Re: [Caml-list] Type inference curiosity

[Grumpy <grumpy@drno.eu>]

This does not explain why the compiler accepts as well typed the
function eval2, after inferring a = int from the first branch.
Do I miss something ?

Le 24/03/2017 à 10:55, Nicolás Ojeda Bär a écrit :
> Hello,
> 
> Recursive definitions need to be explicitly annotated to be
> polymorphic.  The (type a) annotation
> introduces an abstract type a in the body of the function but does not
> make it polymorphic.
> The annotation type a. on the other hand, does both.
> 
> See http://caml.inria.fr/pub/docs/manual-ocaml/extn.html#sec227
> <http://caml.inria.fr/pub/docs/manual-ocaml/extn.html#sec227> for the
> details.
> 
> Cheers,
> Nicolas
> 
> 
> On Fri, Mar 24, 2017 at 10:20 AM, Grumpy <grumpy@drno.eu
> <mailto:grumpy@drno.eu>> wrote:
> 
>     Hello,
> 
>     I think there is an incoherency or a bug somewhere in the type inference
>     engine with the following code with version 4.02.3 (I have not tested in
>     previous versions). Both functions eval and eval2 are identical but the
>     inferred types are different, and the type inferred for eval2 is
>     actually wrong.
> 
>     The function eval is typed correcly ('a exp -> 'a) while the type
>     inferred for funcion function eval2 is (int exp -> int), which is wrong
>     because of the Inc case returning ('a exp -> 'a).
> 
>     It seems the syntax (type a) leads to this incorrect behaviour...
> 
> 
>     type _ exp =
>       | Stop : int exp
>       | Inc : (int exp -> int) exp
> 
>     let rec eval : type a. a exp -> a = function
>       | Stop -> 0
>       | Inc -> (fun (p : int exp) -> 1 + eval p)
> 
>     let rec eval2 (type a) (p : a exp) : a =
>       match p with
>       | Stop -> 0
>       | Inc -> (fun (p : int exp) -> 1 + eval2 p)
> 
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