[fine] Notes on patterns

fine/design.md

@@ -40,6 +40,78 @@ immediately following the `new` keyword is always a type expression,
 so we know that e.g. `<` or whatever means "generic type parameter"
 and not "less than".
 
+## Patterns and Alternate Types
+
+Instead of `enums` or inheritance, we're using an alternate type like
+in Typescript, with `or` between types. See the `alternates.fine` test
+for a work-up of the types and syntax and whatnot. I think it works
+pretty well.
+
+Actually using alternate types involves pattern matching, either one
+at a time, with the `is` operator, or in bulk, with the `match`
+expression. `match` can check for completeness, but `if/is` cannot.
+
+Patterns are VERY simple, and are explicitly *not* destructuring right
+now. (Destructuring brings up a whole lot of complexity that I don't
+want to deal with.)
+
+Patterns are basically:
+
+```
+(identifier ":")? type_expression ("and" <expression>)?
+```
+
+The identifier at the front represents a binding of the value being
+considered as if it were of the same type as the type expression; the
+identifier is in scope for the optional predicate after the "and" and
+so you can use it as if it were the type because, well, that part of
+the pattern already matched.
+
+As a special case, the identifier is *also* in scope for the body of
+an `if` expression when an `is` expression is used as the condition.
+
+```
+  if b is c:Foo {
+    result = result + c.a; // c should still be in scope!
+  }
+```
+
+`match` is the multi-value pattern matching expression, like this:
+
+```
+  match b {
+    c:Foo -> c.a,
+    _ -> 0,
+  }
+```
+
+The special pattern `_` always evaluates to true.
+
+Note that unlike rust we do not allow variable binding, e.g., in rust you can write:
+
+```
+  match b {
+    d -> ...,
+  }
+```
+
+but in fine you need to write:
+
+```
+  match b {
+    d:_ -> ...,
+  }
+```
+
+The reason is that the rust version is ambiguous: if `d` matches some
+value already in scope (e.g., an `enum` arm) then the arm is matching
+if b == d, but if `d` is unbound then `d` becomes a variable
+declaration. This is a spooky action-at-a-distance and I don't approve
+of it.
+
+
+# Complete Garbage
+
 ## Lambdas/Closures/Anonymous Functions
 
 Looking for a syntax here; I want to keep `fun` as a declaration like