斯卡拉的尝试的monadic链接

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 .