traverse v0.1.7 Traverse
Traverse is a toolset to walk arbitrary Elixir Datastructures.
walk
visits all substructures down to atomic elements.
iex> ds = [:a, {:b, 1, 2}, [:c, 3, 4, 5]]
...> collector = fn ele, acc when is_atom(ele) or is_number(ele) -> [ele|acc]
...> _, acc -> acc end
...> Traverse.walk(ds, [], collector)
[5, 4, 3, :c, 2, 1, :b, :a]
One can return the accumulator boxed in a %Cut{}
struct to avoid traversal of the
subtree.
iex> ds = [add: [1, 2], ignore: [3, 4]]
...> collector = fn {:ignore, _}, acc -> %Traverse.Cut{acc: acc}
...> n, acc when is_number(n) -> [n|acc]
...> _, acc -> acc end
...> Traverse.walk(ds, [], collector)
[2, 1]
filter allows to filter arbitrary substructures according to a filter function.
The filter function does not need to be completely defined, undefined values are mapped to false. In other words we need to define the filter functions only for structures and values we want to keep.
iex> number_arrays = fn x when is_number(x) -> true
...> l when is_list(l) -> true end
...> Traverse.filter([:a, {1, 2}, 3, [4, :b]], number_arrays)
[3, [4]]
The same result can be achieved with mapall
and Traverse.Ignore
if that suits
your style better:
iex> not_number_arrays = fn x when is_number(x) or is_list(x) -> x
...> _ -> Traverse.Ignore end
...> Traverse.mapall([:a, {1, 2}, 3, [4, :b]], not_number_arrays)
[3, [4]]
map preserves structure, that is lists remain lists, tuples remain tuples and
maps remain maps with the same keys, unless the transformation returns Traverse.Ignore
(c.f. map1
if you want to transform key
value pairs in maps)
In order to avoid putting unnecessary burden on the transformer function it can only be partially defined, and it will be completed with the identity function for undefined parameters. Here is an example.
iex> Traverse.map([:a, 1, {:b, 2}], fn x when is_number(x) -> x + 1 end)
[:a, 2, {:b, 3}]
The transformer function can also return the special value Traverse.Ignore
, which will remove the value from the result, and in
case of a map it will remove the key, value pair.
iex> require Integer
...> no_odds = fn x when Integer.is_even(x) -> x * 2
...> _ -> Traverse.Ignore end
...> Traverse.map([1, %{a: 1, b: 2}, {3, 4}], no_odds)
[%{b: 4}, {8}]
The more general way to achieve this is to use filter_map
, which however is less efficent as the filter function is also called
on inner nodes.
mapall
like map
perserves the structure of the datastructure passed in.
However it also calls the transformer
function for inner nodes, which allows
us to perform mappings on substructures.
Again the transformer
function can be partially defined and is completed by
the identity function.
And, also again, the special return value Traverse.Ignore
can be used to ignore
values or substructures.
Here is a simple example that eliminates empty sublists
iex> [1, [[]], 2, [3, []]]
...> |> Traverse.mapall(fn [] -> Traverse.Ignore end)
[1, [], 2, [3]]
This example shows that mapall
applies a prewalk strategy by default, we can
change this by providing the option post: true
.
iex> [1, [[]], 2, [3, []]]
...> |> Traverse.mapall(fn [] -> Traverse.Ignore end, post: true)
[1, 2, [3]]
Now, by applying the transformation after having transformed the substructure, empty lists of empty lists go away too.
Link to this section Summary
Link to this section Types
Link to this section Functions
mapall(any(), t_simple_mapper_fn(), Keyword.t()) :: any()
walk(any(), any(), t_simple_walker_fn()) :: any()