From: Lukasz Stafiniak <lukstafi@gmail.com>
To: Caml <caml-list@inria.fr>
Subject: Re: [Caml-list] Polymorphic recursion and GADTs
Date: Tue, 10 Dec 2013 15:24:09 +0100 [thread overview]
Message-ID: <CAJMfKEW9TVbuBWwwA+6zwKS+dr0kmOW_fWDD+Z=PcT0+m7ADqw@mail.gmail.com> (raw)
In-Reply-To: <CAJMfKEUURCt4hs9sUNpextFZOafcRz=cQNm0i0uPwfyeZiNuvA@mail.gmail.com>
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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 14:24 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 [this message]
2013-12-10 19:07 ` Gabriel Scherer
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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