While reviewing the code for the water pouring problem in the Coursera Scala class, I came across two uses of map where I don't understand the syntax.
I'm used to:
a.map(b => b * b)
But I'm seeing:
a map something
For example:
next <- moves map path.extend
does this mean "for each next in moves, path.extend(next)"?
paths #:: from(more, explored ++ (more map (_.endstate)))
does this mean "add the endstate of more to explored"?
In scala, the following expressions are equivalent:
moves map path.extend
moves map(path.extend)
moves.map(path.extend)
moves.map(m => path.extend(m))
And the following are equivalent:
more map (_.endstate)
more.map(_.endstate)
more.map(m => m.endstate)
moves.map(path.extend) and moves.map(m => path.extend(m)) are not equivalent: def a(x: Int)(y: Int) = x + y; List(1).map(a); List(1).map(x => a(x)) (last statement won't compile)To answer your first question, I think you're correct about next <- moves map path.extend
I'm assuming you're talking about a "for comprehension". For example,
val a = List (1, 3, 5)
a.map(b => b * b)
//> res0: List[Int] = List(1, 9, 25)
for {
blah <- a map (b => b * b)
} yield blah
//> res1: List[Int] = List(1, 9, 25)
You can see these two return the same result and are equivalent in Scala.
So we're saying for every element in "a" call the function (b => b * b).
In your example it's more like this scenario:
case class Song(artists:List[String], title: String)
val songs = List(Song(List("Michael Jackson", "Janet Jackson"), "Scream"),
Song(List("Janet"), "Unbreakable"))
for {
song <- songs
artist <- song.artists
if artist startsWith "Mic"
} yield song.title
//> res0: List[String] = List(Scream)
songs.flatMap(
song => song.artists.withFilter(artist => artist startsWith "Mic").map(artist => song.title)
)
//> res1: List[String] = List(Scream)
You can see these are also equivalent but the "for-comprehension" is easier to read.
def from(paths: Set[Path], explored: Set[State]): Stream[Set[Path]] = if (paths.isEmpty) Stream.empty else { val more = for { path <- paths next <- moves map path.extend if !(explored contains next.endState) } yield next paths #:: from(more, explored ++ (more map (_.endState))) }
obj.meth(arg)to be expressed with spacesobj meth arg.