MLton 20210117

# Defunctorize

Defunctorize is a translation pass from the CoreML IntermediateLanguage to the XML IntermediateLanguage.

## Description

This pass converts a CoreML program to an XML program by performing:

- linearization

- MatchCompile

- polymorphic val dec expansion

- datatype lifting (to the top-level)

## Implementation

- defunctorize.sig

- defunctorize.fun

## Details and Notes

This pass is grossly misnamed and does not perform defunctorization.

### Datatype Lifting

This pass moves all datatype declarations to the top level.

Standard ML datatype declarations can contain type variables that are not bound in the declaration itself. For example, the following program is valid.

    fun 'a f (x: 'a) =
       let
          datatype 'b t = T of 'a * 'b
          val y: int t = T (x, 1)
       in
          13
       end

Unfortunately, the datatype declaration can not be immediately moved to the top level, because that would leave 'a free.

    datatype 'b t = T of 'a * 'b
    fun 'a f (x: 'a) =
       let
          val y: int t = T (x, 1)
       in
          13
       end

In order to safely move `datatype`s, this pass must close them, as well as add any free type variables as extra arguments to the type constructor. For example, the above program would be translated to the following.

    datatype ('a, 'b) t = T of 'a * 'b
    fun 'a f (x: 'a) =
       let
          val y: ('a * int) t = T (x, 1)
       in
          13
       end

## Historical Notes

The Defunctorize pass originally eliminated Standard ML functors by duplicating their body at each application. These duties have been adopted by the Elaborate pass.