对于 Haskell 中的列表理解，Scala 中的等价物？

Question

I'm reading the Haskell book "Learn You a Haskell for Great Good!". Chapter 2 explains list comprehension with this little example:

boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]   

Can somebody re-written this list comprehension in Scala, please? Scala has no even or odd function? So i must use

x%2!=0     

for check if the number odd?

Thanks in advance for an elegant solution!

Answer 1

Even if Scala has no even or odd function in its standard library (which I am unsure of), it is trivial to implement either. Assuming this (to keep it closest to the original Haskell version), the Scala code may look like

val boomBangs = for {
  x <- xs
  if odd x
} yield if (x < 10) "BOOM!" else "BANG!"

Disclaimer: I couldn't compile or test it for the time being, so no guarantees that it works as is.

Answer 2

As an alternative to for-comprehensions, here is a solution based on filter and map :

def odd(x: Int) = x % 2 == 1

def boomBangs(xs: Seq[Int]) =
  xs filter odd map {i => if (i < 10) "BOOM!" else "BANG!"}

boomBangs(3 :: 4 :: 5 :: 10 :: 11 :: 12 :: 13 :: Nil)
  // List(BOOM!, BOOM!, BANG!, BANG!)

For-comprehensions actually get translated into withFilter , map and flatMap expressions by the compiler.