I guess a canonical example of the reason behind this restriction would be the following:
let put = let r = ref [] in fun x -> r := x::!r OCaml will tell you that it has type '_a -> unit. It would be unsound (exercise!) to decide it has type 'a -> unit. Of course, your example is perfectly sound, but the typing algorithm doesn't know that. -- Arnaud Spiwack On 10 January 2012 16:45, Romain Bardou <[email protected]> wrote: > Le 10/01/2012 16:29, Dario Teixeira a écrit : > > Hi, >> >> Consider functions foobar1 and foobar2: >> >> >> type 'a t = {id: int; x: 'a} >> >> let foobar1: 'a -> 'a t = >> fun x -> {id = 0; x} >> >> let foobar2: 'a -> 'a t = >> let ctr = ref 0 in >> fun x -> incr ctr; {id = !ctr; x} >> >> >> >> I would expect them to have the same type, because foobar2's >> use of a reference cell is kept private. However, they don't. >> In fact, foobar2 is not really polymorphic: >> >> >> type 'a t = { id : int; x : 'a; } >> val foobar1 : 'a -> 'a t >> val foobar2 : '_a -> '_a t >> >> >> It's easy to get around this issue by putting the reference cell >> outside of foobar2. Function foobar3 does just this, and behaves >> as expected: >> >> >> let next = >> let ctr = ref 0 in >> fun () -> incr ctr; !ctr >> >> let foobar3: 'a -> 'a t = >> fun x -> {id = next (); x} >> >> >> >> Could someone point me to a good explanation of what's going on? >> (I have the feeling I've read about this restriction before.) >> >> Best regards, >> Dario Teixeira >> >> >> >> > Hello, > > This is, basically, the value restriction: you cannot let-generalize > something which is not *syntactically* a value. Function foobar1 is > syntactically a function, and a function is a value. Function foobar2 is > not: it starts with a let-binding. It computes something before returning a > function. It cannot be generalized. > > Cheers, > > -- > Romain Bardou > > > -- > Caml-list mailing list. Subscription management and archives: > https://sympa-roc.inria.fr/**wws/info/caml-list<https://sympa-roc.inria.fr/wws/info/caml-list> > Beginner's list: > http://groups.yahoo.com/group/**ocaml_beginners<http://groups.yahoo.com/group/ocaml_beginners> > Bug reports: > http://caml.inria.fr/bin/caml-**bugs<http://caml.inria.fr/bin/caml-bugs> > > -- Caml-list mailing list. Subscription management and archives: https://sympa-roc.inria.fr/wws/info/caml-list Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
