Why does a Scala for-comprehension have to start with a generator? - scala

Question

According to the Scala Language Specification (§6.19), "An enumerator sequence always starts with a generator". Why?
I sometimes find this restriction to be a hindrance when using for-comprehensions with monads, because it means you can't do things like this:
def getFooValue(): Future[Int] = {
for {
manager = Manager.getManager() // could throw an exception
foo <- manager.makeFoo() // method call returns a Future
value = foo.getValue()
} yield value
}
Indeed, scalac rejects this with the error message '<-' expected but '=' found.
If this was valid syntax in Scala, one advantage would be that any exception thrown by Manager.getManager() would be caught by the Future monad used within the for-comprehension, and would cause it to yield a failed Future, which is what I want. The workaround of moving the call to Manager.getManager() outside the for-comprehension doesn't have this advantage:
def getFooValue(): Future[Int] = {
val manager = Manager.getManager()
for {
foo <- manager.makeFoo()
value = foo.getValue()
} yield value
}
In this case, an exception thrown by foo.getValue() will yield a failed Future (which is what I want), but an exception thrown by Manager.getManager() will be thrown back to the caller of getFooValue() (which is not what I want). Other possible ways of handling the exception are more verbose.
I find this restriction especially puzzling because in Haskell's otherwise similar do notation, there is no requirement that a do block should begin with a statement containing <-. Can anyone explain this difference between Scala and Haskell?
Here's a complete working example showing how exceptions are caught by the Future monad in for-comprehensions:
import scala.concurrent._
import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.util.{Try, Success, Failure}
class Foo(val value: Int) {
def getValue(crash: Boolean): Int = {
if (crash) {
throw new Exception("failed to get value")
} else {
value
}
}
}
class Manager {
def makeFoo(crash: Boolean): Future[Foo] = {
if (crash) {
throw new Exception("failed to make Foo")
} else {
Future(new Foo(10))
}
}
}
object Manager {
def getManager(crash: Boolean): Manager = {
if (crash) {
throw new Exception("failed to get manager")
} else {
new Manager()
}
}
}
object Main extends App {
def getFooValue(crashGetManager: Boolean,
crashMakeFoo: Boolean,
crashGetValue: Boolean): Future[Int] = {
for {
manager <- Future(Manager.getManager(crashGetManager))
foo <- manager.makeFoo(crashMakeFoo)
value = foo.getValue(crashGetValue)
} yield value
}
def waitForValue(future: Future[Int]): Unit = {
val result = Try(Await.result(future, Duration("10 seconds")))
result match {
case Success(value) => println(s"Got value: $value")
case Failure(e) => println(s"Got error: $e")
}
}
val future1 = getFooValue(false, false, false)
waitForValue(future1)
val future2 = getFooValue(true, false, false)
waitForValue(future2)
val future3 = getFooValue(false, true, false)
waitForValue(future3)
val future4 = getFooValue(false, false, true)
waitForValue(future4)
}
Here's the output:
Got value: 10
Got error: java.lang.Exception: failed to get manager
Got error: java.lang.Exception: failed to make Foo
Got error: java.lang.Exception: failed to get value
This is a trivial example, but I'm working on a project in which we have a lot of non-trivial code that depends on this behaviour. As far as I understand, this is one of the main advantages of using Future (or Try) as a monad. What I find strange is that I have to write
manager <- Future(Manager.getManager(crashGetManager))
instead of
manager = Manager.getManager(crashGetManager)
(Edited to reflect #RexKerr's point that the monad is doing the work of catching the exceptions.)

Answer 1

for comprehensions do not catch exceptions. Try does, and it has the appropriate methods to participate in for-comprehensions, so you can
for {
manager <- Try { Manager.getManager() }
...
}
But then it's expecting Try all the way down unless you manually or implicitly have a way to switch container types (e.g. something that converts Try to a List).
So I'm not sure your premises are right. Any assignment you made in a for-comprehension can just be made early.
(Also, there is no point doing an assignment inside a for comprehension just to yield that exact value. Just do the computation in the yield block.)
(Also, just to illustrate that multiple types can play a role in for comprehensions so there's not a super-obvious correct answer for how to wrap an early assignment in terms of later types:
// List and Option, via implicit conversion
for {i <- List(1,2,3); j <- Option(i).filter(_ <2)} yield j
// Custom compatible types with map/flatMap
// Use :paste in the REPL to define A and B together
class A[X] { def flatMap[Y](f: X => B[Y]): A[Y] = new A[Y] }
class B[X](x: X) { def map[Y](f: X => Y): B[Y] = new B(f(x)) }
for{ i <- (new A[Int]); j <- (new B(i)) } yield j.toString
Even if you take the first type you still have the problem of whether there is a unique "bind" (way to wrap) and whether to doubly-wrap things that are already the correct type. There could be rules for all these things, but for-comprehensions are already hard enough to learn, no?)

Answer 2

Haskell translates the equivalent of for { manager = Manager.getManager(); ... } to the equivalent of lazy val manager = Manager.getManager(); for { ... }. This seems to work:
scala> lazy val x: Int = throw new Exception("")
x: Int = <lazy>
scala> for { y <- Future(x + 1) } yield y
res8: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise#fedb05d
scala> Try(Await.result(res1, Duration("10 seconds")))
res9: scala.util.Try[Int] = Failure(java.lang.Exception: )

Answer 3

I think the reason this can't be done is because for-loops are syntactic sugar for flatMap and map methods (except if you are using a condition in the for-loop, in that case it's desugared with the method withFilter). When you are storing in a immutable variable, you can't use these methods. That's the reason you would be ok using Try as pointed out by Rex Kerr. In that case, you should be able to use map and flatMap methods.