BAM Weblog

Roy - Deep Pattern Matching

Brian McKenna — 2011-10-09

Roy now has deep pattern matching. That means you can write things like:

let xor e = match e
  case (Left (Left n)) = 0
  case (Left (Right n)) = n
  case (Right (Left n)) = n
  case (Right (Right n)) = 0

console.log (xor (Left (Left 100)))
console.log (xor (Left (Right 100)))
console.log (xor (Right (Left 100)))
console.log (xor (Right (Right 100)))

Which wil output:


Here’s some code I wrote while trying to break it:

let x a = match a
  case (Some (Some (Some s))) = s
  case (Some (Some None)) = 0
  case (Some None) = 0
  case None = 0

// Doesn't compile:
// console.log (x (Some (Some (Some None))))

console.log (x (Some (Some (Some 42))))
console.log (x (Some (Some None)))
console.log (x (Some None))
console.log (x None)

data Tuple a b c = Tuple2 a b | Tuple3 a b c

let addTuple t = match t
  case (Tuple3 a (Tuple3 b c d) e) = b + c + d
  case (Tuple3 a (Tuple2 b c) d) = b + c
  case (Tuple2 a (Tuple3 b c d)) = b + c + d
  case (Tuple2 a (Tuple2 b c) d) = b + c

console.log (addTuple (Tuple3 "addTuple" (Tuple2 1 2) "addTuple"))
console.log (addTuple (Tuple3 "addTuple" (Tuple3 1 2 3) "adsf"))
console.log (addTuple (Tuple2 "addTuple" (Tuple2 1 2)))
console.log (addTuple (Tuple2 "addTuple" (Tuple3 1 2 3)))

Both functions broke the type checking but now work.

I’ve been working on just deep matching for the past week. I really need to start proving parts of Roy’s type system instead of just writing it in JavaScript and hoping it works (and makes sense).

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