Best syntax for recovering from a for understanding
I have a number of functions that return a future that is the result of understanding, but I need to recover from some possible glitches on the way out. The standard syntax seems to capture comprehension for intermediate results as follows:
def fooBar(): Future[String] = {
val x = for {
x <- foo()
y <- bar(x)
} yield y
x.recover {
case SomeException() => "bah"
}
}
The best alternative I have found is to wrap the integer in parentheses for understanding:
def fooBar(): Future[String] = (for {
x <- foo()
y <- bar(x)
} yield y).recover {
case SomeException() => "bah"
}
This is more like a shortcut than a syntax improvement, so I'm wondering if there is a better way to weave the recovery for understanding?
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2 answers
Some curly brace adjustment helps, although some people prefer curly braces instead of parens for a multi-line expression:
scala> def f = (
| for {
| x <- foo;
| y <- bar(x)
| } yield y
| ) recover {
| case _: NullPointerException => -1
| }
f: scala.concurrent.Future[Int]
if you don't like
scala> foo flatMap bar recover { case _: NullPointerException => -1 }
res9: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@3efe7086
You can use all syntaxes:
object Test extends App {
import concurrent._
import duration.Duration._
import ExecutionContext.Implicits._
type ~>[A, B] = PartialFunction[A, B]
type NPE = NullPointerException
class `recovering future`[A, R >: A](val f: Future[A], val pf: Throwable ~> R) {
def map[B >: A <: R](m: A => B) = new `recovering future`[B, R](f map m, pf)
def flatMap[B >: A <: R](m: A => Future[B]) = new `recovering future`[B, R](f flatMap m, pf)
def recovered: Future[R] = f recover pf
}
object `recovering future` {
implicit def `back to the future`[A, R >: A](x: `recovering future`[A, R]): Future[R] = x.recovered
}
implicit class `inline recoverer`[A](val f: Future[A]) {
def recovering[B >: A](pf: Throwable ~> B) = new `recovering future`(f, pf)
}
def f = Future(8)
def g(i: Int) = Future(42 + i)
def e(i: Int): Future[Int] = Future((null: String).length)
embellishment:
for {
x <- f
y <- g(x)
} Console println y // 50
And with inlined recovery:
def compute: Future[Int] =
for {
x <- f recovering { case _: NPE => -1 }
y <- g(x)
} yield y
Console println (Await result (compute, Inf)) // 50
Or showing a bad case:
def fail: Future[Int] =
for {
x <- f recovering { case _: NPE => -1 }
y <- e(x)
} yield y
Console println (Await result (fail, Inf)) // -1
}
if you rock this way.
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