Re: [Caml-list] Equality between abstract type definitions

[Roberto Di Cosmo <roberto@dicosmo.org>]

Thanks Ivan,
       the potential confusion between type annotations
in the code and type specifications in module interfaces
is a very good point.

Writing

 let f : 'a -> 'a = fun x -> x+1 

will just boil down to defining 

 val f : int -> int = <fun>

On the other side, declaring 

 val f : 'a -> 'a 

in a module signature actually *requires* the implementation
to be at least as generic as 'a -> 'a, so a definition
let f = fun x -> x+1 in the body will not work.

Nevertheless, I would say that the difference is pretty
easy to grasp, as soon as one explains that specifications
are only introduced in modules signatures, with the val
keyword.

One may want to introduce type specifications in the code
like in Haskell, but I am not sure that il will be much
better for newbies... : let's write some code similar
to the above one

succ :: a -> a
succ n = n+1

here is the system's answer

    No instance for (Num a)
      arising from a use of `+'
    In the expression: n + 1
    In an equation for `succ': succ n = n + 1

Is this really more new-user friendly?

--
Roberto

 

On Fri, Oct 25, 2013 at 01:59:26PM +0400, Ivan Gotovchits wrote:
> Roberto Di Cosmo <roberto@dicosmo.org> writes:
> 
> >
> > I am curious to know why you consider this a pitfall: if it is
> > not what people expect, it is probably because nobody explained
> > their meaning to them properly, and I am quite interested in
> > understanding how to explain this better.
> >
> 
> I think that people expect that an expression:
> 
> ```
>   let a : int = b 
> ```
> 
> is a declaration that value `a` has type int (Just like C'ish 
> `int a = b;`). But, indeed, it should be understood as a type
> constraint. Thus the following, will be readily accepted by the
> type checker (because we «constrain» a to be anything):
> 
> ```
>   let a : 'a = 12
> ```
> 
> The root of misunderstanding, I think, lies in that the same syntax is
> used for type annotations and value specifications. Consider the
> following example:
> 
> ```
>    module T : sig
>     val sum: 'a -> 'a -> 'a
>    end = struct
>     let sum: 'a -> 'a -> 'a = 
>         fun x y -> x + y
>    end
> ```
> 
> It looks like that the value sum has the same type in the module
> specification and in the module implementation. So if compiler accepts
> definition, it should accept that it conforms to the specification. 
> 
> Indeed, it's rather intuitional - this types do look the same!
> 
> So, I think, that it should be clarified by someone, who knows OCaml and
> English much better than me, what is the difference between this two
> cases. And it would be great if it will be described in the manual,
> too. 
> 
> 
> 
> 
> -- 
>          (__) 
>          (oo) 
>    /------\/ 
>   / |    ||   
>  *  /\---/\ 
>     ~~   ~~   
> ...."Have you mooed today?"...

-- 
Roberto Di Cosmo
 
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