add optional_opt

gleam.toml

@@ -1,5 +1,5 @@
 name = "clad"
-version = "1.0.1"
+version = "1.1.0"
 
 # Fill out these fields if you intend to generate HTML documentation or publish
 # your project to the Hex package manager.

src/clad.gleam

@@ -213,10 +213,34 @@ pub fn flag(
   short_name: String,
   next: fn(Bool) -> Decoder(final),
 ) -> Decoder(final) {
-  use value <- optional_field(long_name, decode.bool)
+  optional_opt(long_name, short_name, False, decode.bool, next)
+}
+
+/// Decode a command line option by either a long name or short name.
+/// The given default value is returned if the option does not exist.
+/// ```gleam
+/// let decoder = {
+///   use name <- clad.optional_opt("name", "n", "Lucy", decode.string)
+///   decode.success(name)
+/// }
+///
+/// let result = clad.decode(["--name", "Joe"], decoder)
+/// assert result == Ok("Joe")
+///
+/// let result = clad.decode([], decoder)
+/// assert result == Ok("Lucy")
+/// ```
+pub fn optional_opt(
+  long_name: String,
+  short_name: String,
+  default: t,
+  field_decoder: Decoder(t),
+  next: fn(t) -> Decoder(final),
+) -> Decoder(final) {
+  use value <- optional_field(long_name, field_decoder)
   case value {
     Some(v) -> next(v)
-    None -> decode.optional_field(short_name, False, decode.bool, next)
+    None -> decode.optional_field(short_name, default, field_decoder, next)
   }
 }
 

test/clad_test.gleam

@@ -210,3 +210,19 @@ pub fn list_test() {
   clad.decode([], decoder)
   |> should.be_error
 }
+
+pub fn optional_opt_test() {
+  let decoder = {
+    use name <- clad.optional_opt("name", "n", "Lucy", decode.string)
+    decode.success(name)
+  }
+
+  clad.decode(["--name", "Joe"], decoder)
+  |> should.equal(Ok("Joe"))
+
+  clad.decode(["-n", "Joe"], decoder)
+  |> should.equal(Ok("Joe"))
+
+  clad.decode([], decoder)
+  |> should.equal(Ok("Lucy"))
+}