[fine] Design for "intersection" types

fine/tests/expression/alternates.fine

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+// Examples of alternate types/union types/heterogeneous types.
+class Foo {
+  x: f64;
+}
+
+class Bar {
+ y: f64;
+}
+
+fun extract_value(v: Foo or Bar) -> f64 {
+  if match v as Foo {
+    v.x // Magic type binding!
+  } else as Bar {
+    v.y // Same magic re-binding of the variable to the new type
+  } // No error; exhaustivity analysis should work.
+}
+
+// This is Bob Nystrom's example from
+// https://journal.stuffwithstuff.com/2023/08/04/representing-heterogeneous-data/
+//
+class MeleeWeapon {
+  damage: f64;
+}
+
+class RangedWeapon {
+  minRange: f64;
+  maxRange: f64;
+}
+
+class Monster {
+  health: f64;
+}
+
+fun print(x:string) {}
+
+fun in_range(weapon: MeleeWeapon or RangedWeapon, distance: f64) {
+  if match weapon as w : RangedWeapon {
+    distance >= w.minRange and distance <= w.maxRange
+  } else {
+    distance == 1
+  }
+}
+
+fun attack(weapon: MeleeWeapon or RangedWeapon, monster: Monster, distance: f64) {
+  // This is worse but it works.
+  if match weapon as MeleeWeapon and distance > 1 or
+     match weapon as w:RangedWeapon and (distance < w.minRange or distance > w.maxRange) {
+     print("You are out of range")
+     return
+  }
+
+  // Bob says he doesn't want to do flow analysis but we're doing all our
+  // tricks with variables and scoping here, with one *big* bit of magic which
+  // is...
+  //
+  // NOTE: special syntax here: `identifier` as `TypeExpression` ALMOST means
+  //       `identifier as identifier : TypeExpression` as the shorthand for checking
+  //       local variables. The *almost* part is that the effective type of the
+  //       variable changes but not the binding. (Is this what we want?)
+  let damage = if match weapon as w: MeleeWeapon {
+    roll_dice(w.damage)
+  } else as w: RangedWeapon {
+    // This is the trick here: else as re-uses the expression from the match
+    // and so we can do exhaustivity analysis.
+    w.maxRange - w.minRange
+  };
+
+  if monster.health <= damage {
+    print("You kill the monster!");
+    monster.health = 0
+  } else {
+    print("You wound the monster.");
+    monster.health = monster.health - damage
+  }
+}
+
+fun more_examples(weapon: MeleeWeapon or RangedWeapon) -> f64 or () {
+  if match weapon as w: RangedWeapon and w.maxRange > 10 {
+    // w is still in scope here; the `and` is bound into a predicate expression
+    // and breaks exhaustivity
+    w.minRange
+  }
+}
+
+// Some fun with iterators
+
+class Iterator {
+  current: f64;
+
+  fun next(self) -> f64 or () {
+    if self.current < 10 {
+      let result = self.current;
+      self.current = self.current + 1;
+      return result;
+    }
+  }
+}
+
+fun test() -> f64 {
+  let sum = 0;
+
+  // A single step of an iterator...
+  let it = new Iterator { current: 0 };
+  if match it.next() as v: f64 {
+    sum = sum + v;
+  }
+
+  // Looping by hand over an iterator is pretty clean.
+  let it = new Iterator { current: 0 };
+  while match it.next() as v: f64 {
+    sum = sum + v;
+  }
+
+  // Unroll by hand...
+  let it = new Iterator { current: 0 };
+  loop {
+    if match it.next() as v: f64 {
+      sum = sum + v;
+    } else {
+      return sum;
+    }
+  }
+
+  // Not in this test but `for` over an object should turn into something
+  // like the above.
+}
+
+// @ignore WIP
+// @no-errors