Scala for comprehension translation helper

January 22, 2015

“For comprehension” is a another syntaxe to use map, flatMap and withFilter (or filter) methods.

yield keyword is used to aggregate values in the resulting structure.

This composition can be used on any “monadic” type implementing this methods, like List, Option, Future, etc.

Case 1

case class Book(author: String, title: String)
for {
  book <- books
  if book.author startsWith "bob" 
} yield book.title

is the same as

books.withFilter(book => book.author startsWith "bob").map(book => book.title)

Case 2

case class Book(authors: List[String], title: String)
for {
  book <- books
  author <- book.authors 
  if author startsWith "bob" 
} yield book.title

is the same as

books.flatMap(book => book.authors.withFilter(author => author startsWith "bob").map(author => book.title))

Case 3

val optA : Option[String] = Some("a value")
val optB : Option[String] = Some("b value")

I want an option tuple : Option[(String, String) composing this two options, i.e. Some(("a value","b value")) :

for {
     a <- optA
     b <- optB
} yield (a,b)

is the same as

optA.flatMap(a => optB.map(b => (a, b)))

You can also filter options with if/withFilter :

for {
  a <- optA
  if(a startsWith "a")
  b <- optB
} yield (a,b)

Or

optA.withFilter(a=> a startsWith "a").flatMap(a => optB.map(b => (a, b)))

If you change “a” to “c” in the condition, the resulting value will be None instead of Some(("a value","b value")).

Case 4

You can mix options and list types in map/flatMap (Option can be seen as a simple collection) :

val optNumbers = List(Some(1), Some(2), None, Some(3))

We want to remove empty values and increment other values : List(2, 3, 4) :

for {
  optNumber <- optNumbers
  value <- optNumber
} yield value +1

is the same as

optNumbers.flatMap(optNumber => optNumber.map(value => value+1))

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