Add function.constant

This change adds a new helper to the function module.

2 files changed, 14 insertions, 0 deletions

diff --git a/src/gleam/function.gleam b/src/gleam/function.gleam
index a6d2063..5aebb76 100644
--- a/src/gleam/function.gleam
+++ b/src/gleam/function.gleam
@@ -51,3 +51,10 @@ pub fn flip(fun: fn(a, b) -> c) -> fn(b, a) -> c {
 pub fn identity(x: a) -> a {
   x
 }
+
+/// A function that takes a single argument and returns a new function that
+/// ignores its argument and always returns the input value.
+///
+pub fn constant(value: a) -> fn(b) -> a {
+  fn(_) { value }
+}
diff --git a/test/gleam/function_test.gleam b/test/gleam/function_test.gleam
index e86beb3..1495d2a 100644
--- a/test/gleam/function_test.gleam
+++ b/test/gleam/function_test.gleam
@@ -2,6 +2,7 @@ import gleam/should
 import gleam/dynamic
 import gleam/function
 import gleam/int
+import gleam/pair
 import gleam/list
 import gleam/result
 import gleam/string
@@ -107,3 +108,9 @@ pub fn identity_test() {
   |> function.identity
   |> should.equal(#(1, 2.0))
 }
+
+pub fn always_test() {
+  #(1, 2)
+  |> pair.map_first(function.constant(42))
+  |> should.equal(#(42, 2))
+}