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Release of OCaml 5.5.0
We have the pleasure of celebrating the birthday of Blaise Pascal by announcing the release of OCaml version 5.5.0.
Release Highlights
Some of the highlights of OCaml 5.5.0 are:
Module-dependent Functions
Modules can now be used as function arguments in a form of lightweight functors.
For instance, we can define a function for printing a map generated by
the Map.Make functor:
let pp_map (module M: Map.S) pp_key pp_v ppf set =
if M.is_empty set then
Format.fprintf ppf "ø"
else
let pp_sep ppf () = Format.fprintf ppf ",@ " in
let pp_binding ppf (k,v) =
Format.fprintf ppf "@[%a@ =@ %a@]" pp_key k pp_v v
in
Format.fprintf ppf "@[{@ %a@ }@]"
(Format.pp_print_seq ~pp_sep pp_binding) (M.to_seq set)
We can then apply this function on a string map
module String_map = Map.Make(String)
with
let () =
let m = String_map.of_list ["Zero", "Zero"; "One", "Un"] in
let pp_str = Format.pp_print_string in
Format.printf "%a@."
(pp_map (module String_map) pp_str pp_str) m
Compared to first-class modules, the type of the function pp_map
type 'a printer = Format.formatter -> 'a -> unit
val pp_map: (module M: Map.S) -> M.key printer -> 'a printer -> 'a M.t printer
is dependent over the value of the module S, and thus the function can only
applied over a statically known module:
let f (): (module Map.S) =
if Random.bool () then
(module Map.Make(Int))
else
(module Map.Make(Float))
let fail = pp_map (f ())
Error: This expression has type
(module M : Map.S) ->
(Format.formatter -> M.key -> unit) ->
(Format.formatter -> 'a -> unit) -> Format.formatter -> 'a M.t -> unit
but an expression was expected of type (module Map.S) -> 'b
The module M would escape its scope
This function is module-dependent. The dependency is preserved
when the function is passed a static module argument (module M : S)
or (module M). Its argument here is not static, so the type-checker
tried instead to change the function type to be non-dependent.
Relocatable Compiler
A compiler installation can now be moved or copied with no risk
of hard-to-debug errors due to mixing incompatible bytecode runtime interpreters.
In practice, this means that creating a local switch when there is a
global switch with the same compiler version and configuration
available can be done by cloning the global switch rather than
recompiling the whole compiler.
This should considerably reduce the time required to create
new local opam switches out-of-the-box.
Polymorphic Functions as Function Arguments
Higher-rank polymorphic functions can now be defined directly by
using an explicit type annotation in a function argument
let apply_map (map: 'a 'b. ('a -> 'b) -> 'a list -> 'b list) =
map string_of_int [1;2;3], map List.singleton ["x"; "y"]
let _ = apply_map List.map
Previously defining such a function required going through either a record or an object
with a polymorphic field or methods
type map = { map: 'a 'b. ('a -> 'b) -> 'a list -> 'b list }
let apply_map {map} =
map string_of_int [1;2;3], map List.singleton ["x"; "y"]
Search and Replace Substring Functions
The String module has been extended with many functions
for searching and replacing substrings inside a string.
let _true = String.includes ~affix:"aba" "abbaba"
let sentence = String.replace_all ~sub:"𝄽" ~by:"word" "A 𝄽 is re𝄽ed"
The substring search is using the 2-way string matching algorithm
which has the advantage of requiring constant space memory overhead
independently of the needle size.
Generalised Local Definitions
It is now always possible to define locally a type,
a class, a module type or any kind of item that can be defined
globally:
let mandelbrot n x =
let type t = Converge | Escape of int in
...
match orbit n x with
| Converge -> 0
| Exit_at n -> colorize n
External Types
When interfacing with foreign function libraries, it is now possible
to define external type
type int_gmp = external "mpz_t"
type float_gmp = external "mpf_t"
Compared to an abstract type definition, the external type name
"mpz_t" (resp. mpf_t) makes the type distinguishable from any
non-abstract types or external types with a different name.
In particular, this makes FFI types better behaved when combined with
Generalised Abstract Data Types (GADTs). For instance, The typechecker
is able to prove that
let ok: (int_gmp,[` A] ) Type.eq -> _ = function _ -> .
is a total function because the external type int_gmp is not compatible
with a polymorphic variant type.
Warning: Abstract types in the current module
The astute reader has probably noticed in the definition above that,
in OCaml 5.4.0, the typechecker does accept
type int_gmp
let ok: (int_gmp, [` A] ) Type.eq -> _ = function _ -> .
as total.
Indeed until OCaml 5.5.0, abstract types defined in the current module
type a
type b
were considered as unique and provably different
let f: 'x. (a,b) Type.eq -> 'x = function _ -> .
However, this special rule for local definition of abstract types was
very brittle. As soon as one moved outside of the current module, it
was no longer possible to prove that the types were different.
module M = struct
type a
type b
end
let fail: 'x. (M.a,M.b) Type.eq -> 'x = function _ -> .
Error: This match case could not be refuted.
Here is an example of a value that would reach it: Equal
This special typechecking rule has been removed in OCaml 5.5.0. If you were relying on it,
for instance, because you used an abstract type as type-level label in a GADTs, you
can change your abstract type definition to a possibly private abbreviation of a polymorphic variant
type a = private [`A]
type b = [`B]
or a (possibly private) sum type
type a = A
type b = private B
If you were using an abstract type as both a type-level label and a FFI type, you
can now use an external type definition which will give you a provably distinct type
even outside of the current module.
GC improvements
Some of the ongoing work to improve the pacing of the garbage
collector has been integrated in OCaml 5.5.0, two of the important
changes in OCaml 5.5 GC are
* the addition of a sweep-only phase at the start of major GC
* the addition of an idle phase to smooth the behaviour of the GC
at the start.
Many incremental changes
* The Windows implementation is no more reliant on Winpthreads
* Around 60 new standard library functions
* Around 90 various improvements
* A dozen of documentation updates
* Around 40 bug fixes
Please report any unexpected behaviours on the OCaml issue
tracker and post any questions or
comments you might have on our discussion forums.
The full list of changes can be found in the full changelog.
---
Installation Instructions
The base compiler can be installed as an opam switch with the following commands:
opam update
opam switch create 5.5.0
The source code for the release is also directly available on:
* GitHub
* OCaml archives at Inria
Fine-Tuned Compiler Configuration
If you want to tweak the configuration of the compiler, you can switch to the option variant with:
opam update
opam switch create ocaml-variants.5.5.0+options
where is a space separated list of ocaml-option-* packages. For instance, for a flambda and no-flat-float-array switch:
opam switch create 5.5.0+flambda+nffa ocaml-variants.5.5.0+options ocaml-option-flambda ocaml-option-no-flat-float-array
