implement a lazy or function

2 files changed, 41 insertions, 0 deletions

diff --git a/src/gleam/result.gleam b/src/gleam/result.gleam
index 2ebf2ef..76ee0e1 100644
--- a/src/gleam/result.gleam
+++ b/src/gleam/result.gleam
@@ -202,6 +202,29 @@ pub fn or(first: Result(a, e), second: Result(a, e)) -> Result(a, e) {
   }
 }
 
+/// Return the first value if it is Ok, otherwise evaluates the given function for a fallback value.
+///
+/// ## Examples
+///
+///    > or(Ok(1), Ok(2))
+///    Ok(1)
+///
+///    > or(Ok(1), Error("Error 2"))
+///    Ok(1)
+///
+///    > or(Error("Error 1"), Ok(2))
+///    Ok(2)
+///
+///    > or(Error("Error 1"), Error("Error 2"))
+///    Error("Error 2")
+///
+pub fn lazy_or(first: Result(a, e), second: fn() -> Result(a, e)) -> Result(a, e) {
+  case first {
+    Ok(_) -> first
+    Error(_) -> second()
+  }
+}
+
 /// Combine a list of results into a single result.
 /// If all elements in the list are Ok then returns an Ok holding the list of values.
 /// If any element is Error then returns the first error.
diff --git a/test/gleam/result_test.gleam b/test/gleam/result_test.gleam
index 1c585b5..d047496 100644
--- a/test/gleam/result_test.gleam
+++ b/test/gleam/result_test.gleam
@@ -119,6 +119,24 @@ pub fn or_test() {
   |> should.equal(Error("Error 2"))
 }
 
+pub fn lazy_or_test() {
+  Ok(1)
+  |> result.lazy_or(fn() { Ok(2)} )
+  |> should.equal(Ok(1))
+
+  Ok(1)
+  |> result.lazy_or(fn() { Error("Error 2")} )
+  |> should.equal(Ok(1))
+
+  Error("Error 1")
+  |> result.lazy_or(fn() { Ok(2)} )
+  |> should.equal(Ok(2))
+
+  Error("Error 1")
+  |> result.lazy_or(fn() { Error("Error 2")} )
+  |> should.equal(Error("Error 2"))
+}
+
 pub fn all_test() {
   [Ok(1), Ok(2), Ok(3)]
   |> result.all