From e4d09202ab7d3b0a4bc52213858ce939b0a2a6dd Mon Sep 17 00:00:00 2001
From: Sebastian <s@porto5.com>
Date: Thu, 17 Dec 2020 14:57:08 +1100
Subject: Add list.permutations

---
 src/gleam/list.gleam | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

(limited to 'src')

diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 1af34e8..44d1510 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -1052,3 +1052,27 @@ pub fn partition(
 ) -> tuple(List(a), List(a)) {
   do_partition(list, categorise, [], [])
 }
+
+/// Return all the permutations of a list
+/// All values must be unique
+///
+/// ## Examples
+///
+///    > permutations([1, 2])
+///    [[1, 2], [2, 1]]
+///
+pub fn permutations(l: List(a)) -> List(List(a)) {
+  case l {
+    [] -> [[]]
+    _ ->
+      map(
+        l,
+        fn(x) {
+          filter(l, fn(y) { y != x })
+          |> permutations
+          |> map(append([x], _))
+        },
+      )
+      |> flatten
+  }
+}
-- 
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