# Changes

## 2025/02/12

* `visitors` would sometimes produce an empty universal quantification.
  Beginning with OCaml 5.3, this causes an error: `Error: broken invariant in
  parsetree: Explicit universal type quantification cannot be empty.`
  Reported and fixed by Opale Sjöstedt.

## 2021/06/08

* Replace a reference to the `Parser` module in `ppxlib` with a reference to
  the `Parser` module in `compiler-libs`, so as to remain compatible with the
  next version of `ppxlib` (> 0.22.0), where the type `token` is abstract.
  (Contributed by Sonja Heinze.)

## 2021/01/27

* Switch to version 0.22.0 of `ppxlib`, which internally uses the abstract
  syntax tree of OCaml 4.12.
  (Contributed by Sonja Heinze.)

* Relax our requirement from OCaml 4.07 to OCaml 4.05.

## 2021/01/27

* Switch to version 5.0 of `ppx_deriving`.
  (At present, this implies requiring OCaml 4.07, but this may be relaxed
  to OCaml 4.05 in the future, I am told.)
  Depend on `ppxlib` instead of `ppx_tools`.
  Remove the dependency on `cppo`.
  (Contributed by Kate Deplaix.)

## 2020/02/07

* Avoid references to `Pervasives` in the generated code.

* Switch from `ocamlbuild` to `dune` to compile `visitors`.

## 2018/05/13

* Fixes in `_tags` and `META` so as to allow `visitors`
  to be used in `jbuilder` (`dune`) projects.
  (Contributed by Armaël Guéneau.)

## 2018/03/06

* Warn when the visitor methods for two distinct types or two distinct data
  constructors have the same name, as this results in an OCaml type error
  or multiply-defined-method error. (Reported by Gabriel Radanne.)

## 2017/11/24

* Added compatibility with OCaml 4.06.0.

* Fixed the internal function `occurs_type` in the case of polymorphic types.
  This should make no observable difference, as this function is used only
  to produce an error message in a corner case.

## 2017/08/28

* Added compatibility with OCaml 4.05.0.

## 2017/07/25

* Updated `src/Makefile` to allow compilation on systems where `ocamlopt` is
  missing. (Suggested by Ralf Treinen.)

## 2017/04/20

* New settings `visit_prefix`, `build_prefix`, and `fail_prefix` can be used
  to control which prefixes are used in generated method names. (This feature
  was suggested by Philip Hölzenspies.)

## 2017/04/04

* Extended backward compatibility to OCaml 4.02.2. (Thanks to Benjamin Farinier.)

## 2017/03/17

* New attributes `@build` and `@@build` can be attached to record type
  declarations and data constructors, so as to alter the construction code that
  is used in `map`, `endo`, and `mapreduce` visitors. See the documentation for
  details. (This feature was suggested by Reuben Rowe.)

## 2017/03/15

* New attributes `@name` and `@@name` can be attached to types, type declarations,
  and data constructors, so as to alter the names of the generated methods. See
  the documentation for details. (This feature was suggested by Reuben Rowe.)

## 2017/03/08

* A new option `polymorphic = true` allows generating visitor methods with
  polymorphic types. With `polymorphic = true`, a type variable `'a` is
  handled by a visitor *function* `visit_'a`, which is passed as an argument
  to every visitor method; whereas, with `polymorphic = false`, a type
  variable `'a` is handled by a virtual visitor *method* `visit_'a`.
  With `polymorphic = true`, visitor classes compose better,
  and irregular algebraic data types are supported.
  See the documentation for more details.
  (This feature was suggested by Reuben Rowe.)

## 2017/03/03

* A new option `data = false` allows suppressing the generation of visitor
  methods for data constructors. This makes the generated visitor slightly
  simpler and faster, but less customizable.

* A new option `nude = true` allows *not* implicitly inheriting the class
  `VisitorsRuntime.<variety>`.

## 2017/02/15

* `Makefile.preprocess` is now installed with the package, so users can rely on it
  without needing to copy it. See the documentation for instructions.

## 2017/02/13

* Added a new variety of visitors, `mapreduce`. This visitor computes a pair of a
  data structure (like a `map` visitor) and a summary (like a `reduce` visitor).
  This can be used to annotate every tree node with information about the
  subtree that lies below it. See the documentation for an example.

## 2017/02/09

* Documentation: added a new subsection on OCaml objects,
  entitled "Where the expressiveness of OCaml's type system falls short".
  This section explains why `map` cannot be a subclass of `fold`,
  even though it should be.

## 2017/01/31

* Documentation: added an example of constructing a lexicographic ordering.

* Documentation: discussed generating visitors for existing types and `ppx_import`.

## 2017/01/26

* Initial release.
