1 files changed, 14 insertions, 3 deletions

diff --git a/src/map.rs b/src/map.rs
index 77b2feb..c1294bf 100644
--- a/src/map.rs
+++ b/src/map.rs
@@ -27,7 +27,10 @@ impl Roadmap {
 
     // Find steps that nothing depends on.
     fn goals(&self) -> Vec<&Step> {
-        self.steps.iter().filter(|step| self.is_goal(step)).collect()
+        self.steps
+            .iter()
+            .filter(|step| self.is_goal(step))
+            .collect()
     }
 
     /// Count number of steps that nothing depends on.
@@ -125,10 +128,18 @@ impl Roadmap {
     // Validate that the parsed, constructed roadmap is valid.
     pub fn validate(&self) -> RoadmapResult<()> {
         // Is there exactly one goal?
-        match self.count_goals() {
+        let goals = self.goals();
+        let n = goals.len();
+        match n {
             0 => return Err(format!("the roadmap doesn't have a goal").into()),
             1 => (),
-            _ => return Err(format!("must have exactly one goal for roadmap").into()),
+            _ => {
+                let mut names: Vec<&str> = goals.iter().map(|s| s.name()).collect();
+                names.sort();
+                let names = names.join(", ");
+                let msg = format!("only one goal allowed, found {}: {}", n, names);
+                return Err(msg.into());
+            }
         }
 
         // Does every dependency exist?