map_dict:fold

5 files changed, 65 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e6598c9..db37b8d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+- The `map_dict` module gains a `fold` function.
+
 ## v0.2.0 - 2019-05-11
 
 - Library renamed to `gleam_stdlib`.
diff --git a/gen/src/map_dict.erl b/gen/src/map_dict.erl
index 10150b3..e72b92d 100644
--- a/gen/src/map_dict.erl
+++ b/gen/src/map_dict.erl
@@ -1,7 +1,7 @@
 -module(map_dict).
 -compile(no_auto_import).
 
--export([size/1, to_list/1, from_list/1, has_key/2, new/0, fetch/2, put/3, map_values/2, keys/1, values/1, filter/2, take/2, merge/2, delete/2, drop/2, update/3]).
+-export([size/1, to_list/1, from_list/1, has_key/2, new/0, fetch/2, put/3, map_values/2, keys/1, values/1, filter/2, take/2, merge/2, delete/2, drop/2, update/3, fold/3]).
 
 size(A) ->
     maps:size(A).
@@ -74,3 +74,13 @@ update(Dict, Key, F) ->
         {error, _} ->
             put(Dict, Key, F({error, not_found}))
     end.
+
+fold(Dict, Acc, F) ->
+    Kvs = to_list(Dict),
+    case Kvs of
+        [] ->
+            Acc;
+
+        [{K, V} | _] ->
+            fold(delete(Dict, K), F(K, V, Acc), F)
+    end.
diff --git a/gen/test/map_dict_test.erl b/gen/test/map_dict_test.erl
index 24255f1..03be65a 100644
--- a/gen/test/map_dict_test.erl
+++ b/gen/test/map_dict_test.erl
@@ -1,7 +1,7 @@
 -module(map_dict_test).
 -compile(no_auto_import).
 
--export([from_list_test/0, has_key_test/0, new_test/0, fetch_test/0, put_test/0, map_values_test/0, keys_test/0, values_test/0, take_test/0, drop_test/0, merge_test/0, delete_test/0, update_test/0]).
+-export([from_list_test/0, has_key_test/0, new_test/0, fetch_test/0, put_test/0, map_values_test/0, keys_test/0, values_test/0, take_test/0, drop_test/0, merge_test/0, delete_test/0, update_test/0, fold_test/0]).
 
 from_list_test() ->
     expect:equal(map_dict:size(map_dict:from_list([{4, 0}, {1, 0}])), 2).
@@ -115,3 +115,14 @@ update_test() ->
                                      {<<"b">>, 1},
                                      {<<"c">>, 2},
                                      {<<"z">>, 0}])).
+
+fold_test() ->
+    Dict = map_dict:from_list([{<<"a">>, 0},
+                               {<<"b">>, 1},
+                               {<<"c">>, 2},
+                               {<<"d">>, 3}]),
+    Add = fun(_, V, Acc) -> V + Acc end,
+    expect:equal(map_dict:fold(Dict, 0, Add), 6),
+    Concat = fun(K, _, Acc) -> str:append(Acc, K) end,
+    expect:equal(map_dict:fold(Dict, <<"">>, Concat), <<"abcd">>),
+    expect:equal(map_dict:fold(map_dict:from_list([]), 0, Add), 0).
diff --git a/src/map_dict.gleam b/src/map_dict.gleam
index 6033866..c25378f 100644
--- a/src/map_dict.gleam
+++ b/src/map_dict.gleam
@@ -86,3 +86,11 @@ pub fn update(dict, key, f) {
   | Error(_) -> put(dict, key, f(Error(NotFound)))
   }
 }
+
+pub fn fold(dict, acc, f) {
+  let kvs = to_list(dict)
+  case kvs {
+    | [] -> acc
+    | [{k, v} | _] -> fold(delete(dict, k), f(k, v, acc), f)
+  }
+}
diff --git a/test/map_dict_test.gleam b/test/map_dict_test.gleam
index 551a333..6ff9133 100644
--- a/test/map_dict_test.gleam
+++ b/test/map_dict_test.gleam
@@ -1,5 +1,6 @@
 import expect
 import map_dict
+import str
 
 pub fn from_list_test() {
   [
@@ -201,3 +202,32 @@ pub fn update_test() {
   |> map_dict:update(_, "z", inc_or_zero)
   |> expect:equal(_, map_dict:from_list([{"a", 0}, {"b", 1}, {"c", 2}, {"z", 0}]))
 }
+
+pub fn fold_test() {
+  let dict = map_dict:from_list([
+    {"a", 0},
+    {"b", 1},
+    {"c", 2},
+    {"d", 3},
+  ])
+
+  let add = fn(_, v, acc) {
+    v + acc
+  }
+
+  dict
+  |> map_dict:fold(_, 0, add)
+  |> expect:equal(_, 6)
+
+  let concat = fn(k, _, acc) {
+    str:append(acc, k)
+  }
+
+  dict
+  |> map_dict:fold(_, "", concat)
+  |> expect:equal(_, "abcd")
+
+  map_dict:from_list([])
+  |> map_dict:fold(_, 0, add)
+  |> expect:equal(_, 0)
+}