split_once function for strings

2 files changed, 38 insertions, 1 deletions

diff --git a/src/gleam/string.gleam b/src/gleam/string.gleam
index 7c2f188..3c49c87 100644
--- a/src/gleam/string.gleam
+++ b/src/gleam/string.gleam
@@ -239,7 +239,7 @@ pub external fn ends_with(String, String) -> Bool =
 /// ## Examples
 ///
 ///    > split("home/gleam/desktop/", on: "/")
-///    ["home","gleam","desktop", ""]
+///    ["home", "gleam", "desktop", ""]
 ///
 pub fn split(x: String, on substring: String) -> List(String) {
   x
@@ -248,6 +248,29 @@ pub fn split(x: String, on substring: String) -> List(String) {
   |> list.map(with: iodata.to_string)
 }
 
+external fn erl_split(String, String) -> List(String) =
+  "string" "split"
+
+/// Splits a string a single time on the given substring.
+///
+/// Returns an error if substring not present.
+///
+/// ## Examples
+///
+///    > split_once("home/gleam/desktop/", on: "/")
+///    Ok(tuple("home", "gleam/desktop/"))
+///
+///    > split_once("home/gleam/desktop/", on: "?")
+///    Error(Nil)
+///
+pub fn split_once(x: String, on substring: String) -> Result(tuple(String, String), Nil) {
+    case erl_split(x, substring) {
+        [first, rest] -> Ok(tuple(first, rest))
+        _ -> Error(Nil)
+    }
+}
+
+
 /// Create a new string by joining two strings together.
 ///
 /// This function copies both strings and runs in linear time. If you find
diff --git a/test/gleam/string_test.gleam b/test/gleam/string_test.gleam
index 2fbb84f..a9b0d79 100644
--- a/test/gleam/string_test.gleam
+++ b/test/gleam/string_test.gleam
@@ -38,6 +38,20 @@ pub fn split_test() {
   |> should.equal(["Gleam", "Erlang,Elixir"])
 }
 
+pub fn split_first_test() {
+  "Gleam,Erlang,Elixir"
+  |> string.split_once(",")
+  |> should.equal(Ok(tuple("Gleam", "Erlang,Elixir")))
+
+  "Gleam"
+  |> string.split_once(",")
+  |> should.equal(Error(Nil))
+
+  ""
+  |> string.split_once(",")
+  |> should.equal(Error(Nil))
+}
+
 pub fn replace_test() {
   "Gleam,Erlang,Elixir"
   |> string.replace(",", "++")