Scala: Can there be any reason to prefer `filter+map` over `collect`?

Question

Can there be any reason to prefer filter+map:

list.filter (i => aCondition(i)).map(i => fun(i))

over collect?

Answer 1

I guess this is rather opinion based, but given the following definitions:

scala> val l = List(1,2,3,4)
l: List[Int] = List(1, 2, 3, 4)

scala> def f(x: Int) = 2*x
f: (x: Int)Int

scala> def p(x: Int) = x%2 == 0
p: (x: Int)Boolean

Which of the two do you find nicer to read:

l.filter(p).map(f)

or

l.collect{ case i if p(i) => f(i) }

(Note that I had to fix your syntax above, as you need the bracket and case to add the if condition).

I personally find the filter+map much nicer to read and understand. It's all a matter of the exact syntax that you use, but given p and f, you don't have to write anonymous functions when using filter or map, while you do need them when using collect.

You also have the possibility to use flatMap:

l.flatMap(i => if(p(i)) Some(f(i)) else None)

Which is likely to be the most efficient among the 3 solutions, but I find it less nice than map and filter.

Overall, it's very difficult to say which one would be faster, as it depends a lot of which optimizations end up being performed by scalac and then the JVM. All 3 should be pretty close, and definitely not a factor in deciding which one to use.