What is Scala s yield

Question

I understand Ruby and Python s yield  What does Scala s yield do

User · Answer

Yes  as Earwicker said  it s pretty much the equivalent to LINQ s select and has very little to do with Ruby s and Python s yield  Basically  where in C  you would write  from     select        in Scala you have instead   for     yield       It s also important to understand that for-comprehensions don t just work with sequences  but with any type which defines certain methods  just like LINQ    If your type defines just map  it allows for-expressions consisting of a single generator  If it defines flatMap as well as map  it allows for-expressions consisting of several generators  If it defines foreach  it allows for-loops without yield  both with single and multiple generators   If it defines filter  it allows for-filter expressions starting with an if in the for expression

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Consider the following for-comprehension  val A   for  i  lt - Int MinValue to Int MaxValue  if i  gt  3  yield i   It may be helpful to read it out loud as follows   For each integer i  if it is greater than 3  then yield  produce  i and add it to the list A    In terms of mathematical set-builder notation  the above for-comprehension is analogous to    which may be read as   For each integer   if it is greater than   then it is a member of the set     or alternatively as    is the set of all integers   such that each  is greater than

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It is used in sequence comprehensions  like Python s list-comprehensions and generators  where you may use yield too    It is applied in combination with for and writes a new element into the resulting sequence   Simple example  from scala-lang       Turn command line arguments to uppercase    object Main     def main args  Array String         val res   for  a  lt - args  yield a toUpperCase     println  Arguments      res toString          The corresponding expression in F  would be    for a in args - gt  a toUpperCase     or  from a in args select a toUpperCase    in Linq   Ruby s yield has a different effect

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yield is more flexible than map    see example below  val aList   List  1 2 3 4 5    val res3   for   al  lt - aList if al  gt  3   yield al   1  val res4   aList map     1  gt  3     println  res3   println  res4     yield will print result like   List 5  6   which is good  while map   will return result like   List false  false  true  true  true   which probably is not what you intend

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Yield is similar to for loop which has a buffer that we cannot see and for each increment  it keeps adding next item to the buffer  When the for loop finishes running  it would return the collection of all the yielded values  Yield can be used as simple arithmetic operators or even in combination with arrays  Here are two simple examples for your better understanding  scala gt for  i  lt - 1 to 5  yield i   3   res  scala collection immutable IndexedSeq Int    Vector 3  6  9  12  15   scala gt  val nums   Seq 1 2 3  nums  Seq Int    List 1  2  3   scala gt  val letters   Seq  a    b    c   letters  Seq Char    List a  b  c   scala gt  val res   for              n  lt - nums            c  lt - letters          yield  n  c    res  Seq  Int  Char     List  1 a    1 b    1 c    2 a    2 b    2 c    3 a    3 b    3 c    Hope this helps

User · Answer

Unless you get a better answer from a Scala user  which I m not   here s my understanding   It only appears as part of an expression beginning with for  which states how to generate a new list from an existing list   Something like   var doubled   for  n  lt - original  yield n   2   So there s one output item for each input  although I believe there s a way of dropping duplicates    This is quite different from the  imperative continuations  enabled by yield in other languages  where it provides a way to generate a list of any length  from some imperative code with almost any structure    If you re familiar with C   it s closer to LINQ s select operator than it is to yield return

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val aList   List  1 2 3 4 5    val res3   for   al  lt - aList if al  gt  3   yield al   1 val res4   aList filter    gt  3  map     1   println  res3   println  res4     These two pieces of code are equivalent   val res3   for  al  lt - aList  yield al   1  gt  3 val res4   aList map     1  gt  3    println  res3    println  res4     These two pieces of code are also equivalent   Map is as flexible as yield and vice-versa

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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    This  for x  lt - c1  y  lt - c2  z  lt -c3          is translated into  c1 foreach x   gt  c2 foreach y   gt  c3 foreach z   gt            This  for x  lt - c1  y  lt - c2  z  lt - c3  yield         is translated into  c1 flatMap x   gt  c2 flatMap y   gt  c3 map z   gt            This  for x  lt - c  if cond  yield         is translated on Scala 2 7 into  c filter x   gt  cond  map x   gt           or  on Scala 2 8  into  c withFilter x   gt  cond  map x   gt           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  This  for x  lt - c  y        yield         is translated into  c map x   gt   x        map  x y    gt            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   gt  sl filter el   gt  el  gt  0  map el   gt  el toString length     or  for     sl  lt - l   el  lt - sl   if el  gt  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 gt  var found   false found  Boolean   false  scala gt  List range 1 10  filter     2    1  amp  amp   found  foreach x   gt  if  x    5  found   true else println x   1 3 7 9  scala gt  found   false found  Boolean   false  scala gt  Stream range 1 10  filter     2    1  amp  amp   found  foreach x   gt  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  lt - List range 1  10   if x   2    1  amp  amp   found     if  x    5  found   true else println x   for  x  lt - Stream range 1  10   if x   2    1  amp  amp   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 gt  var found   false found  Boolean   false  scala gt  List range 1 10  filter     2    1  amp  amp   found  foreach x   gt  if  x    5  found   true else println x   1 3 7 9  scala gt  found   false found  Boolean   false  scala gt  List range 1 10  withFilter     2    1  amp  amp   found  foreach x   gt  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

User · Answer

The keyword yield in Scala is simply syntactic sugar which can be easily replaced by a map  as Daniel Sobral already explained in detail   On the other hand  yield is absolutely misleading if you are looking for generators  or continuations  similar to those in Python  See this SO thread for more information  What is the preferred way to implement   39 yield  39  in Scala

[scala] What is Scala's yield?

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