Here is the latest Caml Weekly News, for the week of July 09 to 16, 2013.
Archive: https://sympa.inria.fr/sympa/arc/caml-list/2013-07/msg00064.htmlLouis Gesbert announced:
OCamlPro is proud to announce the release of OCaml-Top 1.0.0 OCaml-Top is an interactive editor targeted at education, with a simple and convenient interface: an edition panel, with syntax coloration and automatic indentation, tightly coupled with an ocaml toplevel to easily evaluate the code as you go. It is available from OPAM and as a binary installer for Windows. OCaml-Top is released under the terms of the GNU GPL v3. Home page: http://typerex.org/ocaml-top.html Source on Github: https://github.com/OCamlPro/ocaml-top We hope you appreciate it, and it gives a better experience to students and first-time users, especially on Windows.
Archive: https://sympa.inria.fr/sympa/arc/caml-list/2013-07/msg00065.htmlGoswin von Brederlow asked, Lukasz Stafiniak remarked, and Leo White said:
> > I'm wondering if one can have an ascociative container, like a Hashtbl.t > > with dependent types (GADTs as the key, value depending on the key). > > Something like this: > > > > module H = struct > > type ('a, 'b) t = ('a, 'b) Hashtbl.t > > let create : type a b . int -> (a b, a) t = > > fun x -> Hashtbl.create x > > let add : type a b . (a b, a) t -> a b -> a -> unit = > > fun h k v -> Hashtbl.add h k v > > let find : type a b . (a b, a) t -> a b -> a = > > fun h k -> Hashtbl.find h k > > end > > > > BUT: > > > > let create : type a b . int -> (a b, a) t = > > ^^^ > > Error: Unbound type constructor b > > > > Is there some special syntax I'm missing or is it simply impossible to > > declare such a container in the abstract? > > I think you need higher kinded types, not GADTs. Haskell has them, > for example you can write code that only depends on the type class > of "b" (which is parameterized by "a"), and "b" has signature > "* -> *" or something like that. That type is indeed higher-kinded. OCaml does support higher-kinded types but not in the core type system: you need to use functors. For example, > > let create : type a b . int -> (a b, a) t = > > fun x -> Hashtbl.create x could be written as: module Create (B: sig type 'a t end) = struct let f x : ('a B.t, 'a) Hashtbl.t = Hashtbl.create x end;; Note that this is not actually the correct type for writing an associative container. See Jeremy's post for details.Alain Frisch also suggested:
This thread might of interest to you: https://sympa.inria.fr/sympa/arc/caml-list/2013-02/msg00037.htmlJeremy Yallop suggested:
It is indeed possible to create associative containers where the types of the values depend on the types of the keys. Let's see what can to be done to turn the standard hash table into such a container, using GADTs for keys. We'll start with the interface. The standard hash table interface (Hashtbl.S) looks like this, in part: module type S = sig type key type 'a t val create : int -> 'a t val add : 'a t -> key -> 'a -> unit val remove : 'a t -> key -> unit val find : 'a t -> key -> 'a val iter : (key -> 'a -> unit) -> 'a t -> unit end The type of tables ('a t) is parameterized by the type of values, since each table holds a single type of value. We're aiming instead to have value types depend on key types, so we'll move the type parameter into the key type. Making this change mechanically throughout the interface gives us the following: module type GS = sig type 'a key type t val create : int -> t val add : t -> 'a key -> 'a -> unit val remove : t -> 'a key -> unit val find : t -> 'a key -> 'a val iter : < f: 'a. 'a key -> 'a -> unit > -> t -> unit end Actually, I've made one additional change, in the type of iter. In the regular Hashtbl iter function we can get by with ML-style polymorphism, where all the type variables are implicitly quantified at the outermost point. This constrains the function passed to iter to be monomorphic, which is fine, since regular Hashtbls only support a single value type. In our revised interface, however, the function argument must be polymorphic, since it needs to handle *any* suitable pairing of keys and values. The object type allows quantifier nesting, giving us the polymorphism we need. The type of iter is a hint of things to come: putting things *into* a polymorphic hash table is a doddle, but there's a bit of a knack to getting them *out* again intact, as we'll see further down. Next up is the definition of keys. The standard Hashtbl.Make functor uses a definition of keys that bundles the key type together with equality and hashing operations, like this: module type HashedType = sig type t val equal : t -> t -> bool val hash : t -> int end Of course, it's no good having just any old definitions of equal and hash. It's essential that equal l r implies hash l = hash r, for example, and there are additional fairly obvious constraints on equal. Our analogue to HashedType, GHashedType, comes with some additional operations (and so places additional demands on the creator of hash tables). The first part of the signature looks essentially the same as HashedType: we've added a parameter to the key type, but it's not used as yet, so we can replace it with the don't-care underscore. (Note that this means that our equality is heterogeneous, happy to accept keys of disparate types.) The remainder of the signature deals with packing up key-value pairs into existential boxes, and attempting to get them out again; this will allow us to store different types of key in a single table in our implementation. module type GHashedType = sig type _ key val equal : _ key -> _ key -> bool val hash : _ key -> int type pair = Pair : 'a key * 'a -> pair val unpack : 'a key -> pair -> 'a end As with HashedType there are requirements not captured in the types. In particular, we'd like unpack k (Pair (k', v)) = v whenever equal k k' is true. Time for the implementation. This is mostly straightforward: after some preliminaries (mostly about hiding the type parameter in keys by boxing them up appropriately), there are two functions (add and remove) that put keys and values in boxes and store them in a monomorphic table, and two functions (find and iter) that unbox keys and values to recover the parameterization. module GHashtbl (G : GHashedType) : GS with type 'a key = 'a G.key = struct include G type k = Key : 'a key -> k module H = Hashtbl.Make(struct type t = k let hash (Key k) = hash k let equal (Key l) (Key r) = equal l r end) type t = pair H.t let create n = H.create n let add tbl k v = H.add tbl (Key k) (Pair (k, v)) let remove tbl k = H.remove tbl (Key k) let find tbl key = unpack key (H.find tbl (Key key)) let iter (f : <f: 'a. 'a key -> 'a -> unit>) tbl = H.iter (fun _ (Pair (k, v)) -> f#f k v) tbl end As is often the case, the unboxing is the interesting part. The find function reveals why we introduced the unpack operation for keys (and hence the pair type, which could otherwise have been hidden away in the body of GHashtbl), and shows the secondary purpose of keys as unboxers of values. The iter function makes use of the polymorphism that we introduced in its signature earlier; when we unbox a pair we have no idea how to instantiate the type variable in the contents, so it's just as well we have a function to hand (f#f) that's polymorphic enough to handle any possible instantiation. Time to try it out. Here's a sample implementation of GHashedType that associates ints with int lists (which we'll use to store prime factors) and strings with bools (which we'll use to indicate capitalization). module KeyType (* : GHashedType *) = struct type _ key = Int : int -> int list key | String : string -> bool key let equal : type a b. a key -> b key -> bool = fun l r -> match l, r with | Int x, Int y -> x = y | String x, String y -> x = y | _ -> false let hash = Hashtbl.hash type pair = Pair : 'a key * 'a -> pair let rec unpack : type a. a key -> pair -> a = fun k p -> match k, p with | Int _, Pair (Int _, v) -> v | String _, Pair (String _, v) -> v | _ -> raise Not_found end Using KeyType we can instantiate the functor and set about creating heterogeneous hash tables: # module HT = GHashtbl(KeyType) ... # let ht = HT.create 10;; val ht : HT.t = <abstr> # begin HT.add ht (Int 10) [2; 5]; HT.add ht (Int 12) [2; 2; 3]; HT.add ht (Int 2) ; end;; - : unit = () # begin HT.add ht (String "foo") false; HT.add ht (String "Foo") true; HT.add ht (String "bar") false; HT.add ht (String "Bar") true; end;; - : unit = () # HT.find ht (Int 10), HT.find ht (String "Foo"), HT.find ht (Int 12);; - : int list * bool * int list = ([2; 5], true, [2; 2; 3]) # let o = object method f : type a. a key -> a -> unit = fun k v -> match k with | Int i -> let s = String.concat "*" (List.map string_of_int v) in Printf.printf "%d = %s\n" i s | String s -> Printf.printf "%s is%s capitalized\n" s (if v then "" else " not") end;; val o : < f : 'a. 'a KeyType.key -> 'a -> unit > = <obj> # HT.iter o ht;; 2 = 2 12 = 2*2*3 10 = 2*5 foo is not capitalized bar is not capitalized Bar is capitalized Foo is capitalized - : unit = ()
Archive: https://sympa.inria.fr/sympa/arc/caml-list/2013-07/msg00077.htmlStefan Holdermans announced:
Vector Fabrics has another position open for a functional programmer. See below and http://www.vectorfabrics.com/company/career/functional_programmer for details. This time, we particularly welcome applications from experienced developers, the position providing a fair share of opportunities for growing into the rôle of an architect. Interested? Please contact us at jobs <AT> vectorfabrics.com.
Archive: https://sympa.inria.fr/sympa/arc/caml-list/2013-07/msg00086.htmlMichel Mauny announced:
The OCaml 2013 (preliminary) program is now available: http://ocaml.org/meetings/ocaml/2013/program.html The workshop is just the day before ICFP, and the registration is now open (from http://icfpconference.org/icfp2013/). It is time to plan your trip and to make hotel reservations!
Archive: https://sympa.inria.fr/sympa/arc/caml-list/2013-07/msg00087.htmlMark Shinwell announced:
Linux has a tool called "perf" that enables the display of source code alongside time profiling information and the corresponding assembly code. (See https://perf.wiki.kernel.org/index.php/Tutorial, "perf annotate"). I am pleased to announce an alpha version of the OCaml native code compiler that permits perf to do the same for OCaml code. This compiler works only on x86-64 Linux, although porting it to other Linux targets should be straightforward. The compiler is available in OPAM. If you add the remote repository git://github.com/mshinwell/opam-repo-dev then you should be able to "opam switch" to the 4.01-perf-annotate compiler. Please let me have any reports of problems. After compilation, you can run "perf record" to gather data about your OCaml program, and then use "perf report" to interactively examine it. If you hit Return on a function, then you should be given the option to annotate it, and then you should see the OCaml code as above. Note that line number information is not yet as fine-grained for OCaml as it might be for C code. You need to have the source files available at the same location on the filesystem when you run "perf report" as you did when you compiled the program. This work forms part of a larger project in collaboration with OCaml Labs at Cambridge, UK, to enhance the level of debugging information emitted by the OCaml compiler. The perf-annotate compiler emits debugging sections that aim to be compliant with the DWARF-2 standard. Mark P.S. The eagle-eyed of you will notice that there is another compiler, 4.01-allocation-profiling, also available in that OPAM repo. This provides allocation profiling capabilities for native code, documentation for which I will endeavour to circulate to the list shortly.Maxime Dénès asked and Mark Shinwell replied:
> Is it related to the recently introduced "-with-frame-pointers" configure > flag http://caml.inria.fr/mantis/view.php?id=5721 ? > > The purpose of this flag seemed to be the use of the same perf tool. Compilation with a frame pointer enables the display of callgraph information in perf. It is effectively a workaround for the fact that the in-kernel stack unwinder used by perf does not use the debugging information in the executable. The perf-annotate compiler is designed specifically to make the "perf annotate" feature of the perf tools work in a satisfactory way for OCaml code. It's roughly speaking orthogonal, but you probably want to use both.
Thanks to Alp Mestan, we now include in the Caml Weekly News the links to the recent posts from the ocamlcore planet blog at http://planet.ocaml.org/. Full Time: Software Developer (Functional Programming) at Jane Street in New York, NY; London, UK; Hong Kong: http://jobs.github.com/positions/0a9333c4-71da-11e0-9ac7-692793c00b45 Better Inlining: Progress Report: http://www.ocamlpro.com/blog/2013/07/11/inlining-progress-report.html Experimenting in API Design: Riakc: http://functional-orbitz.blogspot.com/2013/07/experimenting-in-api-design-riakc.html
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