GADTs in OCaml
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Date:   (:) 
From:  oleg@o... 
Subject:  GADTs in OCaml 
We present a simple, pure, magicfree implementation of a form of GADTs in OCaml that is sufficient for the common applications of GADTs such as data structures with embedded invariants, typed printf/scanf, tagless interpreters. The implementation is a simple module, requiring no changes to the OCaml system. The implementation is so trivial that it should work on any ML system (although, like nested data types, GADTs aren't very useful on an SML system without support for polymorphic recursion). Our examples include:  enforcing invariants on data structures: statically ensuring that in a tree representation of an HTML document, a link node is never an ancestor of another link node;  typed printf/scanf sharing the same format descriptor, which is firstclass and can be built incrementally;  simply typed lambdacalculus with constants and higherorder abstract syntax. This is essentially the example of Hongwei Xi's et al POPL 2003 paper. The complete code with the implementation and examples is available at http://okmij.org/ftp/ML/GADT.ml The GADT notation turns out lightweight. However, GADTs are often inductive, and so we need polymorphic recursion to process them. That adds some (relatively small) notational overhead. Mainly, GADTs are often used with existentials  so often that Haskell makes existential quantification implicit in GADT declarations. OCaml, alas, lacks such notational conveniences, and so existentials (which must be encoded via doublenegation) aren't pretty. Smart constructors  which can be build mechanically, perhaps with a suitable camlp4 macro  ease the pain. We learned the lesson that common GADTs are available in OCaml here and now. We can truly write the published examples that motivated GADTs, without too much violence to their notation. We can translate GADT code from Haskell, more or less mechanically. No changes to the OCaml type system or the type checker are necessary. Of course changes such as explicit existential quantification, better support for rank2 types, etc. shall be greatly appreciated  but they are not necessary to start using and enjoying GADTs. Hopefully the present implementation lets one get a taste of GADTs and write code that seems to require them. The end of GADT.ml points out to a more efficient implementation, which perhaps can be the basis for the native OCaml implementation. As shown by Patricia Johann and Neil Ghani: Foundations for Structured Programming with GADT. POPL 2008. the essence of GADTs is the EQ GADT, which implements the following interface: type ('a,'b) eq val refl : ('a,'a) eq val apply_eq : ('a,'b) eq > 'a > 'b The value of the type ('a,'b) eq witnesses the equality of two types. The function apply_eq relies on the witness when performing type coercion. To be precise, the genuine GADTs provide a more general function for the Leibniz principle val apply_eq : ('a,'b) eq > 'a tau > 'b tau for any type tau. Our implementation supports only those tau that are functors (that is, admit a map operation). That seems sufficient however for all the common examples. The following trivial implementation is genuinely safe, meaning it never leads to segmentation faults, even in principle. module EQ = struct type ('a,'b) eq = Refl of 'a option ref * 'b option ref let refl () = let r = ref None in Refl (r,r) let symm : ('a,'b) eq > ('b,'a) eq = function Refl (x,y) > Refl (y,x) let apply_eq : ('a,'b) eq > 'a > 'b = function Refl (rx,ry) > fun x > rx := Some x; match !ry with  Some y > rx := None; y  _ > failwith "Impossible" end;; We show briefly one of the examples from GADT.ml: typed printf/scanf sharing the same format descriptor (which is firstclass and can be built incrementally): let tp2 = sprintf (f_lit "Hello " ^^ f_lit "world" ^^ f_char) '!';; (* val tp2 : string = "Hello world!" *) let ts2 = sscanf tp2 (f_lit "Hello " ^^ f_lit "world" ^^ f_char) (fun x > x);; (* val ts2 : char * string = ('!', "") *) (* Formats are firstclass and can be constructed incrementally *) let fmt31 () = f_lit "The value of " ^^ f_char ^^ f_lit " is ";; (* val fmt31 : unit > ('a, char > 'a) fmt *) let fmt3 () = fmt31 () ^^ f_int;; (* val fmt3 : unit > ('a, char > int > 'a) fmt *) let tp3 = sprintf (fmt3 ()) 'x' 3;; (* val tp3 : string = "The value of x is 3" *) (* What we print, we can parse back *) let ts3 = sscanf tp3 (fmt3 ()) (fun x n > (x,n));; (* val ts3 : (char * int) * string = (('x', 3), "") *) The example is a straightforward reimplementation of the Haskell code http://okmij.org/ftp/typedformatting/PrintScanI.txt http://okmij.org/ftp/typedformatting/PrintScan.hs In particular, the GADT defining a domainspecific language of format descriptors is written in OCaml as follows: type ('a,'b) fmt =  FLit of < m_flit : 'w. (('a,'b) eq > string > 'w) > 'w >  FInt of < m_fint : 'w. ((int > 'a,'b) eq > 'w) > 'w >  FChr of < m_fchr : 'w. ((char > 'a,'b) eq > 'w) > 'w >  FCmp of < m_fcmp : 'w. ('a,'b,'w) fcmp_k > 'w > (* The standard encoding of existentials *) and ('a,'c,'w) fcmp_k = {fcmp_k : 'b. ('b,'c) fmt * ('a,'b) fmt > 'w} ;; We can (mechanically) define smart constructors: let f_lit x = FLit (object method m_flit : 'w. (('a,'b) eq > string > 'w) > 'w = fun k > k (refl ()) x end);; (* val f_lit : string > ('a, 'a) fmt *) let f_int = FInt (object method m_fint : 'w. ((int > 'a,'b) eq > 'w) > 'w = fun k > k (refl ()) end);; (* val f_int : ('a, int > 'a) fmt *) We show one interpreter of the DSL, to format values into a string: type print_sig = {pr: 'a 'b. ('a,'b) fmt > (string > 'a) > 'b};; let rec printer = {pr = function  FLit x > fun k > x#m_flit (fun eq x > apply_eq eq (k x))  FInt x > fun k > x#m_fint (fun eq > apply_eq eq (fun x > k (string_of_int x)))  FChr x > fun k > x#m_fchr (fun eq > apply_eq eq (fun x > k (String.make 1 x)))  FCmp x > fun k > x#m_fcmp {fcmp_k = fun (a,b) > printer.pr a (fun sa > printer.pr b (fun sb > k (sa ^ sb)))} };; let sprintf fmt = printer.pr fmt (fun x > x);; (* val sprintf : (string, 'a) fmt > 'a *) The code is rather straightforward. One can build many similar interpreters, e.g., to direct the output into any suitable data sink. No changes to the format descriptor or the library is needed.