1 files changed, 74 insertions, 0 deletions
diff --git a/src/eval/none.rs b/src/eval/none.rs
new file mode 100644
index 00000000..ab7644a7
--- /dev/null
+++ b/src/eval/none.rs
@@ -0,0 +1,74 @@
+use std::fmt::{self, Debug, Formatter};
+
+use super::{cast, CastInfo, FromValue, IntoValue, Reflect, Value};
+use crate::diag::StrResult;
+
+/// A value that indicates the absence of any other value.
+#[derive(Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct NoneValue;
+
+impl Reflect for NoneValue {
+    fn describe() -> CastInfo {
+        CastInfo::Type("none")
+    }
+
+    fn castable(value: &Value) -> bool {
+        matches!(value, Value::None)
+    }
+}
+
+impl IntoValue for NoneValue {
+    fn into_value(self) -> Value {
+        Value::None
+    }
+}
+
+impl FromValue for NoneValue {
+    fn from_value(value: Value) -> StrResult<Self> {
+        match value {
+            Value::None => Ok(Self),
+            _ => Err(Self::error(&value)),
+        }
+    }
+}
+
+impl Debug for NoneValue {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.pad("none")
+    }
+}
+
+cast! {
+    (),
+    self => Value::None,
+    _: NoneValue => (),
+}
+
+impl<T: Reflect> Reflect for Option<T> {
+    fn describe() -> CastInfo {
+        T::describe() + NoneValue::describe()
+    }
+
+    fn castable(value: &Value) -> bool {
+        NoneValue::castable(value) || T::castable(value)
+    }
+}
+
+impl<T: IntoValue> IntoValue for Option<T> {
+    fn into_value(self) -> Value {
+        match self {
+            Some(v) => v.into_value(),
+            None => Value::None,
+        }
+    }
+}
+
+impl<T: FromValue> FromValue for Option<T> {
+    fn from_value(value: Value) -> StrResult<Self> {
+        match value {
+            Value::None => Ok(None),
+            v if T::castable(&v) => Ok(Some(T::from_value(v)?)),
+            _ => Err(Self::error(&value)),
+        }
+    }
+}