Terminator vs. separator

From HaskellWiki
Jump to navigation Jump to search

There are several concepts for notation of sequences of elements. Usually, we don't think much about it. Programming languages provide different schemes, often different schemes in the same language, we are used to them, and no scheme seems to be better than the other one. However, there are differences and good reasons for preferences.


  • Separator: There is a symbol between each element. This is what the functions Data.List.intersperse and Data.List.unwords generate. In Haskell language, the following syntaxes allow separators only:
    • list sugar: [0,1,2,3]
    • tuples: (0,1,2,3)
    • type class constraints: f :: (Show a, Ord a) => a -> a
    • declaration of named record fields: data T = Cons {a :: Int, b :: String}
    • declaration of data constructors: data T = A | B | C
  • Terminator: There is one symbol after each element.
    • list notation using infixes can be considered an example: 0:1:2:3:[]
  • Liberal choice between separators and terminators:
    • export lists: module A(a,b,c) where and module A(a,b,c,) where (and module A(a,b,c,,,) where ...)
    • import lists: import A(a,b,c) and import A(a,b,c,)
    • let syntax: let a = 'a'; b = 'b' in ... and let a = 'a'; b = 'b'; in ...
    • do syntax: do a ; b and do a; b;
  • Initiator:
    • The whitespace/indentation-delimited format used by let and do statements, among others, is an example of initiator format. Each element is preceded by a newline and the appropriate amount of spaces/indentation.

Terminators are better

  • The theoretical reason: In separator notations there is one comma less than the number of elements. An empty list would need -1 commas, which can't be written, obviously. That is, empty lists must always be handled differently in the separator approach. There is no such problem with terminators.
  • The practical reason: In terminator notation, each list element is followed by the terminator symbol. Thus it is easier to reorder the elements of a list in an editor. If you have written (1:2:3:[]) you can simply cut some elements and the subsequent ':' and then you can insert them whereever you want. If you place every element at a line then it is easy to disable individual lines with a line comment (starting with --). This formatting is also optimal for line oriented version control systems and diff.

See also