Try monad in Java 8

Question

Is there a built-in support for monad that deals with exception handling? Something similar to Scala\'s Try. I am asking because I don\'t like unchecked exceptions.

Answer 1

@Misha is onto something. Obviously you wouldn't do this exact thing in real code, but CompletableFuture provides Haskell-style monads like this:

• return maps to CompletableFuture.completedFuture
• >= maps to thenCompose

So you could rewrite @Misha's example like this:

CompletableFuture.completedFuture(new Scanner(System.in)).thenCompose(scanner ->
CompletableFuture.completedFuture(scanner.nextInt()).thenCompose(divident ->
CompletableFuture.completedFuture(scanner.nextInt()).thenCompose(divisor ->
CompletableFuture.completedFuture(divident / divisor).thenCompose(val -> {
   System.out.printf("%s/%s = %s%n", divident, divisor, val);
   return null;
}))));

which maps to the Haskell-ish:

(return (newScanner SystemIn)) >>= \scanner ->
(return (nextInt scanner)) >>= \divident ->
(return (nextInt scanner)) >>= \divisor ->
(return (divident / divisor)) >>= \val -> do
   SystemOutPrintf "%s/%s = %s%n" divident divisor val
   return Null

or with do syntax

do
   scanner <- return (newScanner SystemIn)
   divident <- return (nextInt scanner)
   divisor <- return (nextInt scanner)
   val <- return (divident / divisor)
   do
       SystemOutPrintf "%s/%s = %s%n" divident divisor val
       return Null

Implementations of fmap and join

I got a little carried away. These are the standard fmap and join implemented in terms of CompletableFuture:

 CompletableFuture fmap(Function f, CompletableFuture m) {
   return m.thenCompose(x -> CompletableFuture.completedFuture(f.apply(x)));
}

 CompletableFuture join(CompletableFuture> n) {
   return n.thenCompose(x -> x);
}