Add list:index_map

5 files changed, 46 insertions, 4 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cebfd5b..ed4d88d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,8 +2,8 @@
 
 ## Unreleased
 
-- `list` module gains `at`, `all`, `any`, `intersperse`, `unique`, `sort`, and
-  `zip` functions.
+- `list` module gains `at`, `all`, `any`, `index_map`, `intersperse`, `sort`,
+  `unique`, and `zip` functions.
 - `str` module gains `append`.
 
 ## v0.1.1 - 2019-04-17
diff --git a/gen/src/list.erl b/gen/src/list.erl
index 10a093a..36fb42d 100644
--- a/gen/src/list.erl
+++ b/gen/src/list.erl
@@ -1,7 +1,7 @@
 -module(list).
 -compile(no_auto_import).
 
--export([length/1, reverse/1, is_empty/1, contains/2, head/1, tail/1, filter/2, map/2, traverse/2, drop/2, take/2, new/0, append/2, flatten/1, fold/3, fold_right/3, find/2, all/2, any/2, zip/2, intersperse/2, at/2, unique/1, sort/1]).
+-export([length/1, reverse/1, is_empty/1, contains/2, head/1, tail/1, filter/2, map/2, index_map/2, traverse/2, drop/2, take/2, new/0, append/2, flatten/1, fold/3, fold_right/3, find/2, all/2, any/2, zip/2, intersperse/2, at/2, unique/1, sort/1]).
 
 length(A) ->
     erlang:length(A).
@@ -70,6 +70,18 @@ do_map(List, Fun, Acc) ->
 map(List, Fun) ->
     do_map(List, Fun, []).
 
+do_index_map(List, Fun, Index, Acc) ->
+    case List of
+        [] ->
+            reverse(Acc);
+
+        [X | Xs] ->
+            do_index_map(Xs, Fun, Index + 1, [Fun(Index, X) | Acc])
+    end.
+
+index_map(List, Fun) ->
+    do_index_map(List, Fun, 0, []).
+
 do_traverse(List, Fun, Acc) ->
     case List of
         [] ->
diff --git a/gen/test/list_test.erl b/gen/test/list_test.erl
index 6105a88..1944d25 100644
--- a/gen/test/list_test.erl
+++ b/gen/test/list_test.erl
@@ -1,7 +1,7 @@
 -module(list_test).
 -compile(no_auto_import).
 
--export([length_test/0, reverse_test/0, is_empty_test/0, contains_test/0, head_test/0, tail_test/0, filter_test/0, map_test/0, traverse_test/0, drop_test/0, take_test/0, new_test/0, append_test/0, flatten_test/0, fold_test/0, fold_right_test/0, find_test/0, all_test/0, any_test/0, zip_test/0, intersperse_test/0, at_test/0, unique_test/0, sort_test/0]).
+-export([length_test/0, reverse_test/0, is_empty_test/0, contains_test/0, head_test/0, tail_test/0, filter_test/0, map_test/0, traverse_test/0, drop_test/0, take_test/0, new_test/0, append_test/0, flatten_test/0, fold_test/0, fold_right_test/0, find_test/0, all_test/0, any_test/0, zip_test/0, intersperse_test/0, at_test/0, unique_test/0, sort_test/0, index_map_test/0]).
 
 length_test() ->
     expect:equal(list:length([]), 0),
@@ -132,3 +132,10 @@ sort_test() ->
     expect:equal(list:sort([4, 3, 6, 5, 4]), [3, 4, 4, 5, 6]),
     expect:equal(list:sort([]), []),
     expect:equal(list:sort([{1, 2}, {4, 5}, {3, 2}]), [{1, 2}, {3, 2}, {4, 5}]).
+
+index_map_test() ->
+    expect:equal(list:index_map([3, 4, 5], fun(I, X) -> {I, X} end),
+                 [{0, 3}, {1, 4}, {2, 5}]),
+    F = fun(I, X) -> str:append(X, str:from_int(I)) end,
+    expect:equal(list:index_map([<<"a">>, <<"b">>, <<"c">>], F),
+                 [<<"a0">>, <<"b1">>, <<"c2">>]).
diff --git a/src/list.gleam b/src/list.gleam
index 1add1be..2571318 100644
--- a/src/list.gleam
+++ b/src/list.gleam
@@ -65,6 +65,17 @@ pub fn map(list, fun) {
   do_map(list, fun, [])
 }
 
+fn do_index_map(list, fun, index, acc) {
+  case list {
+  | [] -> reverse(acc)
+  | [x | xs] -> do_index_map(xs, fun, index + 1, [fun(index, x) | acc])
+  }
+}
+
+pub fn index_map(list, fun) {
+  do_index_map(list, fun, 0, [])
+}
+
 fn do_traverse(list, fun, acc) {
   case list {
   | [] -> Ok(reverse(acc))
diff --git a/test/list_test.gleam b/test/list_test.gleam
index f9882c0..9c510c4 100644
--- a/test/list_test.gleam
+++ b/test/list_test.gleam
@@ -1,5 +1,6 @@
 import expect
 import list
+import str
 
 pub fn length_test() {
   list:length([]) |> expect:equal(_, 0)
@@ -248,3 +249,14 @@ pub fn sort_test() {
   list:sort([{1,2}, {4,5}, {3,2}])
   |> expect:equal(_, [{1,2}, {3,2}, {4,5}])
 }
+
+pub fn index_map_test() {
+  list:index_map([3,4,5], fn(i, x) { {i, x} })
+  |> expect:equal(_, [{0,3},{1,4},{2,5}])
+
+  let f = fn(i, x) {
+    str:append(x, str:from_int(i))
+  }
+  list:index_map(["a","b","c"], f)
+  |> expect:equal(_, ["a0", "b1", "c2"])
+}