To augment Dave’s answer, here is a translation schema for ‘for-comprehensions’ from Scala language specification: A comprehension for (enums) yield e evaluates expression e for each binding generated by the enumerators enums. An enumerator sequence always starts with a generator; this can be followed by further generators, value definitions, or guards. A generator p <- … Read more
From the Scala docs: Note: the difference between c filter p and c withFilter p is that the former creates a new collection, whereas the latter only restricts the domain of subsequent map, flatMap, foreach, and withFilter operations. So filter will take the original collection and produce a new collection, but withFilter will non-strictly (i.e. … Read more
The following suggestion is not using a for-comprehension. But I don’t think it’s a good idea after all, because as you noticed you’d be tied to a certain length of your cartesian product. scala> def cartesianProduct[T](xss: List[List[T]]): List[List[T]] = xss match { | case Nil => List(Nil) | case h :: t => for(xh <- … Read more
As I already said in the other topic, scalac -print prints out scala code, not java. It translates all scala keywords that are not directly compatible with java to normal scala code. It is not possible to let the compiler translate only parts afaik. But basically a for-comprehension is always translated the same way. A … Read more
TL;DR go directly to the final example I’ll try and recap. Definitions The for comprehension is a syntax shortcut to combine flatMap and map in a way that’s easy to read and reason about. Let’s simplify things a bit and assume that every class that provides both aforementioned methods can be called a monad and … Read more
For comprehensions are converted into calls to the map or flatMap method. For example this one: for(x <- List(1) ; y <- List(1,2,3)) yield (x,y) becomes that: List(1).flatMap(x => List(1,2,3).map(y => (x,y))) Therefore, the first loop value (in this case, List(1)) will receive the flatMap method call. Since flatMap on a List returns another List, … Read more
If you’re willing to use Scalaz, it has a handful of tools that make this kind of task more convenient, including a new Validation class and some useful right-biased type class instances for plain old scala.Either. I’ll give an example of each here. Accumulating errors with Validation First for our Scalaz imports (note that we … Read more