option.lazy_unwrap and option.lazy_or (#261)

3 files changed, 67 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 356f433..09b9b16 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@
 - The `dynamic` module gains the `decode2`, `decode3`, `decode4`, `decode5`,
   `decode6`, `decode7`, and `decode8` functions.
 - The `int` module gains the `digits` and `undigits` functions.
+- The `option` module gains the `lazy_or` and `lazy_unwrap` functions.
 
 ## v0.18.1 - 2021-12-19
 
diff --git a/src/gleam/option.gleam b/src/gleam/option.gleam
index 20e85e9..4a097e8 100644
--- a/src/gleam/option.gleam
+++ b/src/gleam/option.gleam
@@ -115,6 +115,23 @@ pub fn unwrap(option: Option(a), or default: a) -> a {
   }
 }
 
+/// Extracts the value from an `Option`, evaluating the default function if the option is `None`.
+///
+/// ## Examples
+///
+///    > lazy_unwrap(Some(1), fn() { 0 })
+///    1
+///
+///    > lazy_unwrap(None, fn() { 0 })
+///    0
+///
+pub fn lazy_unwrap(option: Option(a), or default: fn() -> a) -> a {
+  case option {
+    Some(x) -> x
+    None -> default()
+  }
+}
+
 /// Updates a value held within the `Some` of an `Option` by calling a given function
 /// on it.
 ///
@@ -210,6 +227,29 @@ pub fn or(first: Option(a), second: Option(a)) -> Option(a) {
   }
 }
 
+/// Returns the first value if it is `Some`, otherwise evaluates the given function for a fallback value.
+///
+/// ## Examples
+///
+///    > lazy_or(Some(1), fn() { Some(2) })
+///    Some(1)
+///
+///    > lazy_or(Some(1), fn() { None })
+///    Some(1)
+///
+///    > lazy_or(None, fn() { Some(2) })
+///    Some(2)
+///
+///    > lazy_or(None, fn() { None })
+///    None
+///
+pub fn lazy_or(first: Option(a), second: fn() -> Option(a)) -> Option(a) {
+  case first {
+    Some(_) -> first
+    None -> second()
+  }
+}
+
 /// Given a list of `Option`s,
 /// returns only the values inside `Some`.
 ///
diff --git a/test/gleam/option_test.gleam b/test/gleam/option_test.gleam
index 35c9eb3..36a40f7 100644
--- a/test/gleam/option_test.gleam
+++ b/test/gleam/option_test.gleam
@@ -49,6 +49,14 @@ pub fn unwrap_option_test() {
   |> should.equal(0)
 }
 
+pub fn lazy_unwrap_option_test() {
+  option.lazy_unwrap(Some(1), fn() { 0 })
+  |> should.equal(1)
+
+  option.lazy_unwrap(None, fn() { 0 })
+  |> should.equal(0)
+}
+
 pub fn map_option_test() {
   Some(1)
   |> option.map(fn(x) { x + 1 })
@@ -109,6 +117,24 @@ pub fn or_option_test() {
   |> should.equal(None)
 }
 
+pub fn lazy_or_option_test() {
+  Some(1)
+  |> option.lazy_or(fn() { Some(2) })
+  |> should.equal(Some(1))
+
+  Some(1)
+  |> option.lazy_or(fn() { None })
+  |> should.equal(Some(1))
+
+  None
+  |> option.lazy_or(fn() { Some(2) })
+  |> should.equal(Some(2))
+
+  None
+  |> option.lazy_or(fn() { None })
+  |> should.equal(None)
+}
+
 pub fn values_test() {
   option.values([Some(1), None, Some(3)])
   |> should.equal([1, 3])