Closes #115: Add dynamic.option

4 files changed, 52 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1956083..b45b807 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 - The `list` module gains the `each`, and `partition` functions.
 - The `int` and `float` modules gain the `negate` function.
 - The `result` module gains the `all` function.
+- The `dynamic` module gains the `option` function.
 
 ## v0.11.0 - 2020-08-22
 
diff --git a/gleam b/gleam
new file mode 100644
index 0000000..7f13854
--- /dev/null
+++ b/gleam
diff --git a/src/gleam/dynamic.gleam b/src/gleam/dynamic.gleam
index bf7963f..e4656c4 100644
--- a/src/gleam/dynamic.gleam
+++ b/src/gleam/dynamic.gleam
@@ -1,8 +1,9 @@
 import gleam/bit_string.{BitString} as bit_string_mod
 import gleam/list as list_mod
-import gleam/atom
+import gleam/atom as atom_mod
 import gleam/map.{Map}
 import gleam/result
+import gleam/option.{None, Option, Some}
 
 /// `Dynamic` data is data that we don"t know the type of yet.
 /// We likely get data like this from interop with Erlang, or from
@@ -102,7 +103,7 @@ pub external fn float(from: Dynamic) -> Result(Float, String) =
 ///    > atom(from(123))
 ///    Error("Expected an Atom, got `123`")
 ///
-pub external fn atom(from: Dynamic) -> Result(atom.Atom, String) =
+pub external fn atom(from: Dynamic) -> Result(atom_mod.Atom, String) =
   "gleam_stdlib" "decode_atom"
 
 /// Check to see whether a Dynamic value is an bool, and return the bool if
@@ -182,6 +183,36 @@ pub fn typed_list(
   |> result.then(list_mod.try_map(_, decoder_type))
 }
 
+/// Check to see if a Dynamic value is an Option of a particular type, and return
+/// the Option if it is.
+///
+/// The second argument is a decoder function used to decode the elements of
+/// the list. The list is only decoded if all elements in the list can be
+/// successfully decoded using this function.
+///
+/// ## Examples
+///
+///    > option(from("Hello"), string)
+///    Ok(Some("Hello"))
+///
+///    > option(from(atom.from_string("null")), string)
+///    Ok(None)
+///
+///    > option(from(123), string)
+///    Error("Expected a bit_string, got an int")
+///
+pub fn option(
+  from dynamic: Dynamic,
+  of decoder: Decoder(inner),
+) -> Result(Option(inner), String) {
+  let Ok(null) = atom_mod.from_string("null")
+  case atom(dynamic), decoder(dynamic) {
+    Ok(atom), _ if atom == null -> Ok(None)
+    _, Ok(result) -> Ok(Some(result))
+    _, Error(msg) -> Error(msg)
+  }
+}
+
 /// Check to see if a Dynamic value is a map with a specific field, and return
 /// the value of the field if it is.
 ///
diff --git a/test/gleam/dynamic_test.gleam b/test/gleam/dynamic_test.gleam
index 2b272d4..f4e78a0 100644
--- a/test/gleam/dynamic_test.gleam
+++ b/test/gleam/dynamic_test.gleam
@@ -5,6 +5,7 @@ import gleam/list
 import gleam/should
 import gleam/result
 import gleam/map
+import gleam/option.{None, Some}
 
 pub fn bit_string_test() {
   ""
@@ -220,6 +221,23 @@ pub fn typed_list_test() {
   |> should.be_error
 }
 
+pub fn option_test() {
+  let Ok(null) = atom.from_string("null")
+
+  1
+  |> dynamic.from
+  |> dynamic.option(dynamic.int)
+  |> should.equal(Ok(Some(1)))
+  null
+  |> dynamic.from
+  |> dynamic.option(dynamic.int)
+  |> should.equal(Ok(None))
+  1
+  |> dynamic.from
+  |> dynamic.option(dynamic.string)
+  |> should.be_error
+}
+
 pub fn field_test() {
   let Ok(ok_atom) = atom.from_string("ok")
   let Ok(error_atom) = atom.from_string("error")