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Stream.transform/4 is now aware of the acc’s tail, if presented #5680

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124e25b
`after_fun` might return fun/1
Jan 26, 2017
15c77ff
Documented `after_fun` behaviour + test cleanup.
Jan 26, 2017
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75 changes: 46 additions & 29 deletions lib/elixir/lib/stream.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -160,18 +160,13 @@ defmodule Stream do
def chunk(enum, n, step, leftover \\ nil)
when is_integer(n) and n > 0 and is_integer(step) and step > 0 do
limit = :erlang.max(n, step)
if is_nil(leftover) do
lazy enum, {[], 0}, fn(f1) -> R.chunk(n, step, limit, f1) end
else
lazy enum, {[], 0},
fn(f1) -> R.chunk(n, step, limit, f1) end,
&do_chunk(&1, n, leftover, &2)
end
lazy enum, {[], 0},
fn(f1) -> R.chunk(n, step, limit, f1) end,
&do_chunk(&1, n, leftover, &2)
end

defp do_chunk(acc(_, {_, 0}, _) = acc, _, _, _) do
{:cont, acc}
end
defp do_chunk(acc, _, nil, _), do: skip(acc)
defp do_chunk(acc(_, {_, 0}, _) = acc, _, _, _), do: skip(acc)

defp do_chunk(acc(h, {buffer, count} = old, t), n, leftover, f1) do
buffer = :lists.reverse(buffer, Enum.take(leftover, n - count))
Expand All @@ -197,9 +192,7 @@ defmodule Stream do
&do_chunk_by(&1, &2)
end

defp do_chunk_by(acc(_, nil, _) = acc, _f1) do
{:cont, acc}
end
defp do_chunk_by(acc(_, nil, _) = acc, _f1), do: skip(acc)

defp do_chunk_by(acc(h, {buffer, _}, t), f1) do
next_with_acc(f1, :lists.reverse(buffer), h, nil, t)
Expand Down Expand Up @@ -749,6 +742,30 @@ defmodule Stream do
allows an after function to be given which is invoked when the stream
halts or completes.

After function has an option to return a function of arity 1, in which
case this function will be called back with a current state of an
accumulator given. The value returned by this function will be assigned
to the whole result of `transform/4`. Any other returned value
will be safely ignored, and the accumulated value will be returned as is.

## Examples

iex> [1, 2, 3, 4]
...> |> Stream.transform(
...> fn -> [] end,
...> fn i, acc ->
...> case i do
...> i when rem(i, 2) == 0 -> {[acc], [i]}
...> _ -> {[], acc ++ [i]}
...> end
...> end,
...> fn rest -> # call me back
...> fn acc -> [rest | acc] end # append the tail
...> end)
...> |> Enum.to_list
[[1], [2, 3], [4]]


This function can be seen as a combination of `Stream.resource/3` with
`Stream.transform/3`.
"""
Expand All @@ -769,17 +786,15 @@ defmodule Stream do

defp do_transform(user_acc, _user, _fun, _next_op, next, {:halt, inner_acc}, _inner, after_fun) do
next.({:halt, []})
do_after(after_fun, user_acc)
{:halted, inner_acc}
{:halted, do_after(after_fun, {inner_acc, user_acc})}
end

defp do_transform(user_acc, user, fun, next_op, next, {:suspend, inner_acc}, inner, after_fun) do
{:suspended, inner_acc, &do_transform(user_acc, user, fun, next_op, next, &1, inner, after_fun)}
end

defp do_transform(user_acc, _user, _fun, :halt, _next, {_, inner_acc}, _inner, after_fun) do
do_after(after_fun, user_acc)
{:halted, inner_acc}
{:halted, do_after(after_fun, {inner_acc, user_acc})}
end

defp do_transform(user_acc, user, fun, :cont, next, inner_acc, inner, after_fun) do
Expand All @@ -788,7 +803,7 @@ defmodule Stream do
catch
kind, reason ->
stacktrace = System.stacktrace
do_after(after_fun, user_acc)
do_after(after_fun, {nil, user_acc})
:erlang.raise(kind, reason, stacktrace)
else
{:suspended, [val], next} ->
Expand All @@ -806,7 +821,7 @@ defmodule Stream do
kind, reason ->
stacktrace = System.stacktrace
next.({:halt, []})
do_after(after_fun, user_acc)
do_after(after_fun, {nil, user_acc})
:erlang.raise(kind, reason, stacktrace)
else
{[], user_acc} ->
Expand All @@ -816,8 +831,7 @@ defmodule Stream do
&Enumerable.List.reduce(list, &1, fun), after_fun)
{:halt, user_acc} ->
next.({:halt, []})
do_after(after_fun, user_acc)
{:halted, elem(inner_acc, 1)}
{:halted, do_after(after_fun, {elem(inner_acc, 1), user_acc})}
{other, user_acc} ->
do_enum_transform(user_acc, user, fun, next_op, next, inner_acc, inner,
&Enumerable.reduce(other, &1, inner), after_fun)
Expand All @@ -830,15 +844,14 @@ defmodule Stream do
kind, reason ->
stacktrace = System.stacktrace
next.({:halt, []})
do_after(after_fun, user_acc)
do_after(after_fun, {nil, user_acc})
:erlang.raise(kind, reason, stacktrace)
else
{:done, acc} ->
do_transform(user_acc, user, fun, next_op, next, {:cont, acc}, inner, after_fun)
{:halted, acc} ->
next.({:halt, []})
do_after(after_fun, user_acc)
{:halted, acc}
{:halted, do_after(after_fun, {acc, user_acc})}
{:suspended, acc, c} ->
{:suspended, acc, &do_list_transform(user_acc, user, fun, next_op, next, &1, inner, c, after_fun)}
end
Expand All @@ -851,7 +864,7 @@ defmodule Stream do
kind, reason ->
stacktrace = System.stacktrace
next.({:halt, []})
do_after(after_fun, user_acc)
do_after(after_fun, {nil, user_acc})
:erlang.raise(kind, reason, stacktrace)
else
# Only take into account outer halts when the op is not halt itself.
Expand All @@ -860,17 +873,21 @@ defmodule Stream do
do_transform(user_acc, user, fun, next_op, next, {:cont, acc}, inner, after_fun)
{:halted, [_ | acc]} ->
next.({:halt, []})
do_after(after_fun, user_acc)
{:halted, acc}
{:halted, do_after(after_fun, {acc, user_acc})}
{:done, [_ | acc]} ->
do_transform(user_acc, user, fun, next_op, next, {:cont, acc}, inner, after_fun)
{:suspended, [_ | acc], c} ->
{:suspended, acc, &do_enum_transform(user_acc, user, fun, next_op, next, &1, inner, c, after_fun)}
end
end

defp do_after(nil, _user_acc), do: :ok
defp do_after(fun, user_acc), do: fun.(user_acc)
defp do_after(nil, {acc, _user_acc}), do: acc
defp do_after(fun, {acc, user_acc}) do
case fun.(user_acc) do
finalize when is_function(finalize, 1) -> finalize.(acc)
_ -> acc
end
end

defp do_transform_each(x, [:outer | acc], f) do
case f.(x, acc) do
Expand Down
22 changes: 22 additions & 0 deletions lib/elixir/test/elixir/stream_test.exs
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Original file line number Diff line number Diff line change
Expand Up @@ -654,6 +654,28 @@ defmodule StreamTest do
assert Process.get(:stream_transform)
end

test "transform/4 with a tail" do
assert [1, 2, 3, 4] |> Stream.transform(fn -> [] end,
fn i, acc ->
case i do
i when rem(i, 2) == 0 -> {[acc], [i]}
_ -> {[], acc ++ [i]}
end
end,
fn rest -> fn acc -> [rest | acc] end end) |> Enum.to_list == [[1], [2, 3], [4]]
end

test "transform/4 with a completely changed accumulator" do
assert [1, 2, 3, 4] |> Stream.transform(fn -> [] end,
fn i, acc ->
case i do
i when rem(i, 2) == 0 -> {[acc], [i]}
_ -> {[], acc ++ [i]}
end
end,
fn rest -> fn acc -> rest end end) |> Enum.to_list == [4]
end

test "transform/4 with early halt" do
stream = Stream.repeatedly(fn -> throw(:error) end)
|> Stream.transform(fn -> nil end, &{[&1, &2], &1},
Expand Down