add pair:map_first and pair:map_second

5 files changed, 69 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2ff57af..d31575e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- The `pair` module gains the `map_first`, and `map_second` functions.
 - The `string` module gains the `compare` function.
 
 ## v0.4.0 - 2019-09-19
diff --git a/gen/src/gleam@pair.erl b/gen/src/gleam@pair.erl
index 887adb9..3361dc7 100644
--- a/gen/src/gleam@pair.erl
+++ b/gen/src/gleam@pair.erl
@@ -1,7 +1,7 @@
 -module(gleam@pair).
 -compile(no_auto_import).
 
--export([first/1, second/1, swap/1]).
+-export([first/1, second/1, swap/1, map_first/2, map_second/2]).
 
 first(Tup) ->
     {A, _} = Tup,
@@ -14,3 +14,11 @@ second(Tup) ->
 swap(Tup) ->
     {A, B} = Tup,
     {B, A}.
+
+map_first(Tup, F) ->
+    {A, B} = Tup,
+    {F(A), B}.
+
+map_second(Tup, F) ->
+    {A, B} = Tup,
+    {A, F(B)}.
diff --git a/gen/test/gleam@pair_test.erl b/gen/test/gleam@pair_test.erl
index 775f161..6a2c856 100644
--- a/gen/test/gleam@pair_test.erl
+++ b/gen/test/gleam@pair_test.erl
@@ -1,7 +1,7 @@
 -module(gleam@pair_test).
 -compile(no_auto_import).
 
--export([first_test/0, second_test/0, swap_test/0]).
+-export([first_test/0, second_test/0, swap_test/0, map_first_test/0, map_second_test/0]).
 
 first_test() ->
     gleam@expect:equal(gleam@pair:first({1, 2}), 1),
@@ -13,3 +13,17 @@ second_test() ->
 
 swap_test() ->
     gleam@expect:equal(gleam@pair:swap({1, <<"2">>}), {<<"2">>, 1}).
+
+map_first_test() ->
+    Inc = fun(A) -> A + 1 end,
+    gleam@expect:equal(gleam@pair:map_first({1, 2}, Inc), {2, 2}),
+    gleam@expect:equal(gleam@pair:map_first({8, 2}, Inc), {9, 2}),
+    gleam@expect:equal(gleam@pair:map_first({0, -2}, Inc), {1, -2}),
+    gleam@expect:equal(gleam@pair:map_first({-10, 20}, Inc), {-9, 20}).
+
+map_second_test() ->
+    Dec = fun(A) -> A - 1 end,
+    gleam@expect:equal(gleam@pair:map_second({1, 2}, Dec), {1, 1}),
+    gleam@expect:equal(gleam@pair:map_second({8, 2}, Dec), {8, 1}),
+    gleam@expect:equal(gleam@pair:map_second({0, -2}, Dec), {0, -3}),
+    gleam@expect:equal(gleam@pair:map_second({-10, 20}, Dec), {-10, 19}).
diff --git a/src/gleam/pair.gleam b/src/gleam/pair.gleam
index 9074330..32fc6d1 100644
--- a/src/gleam/pair.gleam
+++ b/src/gleam/pair.gleam
@@ -17,3 +17,13 @@ pub fn swap(tup) {
   let Pair(a, b) = tup
   Pair(b, a)
 }
+
+pub fn map_first(tup, f) {
+    let Pair(a, b) = tup
+    Pair(f(a), b)
+}
+
+pub fn map_second(tup, f) {
+    let Pair(a, b) = tup
+    Pair(a, f(b))
+}
diff --git a/test/gleam/pair_test.gleam b/test/gleam/pair_test.gleam
index 54a12af..5dd2690 100644
--- a/test/gleam/pair_test.gleam
+++ b/test/gleam/pair_test.gleam
@@ -26,3 +26,37 @@ pub fn swap_test() {
   |> pair.swap
   |> expect.equal(_, pair.Pair("2", 1))
 }
+
+pub fn map_first_test() {
+    let inc = fn(a) {
+        a + 1
+    }
+    pair.map_first(pair.Pair(1, 2), inc)
+    |> expect.equal(_, pair.Pair(2, 2))
+
+    pair.map_first(pair.Pair(8,2), inc)
+    |> expect.equal(_, pair.Pair(9, 2))
+
+    pair.map_first(pair.Pair(0,-2), inc)
+    |> expect.equal(_, pair.Pair(1, -2))
+
+    pair.map_first(pair.Pair(-10, 20), inc)
+    |> expect.equal(_, pair.Pair(-9, 20))
+}
+
+pub fn map_second_test() {
+    let dec = fn(a) {
+        a - 1
+    }
+    pair.map_second(pair.Pair(1, 2), dec)
+    |> expect.equal(_, pair.Pair(1, 1))
+
+    pair.map_second(pair.Pair(8,2), dec)
+    |> expect.equal(_, pair.Pair(8, 1))
+
+    pair.map_second(pair.Pair(0,-2), dec)
+    |> expect.equal(_, pair.Pair(0, -3))
+
+    pair.map_second(pair.Pair(-10, 20), dec)
+    |> expect.equal(_, pair.Pair(-10, 19))
+}