Scala's monadic chaining of Try

Question

Consider the following chaining of function f, g and h using monadic for-comprehensions.

  for {
    x <- List ( 11, 22, 33, 44, 55 )
    y <- f ( x )
    z <- g ( y )
    a <- h ( z )
  } yield a

If f, g and h all have the signature:

  Int => Option [ Int ] 

then for-comprehension compiles fine. However if I replace Option [ Int ] by Try [ Int ], Scala's type-inferencer complains about the line

  y <- f ( x )

with the following error message.

  error: type mismatch;
  found   : scala.util.Try[Int]
  required: scala.collection.GenTraversableOnce[?]
      y <- f ( x )

Why? Both Option [ _ ] and Try [ _ ] are (or should be) monads, and should work smoothly as sketched.

Answer 1

You can only use monads of the same kind in a for comprehension. In this case all of your values have to be GenTraversableOnce, because the first one is. It works with Option, because there is an implicit conversion from Option to Seq, but this is not possible for Try.