Scala Language For Expressions Monadic for comprehensions
Example
If you have several objects of monadic types, we can achieve combinations of the values using a 'for comprehension':
for {
x <- Option(1)
y <- Option("b")
z <- List(3, 4)
} {
// Now we can use the x, y, z variables
println(x, y, z)
x // the last expression is *not* the output of the block in this case!
}
// This prints
// (1, "b", 3)
// (1, "b", 4)
The return type of this block is Unit.
If the objects are of the same monadic type M (e.g. Option) then using yield will return an object of type M instead of Unit.
val a = for {
x <- Option(1)
y <- Option("b")
} yield {
// Now we can use the x, y variables
println(x, y)
// whatever is at the end of the block is the output
(7 * x, y)
}
// This prints:
// (1, "b")
// The val `a` is set:
// a: Option[(Int, String)] = Some((7,b))
Note that the yield keyword cannot be used in the original example, where there is a mix of monadic types (Option and List). Trying to do so will yield a compile-time type mismatch error.
