[fine] Increasingly elaborate alternate types

fine/src/semantics.rs

@@ -148,7 +148,7 @@ pub enum Type {
     Object(TreeRef, Rc<str>),
 
     // An alternate is one or another type.
-    Alternate(Box<Type>, Box<Type>),
+    Alternate(Box<[Type]>),
 }
 
 impl Type {
@@ -158,6 +158,26 @@ impl Type {
             _ => false,
         }
     }
+
+    fn discriminant_number(&self) -> i8 {
+        match self {
+            Type::Error => 0,
+            Type::Unreachable => 1,
+            Type::Assignment(_) => 2,
+            Type::TypeVariable(_) => 3,
+            Type::Nothing => 4,
+            Type::F64 => 5,
+            Type::I64 => 6,
+            Type::String => 7,
+            Type::Bool => 8,
+            Type::Function(_, _) => 9,
+            Type::Method(_, _, _) => 10,
+            Type::List(_) => 11,
+            Type::Class(_, _) => 12,
+            Type::Object(_, _) => 13,
+            Type::Alternate(_) => 14,
+        }
+    }
 }
 
 impl fmt::Debug for Type {
@@ -211,7 +231,70 @@ impl fmt::Display for Type {
             List(t) => write!(f, "list<{t}>"),
             Object(_, name) => write!(f, "{} instance", name),
             Class(_, name) => write!(f, "class {}", name),
-            Alternate(l, r) => write!(f, "{l} or {r}"),
+            Alternate(ts) => {
+                let mut first = true;
+                for t in ts.iter() {
+                    if !first {
+                        write!(f, " or ")?;
+                    }
+                    write!(f, "{t}")?;
+                    first = false;
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
+impl std::cmp::PartialEq for Type {
+    fn eq(&self, other: &Self) -> bool {
+        self.cmp(other).is_eq()
+    }
+}
+
+impl std::cmp::Eq for Type {}
+
+impl std::cmp::PartialOrd for Type {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl std::cmp::Ord for Type {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        use std::cmp::Ordering;
+
+        let li = self.discriminant_number();
+        let ri = other.discriminant_number();
+        if li < ri {
+            Ordering::Less
+        } else if li > ri {
+            Ordering::Greater
+        } else {
+            match (self, other) {
+                (Type::Assignment(x), Type::Assignment(y)) => x.cmp(y),
+
+                // NOTE: This is wrong! Type variables *cannot* be compared
+                //       like this without binding information.
+                (Type::TypeVariable(x), Type::TypeVariable(y)) => x.index().cmp(&y.index()),
+
+                (Type::Function(la, lr), Type::Function(ra, rr)) => {
+                    let lv = (la, lr);
+                    let rv = (ra, rr);
+                    lv.cmp(&rv)
+                }
+                (Type::Method(lo, la, lr), Type::Method(ro, ra, rr)) => {
+                    let lv = (lo, la, lr);
+                    let rv = (ro, ra, rr);
+                    lv.cmp(&rv)
+                }
+                (Type::List(x), Type::List(y)) => x.cmp(y),
+                (Type::Class(lt, _), Type::Class(rt, _)) => lt.index().cmp(&rt.index()),
+                (Type::Object(lt, _), Type::Object(rt, _)) => lt.index().cmp(&rt.index()),
+                (Type::Alternate(ll), Type::Alternate(rr)) => ll.cmp(rr),
+
+                _ => Ordering::Equal,
+            }
         }
     }
 }
@@ -1070,6 +1153,34 @@ impl<'a> Semantics<'a> {
         result
     }
 
+    pub fn build_alternate(&self, left: &Type, right: &Type) -> Type {
+        if left.is_error() {
+            left.clone()
+        } else if right.is_error() {
+            right.clone()
+        } else if self.can_convert(left, right) {
+            right.clone()
+        } else if self.can_convert(right, left) {
+            left.clone()
+        } else {
+            let mut types: Vec<Type> = Vec::new();
+            if let Type::Alternate(ts) = left {
+                types.extend_from_slice(&*ts);
+            } else {
+                types.push(left.clone());
+            }
+            if let Type::Alternate(ts) = right {
+                types.extend_from_slice(&*ts);
+            } else {
+                types.push(right.clone());
+            }
+            types.sort();
+            types.dedup();
+
+            Type::Alternate(types.into())
+        }
+    }
+
     pub fn can_convert(&self, from: &Type, to: &Type) -> bool {
         // TODO: This is wrong; we because of numeric literals etc.
         match (from, to) {
@@ -1105,8 +1216,22 @@ impl<'a> Semantics<'a> {
             (Type::Unreachable, _) => true,
 
             // Alternates convert if either side can convert.
-            (_, Type::Alternate(left, right)) => {
-                self.can_convert(from, left) || self.can_convert(from, right)
+            (Type::Alternate(lts), Type::Alternate(_)) => {
+                for lt in lts.iter() {
+                    if !self.can_convert(lt, to) {
+                        return false;
+                    }
+                }
+                true
+            }
+
+            (_, Type::Alternate(rts)) => {
+                for rt in rts.iter() {
+                    if self.can_convert(from, rt) {
+                        return true;
+                    }
+                }
+                false
             }
 
             // TODO: Unification on type variables! :D
@@ -1948,7 +2073,7 @@ impl<'a> Semantics<'a> {
         // TODO: IDEA: nth_tree returns a bogus tree if not a tree, stop returning Option?
         let left = self.type_of(tree.nth_tree(0)?);
         let right = self.type_of(tree.nth_tree(2)?);
-        Some(Type::Alternate(Box::new(left), Box::new(right)))
+        Some(self.build_alternate(&left, &right))
     }
 
     fn type_of_match_expression(&self, tree: &Tree) -> Option<Type> {