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Here is the latest OCaml Weekly News, for the week of May 19 to 26, 2015.

Archive: https://sympa.inria.fr/sympa/arc/caml-list/2015-05/msg00081.html

> Is it possible to encode a recursive, heterogenous map and list datastructure > with GADTs? > > I want to encode JSON (there are already a couple of great libraries out there > so this is kind of an academic question). I would like to have functions that > can only take a JSON map type, for example to take a json map and return a > value. The key here is that the map can hold values of type int,string and also > maps. Is this possible? > > eg get : (map : map json) -> key:string -> 'a json > > Thank you. > > Trevor You should probably have a look at the examples given for GADTs on the page about language extensions (advanced examples section): http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc85 Not so long ago I used equality types to encode collections of object of different types. Here is an example with lists and a function for obtaining the n-th element. ########## type (_,_) eq = Eq : ('a,'a) eq type 'a typ = | Int : int typ | Float : float typ | Pair : 'a typ * 'b typ -> ('a * 'b) typ let rec equal : type a b. a typ -> b typ -> (a,b) eq option = fun t1 t2 -> match (t1,t2) with | (Int , Int ) -> Some Eq | (Float , Float ) -> Some Eq | (Pair(a,b) , Pair(c,d)) -> begin match (equal a c, equal b d) with | (Some Eq, Some Eq) -> Some Eq | _ -> None end | _ -> None type _ tlist' = | Nil : unit tlist' | Cons : ('a typ * 'a * unit tlist') -> unit tlist' type tlist = unit tlist' let rec nth : type a. tlist -> int -> a typ -> a = fun l n ty -> match (l, n) with | (Nil , _) -> raise Not_found | (Cons(ty',t,_), 0) -> begin match equal ty ty' with | Some Eq -> t | None -> invalid_arg "Ill type in nth" end | (Cons(_,_,l) , _) -> nth l (n-1) ty let l = Cons(Int, 3, Cons(Float, 4.2, Nil)) let n : int = nth l 0 Int let f : float = nth l 1 Float ##########

Archive: https://sympa.inria.fr/sympa/arc/caml-list/2015-05/msg00083.html

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Thanks to Alp Mestan, we now include in the OCaml Weekly News the links to the recent posts from the ocamlcore planet blog at http://planet.ocaml.org/. ICFP programming contest 2014: a retrospective (part 2/2): http://gallium.inria.fr/blog/icfp-contest-2014-retrospective-2 Church Numerals: http://blog.shaynefletcher.org/2015/05/church-numerals.html On the book "More OCaml": http://www.amazon.com/review/R7DMDHZMOKS8X ICFP programming contest 2014: a retrospective (part 1/2): http://gallium.inria.fr/blog/icfp-contest-2014-retrospective-1 Labeled and optional arguments: http://blog.shaynefletcher.org/2015/03/labeled-and-optional-arguments.html The troublesome reflection rule (TYPES 2015 slides): http://math.andrej.com/2015/05/19/the-troublesome-reflection-rule-types-2015-slides/

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