a value. See the module documentation. --- *Consult [api-reference.md](api-reference.md) for complete listing*