result.guard

2 files changed, 47 insertions, 0 deletions

diff --git a/src/gleam/result.gleam b/src/gleam/result.gleam
index e64e96a..c2d92a1 100644
--- a/src/gleam/result.gleam
+++ b/src/gleam/result.gleam
@@ -411,3 +411,36 @@ pub fn replace_error(result: Result(a, e1), error: e2) -> Result(a, e2) {
 pub fn values(results: List(Result(a, e))) -> List(a) {
   list.filter_map(results, fn(r) { r })
 }
+
+/// Run a callback function if the given bool is `True`, otherwise return a
+/// default error value.
+///
+/// This function is suitable for `use` expressions.
+///
+/// ## Examples
+///
+/// ```gleam
+/// > let name = "Kamaka"
+/// > use <- guard(name != "", or: "Missing name")
+/// > "Hello, " <> name
+/// Ok("Hello, Kamaka")
+/// ```
+///
+/// ```gleam
+/// > let name = ""
+/// > use <- guard(name != "", or: "Missing name")
+/// > "Hello, " <> name
+/// Error("Missing name")
+/// ```
+///
+///
+pub fn guard(
+  requirement: Bool,
+  or error: e,
+  then consequence: fn() -> Result(a, e),
+) -> Result(a, e) {
+  case requirement {
+    True -> consequence()
+    False -> Error(error)
+  }
+}
diff --git a/test/gleam/result_test.gleam b/test/gleam/result_test.gleam
index 9075977..dc50dd8 100644
--- a/test/gleam/result_test.gleam
+++ b/test/gleam/result_test.gleam
@@ -205,3 +205,17 @@ pub fn values_test() {
   result.values([Ok(1), Error(""), Ok(3)])
   |> should.equal([1, 3])
 }
+
+pub fn guard_test() {
+  assert Ok(1) = {
+    let x = 1
+    use <- result.guard(True, or: "nope")
+    Ok(x)
+  }
+
+  assert Error("nope") = {
+    let x = 1
+    use <- result.guard(False, or: "nope")
+    Ok(x)
+  }
+}