# OCaml: what changed from 4 to 5

- Product: OCaml (https://whatsnew.fyi/product/ocaml)
- Vendor: OCaml
- Range: changelog entries numbered after 4.14.4 up to and including 5.5.1, stable releases only
- Entries below: 8 releases (newest first)
- Resolved: 4 is 4.14.4 and 5 is 5.5.1, the newest stable release of each major we track
- Carrying security changes: 1 · CVEs mentioned: 0 · Mentioning breaking changes: 1 · Removing or deprecating something: 1
- Page: https://whatsnew.fyi/product/ocaml/compare/4...5

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

## What changed (45 changes, grouped by kind)

### Added

#### 5.5.0 (2026-06-19)

- Module-dependent functions
- Polymorphic functions as function arguments
- Relocatable compiler
- Generalized local definitions
- Search and replace substring functions
- External types
- Around 60 new functions in the standard library

#### 5.4.0 (2025-10-09)

- Labelled tuples
- Immutable arrays
- Array literal syntax support for immutable arrays and floatarrays through type-directed disambiguation
- Atomic record fields
- Four new standard library modules: Pair, Pqueue, Repr, and Iarray
- More than thirty new standard library functions
- New chapter in the manual on profiling OCaml programs on Linux and macOS

#### 5.3.0 (2025-01-08)

- Syntax for deep effect handlers
- Statistical memory profiling (statmemprof)
- UTF-8 encoded Unicode source files and support for Unicode identifiers
- Around 20 new functions in the standard library (also in 5.2.0)

#### 5.2.0 (2024-05-13)

- Re-introduced GC compaction
- Restored native backend for POWER 64 bits
- Thread sanitizer support
- New Dynarray module
- New -H flag for hidden include directories
- Project-wide occurence metadata support for developer tools
- Raw identifiers
- Local open in type expressions

### Changed

#### 5.5.0 (2026-06-19)

- Garbage collector improvements

#### 5.4.0 (2025-10-09)

- Restored memory cleanup upon exit mode
- Many runtime and code generation improvements
- Nearly a dozen improved error messages

#### 5.3.0 (2025-01-08)

- More space-efficient implementation of Dynarray
- Improved metadata on pairs of declarations and definitions for merlin
- Improved error messages for first-class modules, functors, labeled arguments, and type clashes

#### 5.2.0 (2024-05-13)

- Many fixes and improvements in the runtime

### Fixed

#### 5.5.1 (2026-09-04)

- Type system bug fix for module-dependent functions
- Various runtime fixes
- Restore cloning support on macOS

#### 5.4.0 (2025-10-09)

- Around fifty bug fixes

#### 5.3.0 (2025-01-08)

- Restored MSVC port
- Many fixes and improvements in the runtime
- Numerous bug fixes

#### 5.2.0 (2024-05-13)

- Many bug fixes

### Removed

#### 5.1.1 (2023-12-07)

- Remove Marshal.Compression module, compressed marshaling has been moved to an internal compiler library

### Security

#### 5.5.1 (2026-09-04)

- Fix overflow in Marshal
- Fix overflow in bytecode loading

_2 releases carry no categorized changes yet: 5.4.1, 5.2.1._

## Release notes

### 5.5.1 — OCaml 5.5.1

- Date: 2026-09-04
- Version: 5.5.1
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.5.1
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.5.1

- **fixed** — Type system bug fix for module-dependent functions
- **security** — Fix overflow in Marshal
- **security** — Fix overflow in bytecode loading
- **fixed** — Various runtime fixes
- **fixed** — Restore cloning support on macOS

- Type system bug fix for module-dependent functions
- Security fix: overflow in Marshal
- Security fix: overflow in bytecode loading
- Various runtime fixes
- Restore cloning support on macOS 

See [detailed list of changes](https://github.com/ocaml/ocaml/blob/5.5/Changes#7).

### 5.5.0 — OCaml 5.5.0

- Date: 2026-06-19
- Version: 5.5.0
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.5.0
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.5.0

- **added** — Module-dependent functions
- **added** — Polymorphic functions as function arguments
- **added** — Relocatable compiler
- **added** — Generalized local definitions
- **added** — Search and replace substring functions
- **added** — External types
- **changed** — Garbage collector improvements
- **added** — Around 60 new functions in the standard library

Some of the highlights of OCaml 5.5.0 are:

  - Module-dependent functions
  - Polymorphic functions as function arguments
  - Relocatable compiler
  - Generalized local definitions
  - Search and replace substring functions
  - External types
  - Garbage collector improvements
  - Around 60 new functions in the standard library

### 5.4.1 — OCaml 5.4.1

- Date: 2026-02-17
- Version: 5.4.1
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.4.1
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.4.1

Bug fixes. See [detailed list of changes](https://github.com/ocaml/ocaml/blob/5.4/Changes).

### 5.4.0 — OCaml 5.4.0

- Date: 2025-10-09
- Version: 5.4.0
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.4.0
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.4.0

- **added** — Labelled tuples
- **added** — Immutable arrays
- **added** — Array literal syntax support for immutable arrays and floatarrays through type-directed disambiguation
- **added** — Atomic record fields
- **added** — Four new standard library modules: Pair, Pqueue, Repr, and Iarray
- **added** — More than thirty new standard library functions
- **added** — New chapter in the manual on profiling OCaml programs on Linux and macOS
- **changed** — Restored memory cleanup upon exit mode
- **changed** — Many runtime and code generation improvements
- **changed** — Nearly a dozen improved error messages
- **fixed** — Around fifty bug fixes

Some of the highlights of OCaml 5.4.0 are:

- Labelled tuples
- Immutable arrays
- Array literal syntax support for immutable arrays and `floatarray`s
  (through type-directed disambiguation)
- Atomic record fields
- Four new standard library modules: Pair, Pqueue, Repr, and Iarray
- Restored "memory cleanup upon exit" mode
- New chapter in the manual on profiling OCaml programs on Linux and macOS

And a lot of incremental changes:

- Many runtime and code generation improvements
- More than thirty new standard library functions
- Nearly a dozen improved error messages
- Around fifty bug fixes

### 5.3.0 — OCaml 5.3.0

- Date: 2025-01-08
- Version: 5.3.0
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.3.0
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.3.0

- **added** — Syntax for deep effect handlers
- **added** — Statistical memory profiling (statmemprof)
- **added** — UTF-8 encoded Unicode source files and support for Unicode identifiers
- **added** — Around 20 new functions in the standard library
- **changed** — More space-efficient implementation of Dynarray
- **changed** — Improved metadata on pairs of declarations and definitions for merlin
- **changed** — Improved error messages for first-class modules, functors, labeled arguments, and type clashes
- **fixed** — Restored MSVC port
- **fixed** — Many fixes and improvements in the runtime
- **fixed** — Numerous bug fixes

Some of the highlights in OCaml 5.3.0 are:

- Syntax for deep effect handlers
- Restored MSVC port
- Re-introduced statistical memory profiling (statmemprof)
- utf-8 encoded Unicode source files and modest support of Unicode identifiers
- More space-efficient implementation of Dynarray
- Improved metadata on the pairs of declarations and definitions for merlin.

And a lot of incremental changes:

- Around 20 new functions in the standard library
- Many fixes and improvements in the runtime
- Improved error messages for first-class modules, functors, labeled arguments,
type clashes.
- Numerous bug fixes

### 5.2.1 — OCaml 5.2.1

- Date: 2024-11-18
- Version: 5.2.1
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.2.1
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.2.1

Bug fixes. 
See [detailed list of changes](https://github.com/ocaml/ocaml/blob/5.2/Changes).

### 5.2.0 — OCaml 5.2.0

- Date: 2024-05-13
- Version: 5.2.0
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.2.0
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.2.0

- **added** — Re-introduced GC compaction
- **added** — Restored native backend for POWER 64 bits
- **added** — Thread sanitizer support
- **added** — New Dynarray module
- **added** — New -H flag for hidden include directories
- **added** — Project-wide occurence metadata support for developer tools
- **added** — Raw identifiers
- **added** — Local open in type expressions
- **added** — Around 20 new functions in the standard library
- **changed** — Many fixes and improvements in the runtime
- **fixed** — Many bug fixes

OCaml 5.2.0 is still a somewhat experimental release compared to the OCaml 4.14 branch. Some of the highlights in OCaml 5.2.0 are:

- Re-introduced GC compaction
- Restored native backend for POWER 64 bits
- Thread sanitizer support
- New Dynarray module
- New -H flag for hidden include directories
- Project-wide occurence metadata support for developer tools
- Raw identifiers
- Local open in type expressions

And a lot of incremental changes:

- Around 20 new functions in the standard library
- Many fixes and improvements in the runtime
- Many bug fixes

### 5.1.1 — OCaml 5.1.1

- Date: 2023-12-07
- Version: 5.1.1
- Original notes: https://github.com/ocaml/ocaml/releases/tag/5.1.1
- Permalink: https://whatsnew.fyi/product/ocaml/releases/5.1.1

- **removed** — Remove Marshal.Compression module, compressed marshaling has been moved to an internal compiler library

- Breaking change: `Marshal.Compression` has been removed, compressed marshaling has been moved to an internal compiler library
- Bug fixes. See [detailed list of changes](https://github.com/ocaml/ocaml/blob/5.1/Changes).