feat: add sum method to Outcome

01.workspace/oxidice_lib/src/imp/mod.rs

@@ -2,3 +2,4 @@ pub mod die_new;
 pub mod die_roll;
 pub mod handful_grab;
 pub mod handful_roll;
+pub mod outcome_sum;

01.workspace/oxidice_lib/src/imp/outcome_sum.rs

@@ -0,0 +1,37 @@
+use crate::*;
+
+fn sum_of(values: Vec<u16>) -> Result<Outcome, FailedTo> {
+    if values.is_empty() {
+        return Ok(Outcome::Scalar(0));
+    }
+    let mut ret = Ok(Outcome::Scalar(0));
+    for value in values {
+        ret = ret.and_then(|outcome: Outcome| match outcome {
+            Outcome::List(_) => Err(FailedTo::ProcessInput),
+            Outcome::Scalar(acc) => acc
+                .checked_add(value)
+                .map(|new_acc: u16| Outcome::Scalar(new_acc))
+                .ok_or(FailedTo::SumValues),
+        })
+    }
+    ret
+}
+
+impl Outcome {
+    pub fn sum(self) -> Result<Outcome, FailedTo> {
+        match self {
+            Outcome::Scalar(value) => Ok(Outcome::Scalar(value)),
+            Outcome::List(values) => sum_of(values),
+        }
+    }
+}
+
+#[cfg(test)]
+mod unit_tests {
+    use super::*;
+    #[test]
+    fn check_sum() {
+        let outcome = Handful::grab(5, 1).roll().sum();
+        assert_eq!(outcome, Ok(Outcome::Scalar(5)));
+    }
+}