Scala list concatenation vs

Question

Is there any difference between     and    for concatenating lists in Scala   scala gt  List 1 2 3     List 4 5  res0  List Int    List 1  2  3  4  5   scala gt  List 1 2 3      List 4 5  res1  List Int    List 1  2  3  4  5   scala gt  res0    res1 res2  Boolean   true   From the documentation it looks like    is more general whereas     is List-specific  Is the latter provided because it s used in other functional languages

User · Answer

works only with lists  while    can be used with any traversable  In the current implementation  2 9 0      falls back on     if the argument is also a List

User · Answer

A different point is that the first sentence is parsed as   scala gt  List 1 2 3     List 4 5   res0  List Int    List 1  2  3  4  5    Whereas the second example is parsed as   scala gt  List 4 5      List 1 2 3   res1  List Int    List 1  2  3  4  5    So if you are using macros  you should take care   Besides     for two lists is calling     but with more overhead because it is asking for an implicit value to have a builder from List to List  But microbenchmarks did not prove anything useful in that sense  I guess that the compiler optimizes such calls   Micro-Benchmarks after warming up   scala gt def time a    gt  Unit   Long     val t   System currentTimeMillis  a  System currentTimeMillis - t  scala gt def average a       gt  Long     for i lt -1 to 100  yield a    sum 100  scala gt average       gt  time    List Int        1 to 1000     case  l  e    gt  l    List e       res1  Long   46 scala gt average       gt  time    List Int        1 to 1000     case  l  e    gt  l     List e        res2  Long   46   As Daniel C  Sobrai said  you can append the content of any collection to a list using     whereas with     you can only concatenate lists

User · Answer

Always use      There are two reasons  efficiency and type safety   Efficiency  x     y     z is faster than x    y    z  because     is right associative  x     y     z is parsed as x      y     z   which is algorithmically faster than  x     y      z  the latter requires O  x   more steps    Type safety  With     you can only concatenate two Lists  With    you can append any collection to List  which is terrible   scala gt  List 1  2  3      ab  res0  List AnyVal    List 1  2  3  a  b       is also easy to mix up with     scala gt  List 1  2  3     ab  res1  String   List 1  2  3 ab

User · Answer

Legacy  List was originally defined to be functional-languages-looking   1    2    Nil    a list list1     list2     concatenation of two lists  list match     case head    tail   gt   non-empty    case Nil            gt   empty      Of course  Scala evolved other collections  in an ad-hoc manner  When 2 8 came out  the collections were redesigned for maximum code reuse and consistent API  so that you can use    to concatenate any two collections -- and even iterators  List  however  got to keep its original operators  aside from one or two which got deprecated

[list] Scala list concatenation, ::: vs ++

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