MLton 20210117

# Overloading

In Standard ML, constants (like 13, 0w13, 13.0) are overloaded, meaning that they can denote a constant of the appropriate type as determined by context. SML defines the overloading classes Int, Real, and Word, which denote the sets of types that integer, real, and word constants may take on. In MLton, these are defined as follows.

Int | Int2.int, Int3.int, … Int32.int, Int64.int, Int.int, IntInf.int, LargeInt.int, FixedInt.int, Position.int
Real | Real32.real, Real64.real, Real.real, LargeReal.real
Word | Word2.word, Word3.word, … Word32.word, Word64.word, Word.word, LargeWord.word, SysWord.word

The Definition allows flexibility in how much context is used to resolve overloading. It says that the context is no larger than the smallest enclosing structure-level declaration, but that an implementation may require that a smaller context determines the type. MLton uses the largest possible context allowed by SML in resolving overloading. If the type of a constant is not determined by context, then it takes on a default type. In MLton, these are defined as follows.

Int | Int.int
Real | Real.real
Word | Word.word

Other implementations may use a smaller context or different default types.

## Also see

- discussion of overloading in the Basis Library

## Examples

- The following program is rejected.

      structure S:
         sig
            val x: Word8.word
         end =
         struct
            val x = 0w0
         end

  The smallest enclosing structure declaration for 0w0 is val x = 0w0. Hence, 0w0 receives the default type for words, which is Word.word.