[fine] Suggestion for improvement in match

No special keyword. Exhaustivity is harder.

fine/tests/expression/alternates.fine

@@ -8,9 +8,9 @@ class Bar {
 }
 
 fun extract_value(v: Foo or Bar) -> f64 {
-  if match v as Foo {
+  if v is Foo {
     v.x // Magic type binding!
-  } else as Bar {
+  } else if v is Bar {
     v.y // Same magic re-binding of the variable to the new type
   } // No error; exhaustivity analysis should work.
 }
@@ -34,7 +34,7 @@ class Monster {
 fun print(x:string) {}
 
 fun in_range(weapon: MeleeWeapon or RangedWeapon, distance: f64) {
-  if match weapon as w : RangedWeapon {
+  if weapon is w : RangedWeapon {
     distance >= w.minRange and distance <= w.maxRange
   } else {
     distance == 1
@@ -42,26 +42,29 @@ fun in_range(weapon: MeleeWeapon or RangedWeapon, distance: f64) {
 }
 
 fun attack(weapon: MeleeWeapon or RangedWeapon, monster: Monster, distance: f64) {
-  // This is worse but it works.
+  // This is worse than Bob's final version but but it works. `is` operator
-  if match weapon as MeleeWeapon and distance > 1 or
+  // should be the same precedence as `and` and left-associative, so the
-     match weapon as w:RangedWeapon and (distance < w.minRange or distance > w.maxRange) {
+  // `and` that follows the declaration in the `is` still has the variables
+  // from the `is` binding in scope.
+  if weapon is MeleeWeapon and distance > 1 or
+     weapon is 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
+  // Bob says he doesn't want to do flow analysis and we're not, we're using
-  // tricks with variables and scoping here, with one *big* bit of magic which
+  // scoping rules and associativity to make the re-bindings work.
-  // is...
   //
-  // NOTE: special syntax here: `identifier` as `TypeExpression` ALMOST means
+  // NOTE: special syntax here: `identifier` is `TypeExpression` ALMOST means
-  //       `identifier as identifier : TypeExpression` as the shorthand for checking
+  //       `identifier is 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 {
+  //
+  // TODO: What do we do about captured variables?
+  //
+  let damage = if weapon is w: MeleeWeapon {
     roll_dice(w.damage)
-  } else as w: RangedWeapon {
+  } else if weapon is 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
   };
 
@@ -75,7 +78,7 @@ fun attack(weapon: MeleeWeapon or RangedWeapon, monster: Monster, distance: f64)
 }
 
 fun more_examples(weapon: MeleeWeapon or RangedWeapon) -> f64 or () {
-  if match weapon as w: RangedWeapon and w.maxRange > 10 {
+  if weapon is 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
@@ -105,20 +108,20 @@ fun test() -> f64 {
 
   // A single step of an iterator...
   let it = new Iterator { current: 0 };
-  if match it.next() as v: f64 {
+  if it.next() is 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 {
+  while it.next() is v: f64 {
     sum = sum + v;
   }
 
   // Unroll by hand...
   let it = new Iterator { current: 0 };
   loop {
-    if match it.next() as v: f64 {
+    if it.next() is v: f64 {
       sum = sum + v;
     } else {
       return sum;