From: Gabriel Scherer <gabriel.scherer@gmail.com>
To: Lukasz Stafiniak <lukstafi@gmail.com>
Cc: Caml <caml-list@inria.fr>
Subject: Re: [Caml-list] Polymorphic recursion and GADTs
Date: Tue, 10 Dec 2013 20:07:44 +0100 [thread overview]
Message-ID: <CAPFanBEpbxsBmvygWnOWnDydmgw7JO-csZVt=xq8Ei94G6eqjw@mail.gmail.com> (raw)
In-Reply-To: <CAJMfKEW9TVbuBWwwA+6zwKS+dr0kmOW_fWDD+Z=PcT0+m7ADqw@mail.gmail.com>
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You must not use a type annotation here ('t47), because its (implicit)
scope is above. Removing the annotation or using _ in lieu of the free type
variable solves your problem. See this recent bugreport where the same
question was raised:
http://caml.inria.fr/mantis/view.php?id=6264
On Tue, Dec 10, 2013 at 3:24 PM, Lukasz Stafiniak <lukstafi@gmail.com>wrote:
> I have another challenge for you. Without the redundant annotations (i.e.
> with only an annotation on let-rec) it type-checks nicely. I am providing
> more annotations as an option.
> >>>
> type _ term =
> | Lit : integer -> integer term
> | Plus : integer term * integer term -> integer term
> | IsZero : integer term -> boolean term
> | If : (*∀'a.*)boolean term * 'a term * 'a term -> 'a term
> | Pair : (*∀'a, 'b.*)'a term * 'b term -> (('a * 'b)) term
> | Fst : (*∀'a, 'b.*)(('a * 'b)) term -> 'a term
> | Snd : (*∀'a, 'b.*)(('a * 'b)) term -> 'b term
> and integer
> and boolean
>
> external plus : (integer -> integer -> integer) = "plus"
> external is_zero : (integer -> boolean) = "is_zero"
> external if_then : (boolean -> 'a -> 'a -> 'a) = "if_then"
> let rec eval : type a . (a term -> a) =
> ((function Lit i -> i | IsZero x -> is_zero (eval x)
> | Plus (x, y) -> plus (eval x) (eval y)
> | If (b, t, e) -> if_then (eval b) (eval t) (eval e)
> | Pair (x, y) -> (eval x, eval y)
> | Fst p -> let ((x, y): (a * 't47)) = eval p in x
> | Snd p -> let ((x, y): ('t59 * a)) = eval p in y): a term -> a)
> <<<
> We get at "eval p" to the right of "let ((x, y): (a * 't47))":
> Error: This expression has type a * b#0
> but an expression was expected of type a * b#0
> The type constructor b#0 would escape its scope
>
>
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next prev parent reply other threads:[~2013-12-10 19:08 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-09 17:16 Lukasz Stafiniak
2013-12-09 17:24 ` Lukasz Stafiniak
2013-12-09 21:34 ` Lukasz Stafiniak
2013-12-10 13:20 ` Lukasz Stafiniak
2013-12-10 13:21 ` Lukasz Stafiniak
2013-12-10 14:24 ` Lukasz Stafiniak
2013-12-10 19:07 ` Gabriel Scherer [this message]
2013-12-10 22:43 ` Jacques Garrigue
2013-12-10 22:51 ` Lukasz Stafiniak
2013-12-10 22:55 ` Lukasz Stafiniak
2013-12-09 17:25 ` Gabriel Scherer
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