doc/guide/src/Overloading.adoc

   1 Overloading
   2 ===========
   3
   4 In <:StandardML:Standard ML>, constants (like `13`, `0w13`, `13.0`)
   5 are overloaded, meaning that they can denote a constant of the
   6 appropriate type as determined by context.  SML defines the
   7 overloading classes _Int_, _Real_, and _Word_, which denote the sets
   8 of types that integer, real, and word constants may take on.  In
   9 MLton, these are defined as follows.
  10
  11 [cols="^25%,<75%"]
  12 |=====
  13 | _Int_  | `Int2.int`, `Int3.int`, ... `Int32.int`, `Int64.int`, `Int.int`, `IntInf.int`, `LargeInt.int`, `FixedInt.int`, `Position.int`
  14 | _Real_ | `Real32.real`, `Real64.real`, `Real.real`, `LargeReal.real`
  15 | _Word_ | `Word2.word`, `Word3.word`, ... `Word32.word`, `Word64.word`, `Word.word`, `LargeWord.word`, `SysWord.word`
  16 |=====
  17
  18 The <:DefinitionOfStandardML:Definition> allows flexibility in how
  19 much context is used to resolve overloading.  It says that the context
  20 is _no larger than the smallest enclosing structure-level
  21 declaration_, but that _an implementation may require that a smaller
  22 context determines the type_.  MLton uses the largest possible context
  23 allowed by SML in resolving overloading.  If the type of a constant is
  24 not determined by context, then it takes on a default type.  In MLton,
  25 these are defined as follows.
  26
  27 [cols="^25%,<75%"]
  28 |=====
  29 | _Int_ | `Int.int`
  30 | _Real_ | `Real.real`
  31 | _Word_ | `Word.word`
  32 |=====
  33
  34 Other implementations may use a smaller context or different default
  35 types.
  36
  37 == Also see ==
  38
  39  * http://www.standardml.org/Basis/top-level-chapter.html[discussion of overloading in the Basis Library]
  40
  41 == Examples ==
  42
  43  * The following program is rejected.
  44 +
  45 [source,sml]
  46 ----
  47 structure S:
  48    sig
  49       val x: Word8.word
  50    end =
  51    struct
  52       val x = 0w0
  53    end
  54 ----
  55 +
  56 The smallest enclosing structure declaration for `0w0` is
  57 `val x = 0w0`.  Hence, `0w0` receives the default type for words,
  58 which is `Word.word`.