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[Caml-list] kprintf with user formatters
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Date: | 2004-07-16 (06:02) |
From: | Pierre Weis <pierre.weis@i...> |
Subject: | Re: [Caml-list] kprintf with user formatters |
[...] > I had the same problem a month ago, > I ended with the following ugly function: > > let log b fmt = > if b then Printf.printf fmt > else > let rec f x = Obj.magic f in > f fmt > (* val f: bool -> ('a, out_channel, unit) format -> 'a *) > > (I think this is safe since f just eats its arguments, > and gets typed with "-rectypes" : ('b->'a) as 'a) No, this is not safe: you break the type system and create buggy values like that. # let log b fmt = if b then Printf.printf fmt else let rec f x = Obj.magic f in f fmt;; val log : bool -> ('a, out_channel, unit) format -> 'a = <fun> # log false "";; - : unit = <unknown constructor> # let x = log false "";; val x : unit = <unknown constructor> # Marshal.to_string;; - : 'a -> Marshal.extern_flags list -> string = <fun> # Marshal.to_string x [];; Exception: Invalid_argument "output_value: abstract value (outside heap)". There is clearly something wrong! > maybe a function "zprintf" could be added to Printf and Format to do > this job ? It is overkill to introduce a new printf function just for that case. I suggest we try a bit harder to find an acceptable solution without introducing zprintf :) Best regards, Pierre Weis INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/ ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners