BAM Weblog

Roy - Simple Pattern Matching

Brian McKenna — 2011-06-05

This week I worked on adding simple tagged unions (sum types) and pattern matching to Roy. The diff is fairly disgusting but I’m happy that it works. Here is the example I committed:

data Maybe a =
  Some a | None

let printSomething m = match m
  case (Some x) = console.log x
  case None = console.log "Empty"

printSomething (Some 1)
printSomething None

data Either a b =
  Left a | Right b

let printString (s : String) =
  console.log s

let printResult e = match e
  case (Left x) = console.log x
  case (Right x) = printString x

printResult (Left 10)
printResult (Right "Error")

// Won't compile:
// printResult (Right 10)

data Bool =
  True | False

let getBool b ifTrue ifFalse = match b
  case True = ifTrue
  case False = ifFalse

console.log (getBool True 1 2)
console.log (getBool False 1 2)

Running the above example (with ./roy examples/data.roy && node examples/data.js) will print the following:


The current version of pattern matching has no exhaustiveness check. I’ll try to figure out how to do that later on.

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