doc/guide/src/Closure.adoc

   1 Closure
   2 =======
   3
   4 A closure is a data structure that is the run-time representation of a
   5 function.
   6
   7
   8 == Typical Implementation ==
   9
  10 In a typical implementation, a closure consists of a _code pointer_
  11 (indicating what the function does) and an _environment_ containing
  12 the values of the free variables of the function.  For example, in the
  13 expression
  14
  15 [source,sml]
  16 ----
  17 let
  18    val x = 5
  19 in
  20    fn y => x + y
  21 end
  22 ----
  23
  24 the closure for `fn y => x + y` contains a pointer to a piece of code
  25 that knows to take its argument and add the value of `x` to it, plus
  26 the environment recording the value of `x` as `5`.
  27
  28 To call a function, the code pointer is extracted and jumped to,
  29 passing in some agreed upon location the environment and the argument.
  30
  31
  32 == MLton's Implementation ==
  33
  34 MLton does not implement closures traditionally.  Instead, based on
  35 whole-program higher-order control-flow analysis, MLton represents a
  36 function as an element of a sum type, where the variant indicates
  37 which function it is and carries the free variables as arguments.  See
  38 <:ClosureConvert:> and <!Cite(CejtinEtAl00)> for details.