What is Scala's yield?

Question

I understand Ruby and Python's yield. What does Scala's yield do?

Answer 1

I think the accepted answer is great, but it seems many people have failed to grasp some fundamental points.

First, Scala's for comprehensions are equivalent to Haskell's do notation, and it is nothing more than a syntactic sugar for composition of multiple monadic operations. As this statement will most likely not help anyone who needs help, let's try again… :-)

Scala's for comprehensions is syntactic sugar for composition of multiple operations with map, flatMap and filter. Or foreach. Scala actually translates a for-expression into calls to those methods, so any class providing them, or a subset of them, can be used with for comprehensions.

First, let's talk about the translations. There are very simple rules:

1. This

   for(x <- c1; y <- c2; z <-c3) {...}
   

   is translated into

   c1.foreach(x => c2.foreach(y => c3.foreach(z => {...})))
   

2. This

   for(x <- c1; y <- c2; z <- c3) yield {...}
   

   is translated into

   c1.flatMap(x => c2.flatMap(y => c3.map(z => {...})))
   

3. This

   for(x <- c; if cond) yield {...}
   

   is translated on Scala 2.7 into

   c.filter(x => cond).map(x => {...})
   

   or, on Scala 2.8, into

   c.withFilter(x => cond).map(x => {...})
   

   with a fallback into the former if method withFilter is not available but filter is. Please see the section below for more information on this.

4. This

   for(x <- c; y = ...) yield {...}
   

   is translated into

   c.map(x => (x, ...)).map((x,y) => {...})
   

When you look at very simple for comprehensions, the map/foreach alternatives look, indeed, better. Once you start composing them, though, you can easily get lost in parenthesis and nesting levels. When that happens, for comprehensions are usually much clearer.

I'll show one simple example, and intentionally omit any explanation. You can decide which syntax was easier to understand.

l.flatMap(sl => sl.filter(el => el > 0).map(el => el.toString.length))

or

for {
  sl <- l
  el <- sl
  if el > 0
} yield el.toString.length

---

withFilter

Scala 2.8 introduced a method called withFilter, whose main difference is that, instead of returning a new, filtered, collection, it filters on-demand. The filter method has its behavior defined based on the strictness of the collection. To understand this better, let's take a look at some Scala 2.7 with List (strict) and Stream (non-strict):

scala> var found = false
found: Boolean = false

scala> List.range(1,10).filter(_ % 2 == 1 && !found).foreach(x => if (x == 5) found = true else println(x))
1
3
7
9

scala> found = false
found: Boolean = false

scala> Stream.range(1,10).filter(_ % 2 == 1 && !found).foreach(x => if (x == 5) found = true else println(x))
1
3

The difference happens because filter is immediately applied with List, returning a list of odds -- since found is false. Only then foreach is executed, but, by this time, changing found is meaningless, as filter has already executed.

In the case of Stream, the condition is not immediatelly applied. Instead, as each element is requested by foreach, filter tests the condition, which enables foreach to influence it through found. Just to make it clear, here is the equivalent for-comprehension code:

for (x <- List.range(1, 10); if x % 2 == 1 && !found) 
  if (x == 5) found = true else println(x)

for (x <- Stream.range(1, 10); if x % 2 == 1 && !found) 
  if (x == 5) found = true else println(x)

This caused many problems, because people expected the if to be considered on-demand, instead of being applied to the whole collection beforehand.

Scala 2.8 introduced withFilter, which is always non-strict, no matter the strictness of the collection. The following example shows List with both methods on Scala 2.8:

scala> var found = false
found: Boolean = false

scala> List.range(1,10).filter(_ % 2 == 1 && !found).foreach(x => if (x == 5) found = true else println(x))
1
3
7
9

scala> found = false
found: Boolean = false

scala> List.range(1,10).withFilter(_ % 2 == 1 && !found).foreach(x => if (x == 5) found = true else println(x))
1
3

This produces the result most people expect, without changing how filter behaves. As a side note, Range was changed from non-strict to strict between Scala 2.7 and Scala 2.8.