function.identity

4 files changed, 56 insertions, 27 deletions

diff --git a/gen/src/gleam@function.erl b/gen/src/gleam@function.erl
index f014d37..b376ed0 100644
--- a/gen/src/gleam@function.erl
+++ b/gen/src/gleam@function.erl
@@ -1,10 +1,13 @@
 -module(gleam@function).
 -compile(no_auto_import).
 
--export([compose/2, flip/1]).
+-export([compose/2, flip/1, identity/1]).
 
 compose(Fun1, Fun2) ->
     fun(A) -> Fun2(Fun1(A)) end.
 
 flip(Fun) ->
     fun(B, A) -> Fun(A, B) end.
+
+identity(X) ->
+    X.
diff --git a/gen/test/gleam@function_test.erl b/gen/test/gleam@function_test.erl
index 47758c5..58f6a57 100644
--- a/gen/test/gleam@function_test.erl
+++ b/gen/test/gleam@function_test.erl
@@ -1,7 +1,7 @@
 -module(gleam@function_test).
 -compile(no_auto_import).
 
--export([compose_test/0, flip_test/0]).
+-export([compose_test/0, flip_test/0, identity_test/0]).
 
 compose_test() ->
     AddTwo = fun(Int) -> Int + 2 end,
@@ -9,23 +9,24 @@ compose_test() ->
     AddFive = gleam@function:compose(AddTwo, AddThree),
     gleam@expect:equal(AddFive(1), 6),
     HeadToString = gleam@function:compose(
-        fun gleam@list:head/1,
-        fun(IntResult) ->
-            gleam@int:to_string(gleam@result:unwrap(IntResult, 0))
-        end
+        gleam@function:compose(
+            fun gleam@list:head/1,
+            fun(Capture1) -> gleam@result:unwrap(Capture1, 0) end
+        ),
+        fun gleam@int:to_string/1
     ),
     gleam@expect:equal(HeadToString([1]), <<"1">>),
     gleam@expect:equal(HeadToString([]), <<"0">>).
 
 flip_test() ->
-    Fun = fun(String, Int) ->
+    Fun = fun(S, I) ->
         gleam@string:append(
             gleam@string:append(
                 gleam@string:append(
-                    gleam@string:append(<<"String: '">>, String),
+                    gleam@string:append(<<"String: '">>, S),
                     <<"', Int: '">>
                 ),
-                gleam@int:to_string(Int)
+                gleam@int:to_string(I)
             ),
             <<"'">>
         )
@@ -36,3 +37,9 @@ flip_test() ->
         FlippedFun(2, <<"Alice">>),
         <<"String: 'Alice', Int: '2'">>
     ).
+
+identity_test() ->
+    gleam@expect:equal(gleam@function:identity(1), 1),
+    gleam@expect:equal(gleam@function:identity(<<"">>), <<"">>),
+    gleam@expect:equal(gleam@function:identity([]), []),
+    gleam@expect:equal(gleam@function:identity({1, 2.0}), {1, 2.0}).
diff --git a/src/gleam/function.gleam b/src/gleam/function.gleam
index 41ccf67..68fc619 100644
--- a/src/gleam/function.gleam
+++ b/src/gleam/function.gleam
@@ -9,3 +9,8 @@ pub fn compose(fun1: fn(a) -> b, fun2: fn(b) -> c) -> fn(a) -> c {
 pub fn flip(fun: fn(a, b) -> c) -> fn(b, a) -> c {
   fn(b, a) { fun(a, b) }
 }
+
+// A function that always returns its input value.
+pub fn identity(x: a) -> a {
+  x
+}
diff --git a/test/gleam/function_test.gleam b/test/gleam/function_test.gleam
index 5b3ca42..1c3b4b8 100644
--- a/test/gleam/function_test.gleam
+++ b/test/gleam/function_test.gleam
@@ -1,15 +1,15 @@
 import gleam/expect
-import gleam/function.{compose, flip}
-import gleam/int as int_mod
+import gleam/function
+import gleam/int
 import gleam/list
 import gleam/result
-import gleam/string as string_mod
+import gleam/string
 
 pub fn compose_test() {
   let add_two = fn(int: Int) { int + 2 }
   let add_three = fn(int: Int) { int + 3 }
 
-  let add_five = compose(add_two, add_three)
+  let add_five = function.compose(add_two, add_three)
 
   1
   |> add_five
@@ -18,13 +18,9 @@ pub fn compose_test() {
   // Takes a list of ints and returns the head as a string (if there is one, or
   // else "0" if there is not)
   let head_to_string =
-    compose(
-      list.head,
-      fn(int_result: Result(Int, Nil)) {
-        result.unwrap(int_result, 0)
-        |> int_mod.to_string
-      }
-    )
+    list.head
+    |> function.compose(_, result.unwrap(_, 0))
+    |> function.compose(_, int.to_string)
 
   [1]
   |> head_to_string
@@ -36,15 +32,15 @@ pub fn compose_test() {
 }
 
 pub fn flip_test() {
-  let fun = fn(string: String, int: Int) {
-    string
-    |> string_mod.append("String: '", _)
-    |> string_mod.append(_, "', Int: '")
-    |> string_mod.append(_, int_mod.to_string(int))
-    |> string_mod.append(_, "'")
+  let fun = fn(s: String, i: Int) {
+    s
+    |> string.append("String: '", _)
+    |> string.append(_, "', Int: '")
+    |> string.append(_, int.to_string(i))
+    |> string.append(_, "'")
   }
 
-  let flipped_fun = flip(fun)
+  let flipped_fun = function.flip(fun)
 
   fun("Bob", 1)
   |> expect.equal(_, "String: 'Bob', Int: '1'")
@@ -52,3 +48,21 @@ pub fn flip_test() {
   flipped_fun(2, "Alice")
   |> expect.equal(_, "String: 'Alice', Int: '2'")
 }
+
+pub fn identity_test() {
+  1
+  |> function.identity
+  |> expect.equal(_, 1)
+
+  ""
+  |> function.identity
+  |> expect.equal(_, "")
+
+  []
+  |> function.identity
+  |> expect.equal(_, [])
+
+  tuple(1, 2.0)
+  |> function.identity
+  |> expect.equal(_, tuple(1, 2.0))
+}