Re: Problem with information hiding.

Could you provide a self-contained piece of code that fails to compile? It
is easier to debug such problems when we see the error source, and when it
is possible to experiment (check that a fix suggestion work) before sending
a reply. Note that you don't need to make it minimal.

On Wed, Sep 30, 2015 at 7:51 PM, Hendrik Boom [email protected]
[ocaml_beginners] <[email protected]> wrote:

> My modules are hiding too much type information.
>
> I have a module that parses according to a grammar
> that is provided s a module parameter..
>
> The signature of the grammars involves several types
> relating to this grammar, such as the type of tokens
> produced by the lexer.
>
> module type GRAMMAR =
>   sig
>     type token
>     type gramon = {symbol : token; next : gramon option; alt : gramon
> option;}
>     type phrase
>     ...
>     ...
>   end;;
>
>
> And a parser, that uses this grammar description:
>
>
> module Mixfix(Grammar : GRAMMAR) =
>   struct
>     ...
>     ...
>     let rec read
>             (g : Grammar.gramon option)
>             (prio : int)
>             (input : Grammar.token Stream.t)
>             (psf : Grammar.phrase list)
>             (return : Grammar.phrase -> Grammar.token option ->
> Grammar.token Stream.t -> Grammar.phrase) =
>     ...
>     ...
>   end
>
> In general there can be a lot of different types
> that act as tokens.  But to test this parser, I define
> a specific grammar with token = char:
>
>
> module Chargram : GRAMMAR with token = char =
>   struct
>     type token = char;; (* Should really be a module's type parameter *)
>     ...
>     ...
>   end;;
>
>
> and set up a test parser by giving Mixfix this grammar:
>
>
> module Test = Mixfix(Chargram);;
>
>
> Now this doesn't work because of information-hiding.
> Test.token is an abstract type.  Even though in this
> case it *is* char, that information is not available
> when I call Test's functions outside the module, so
> the type check fails when I try to call Test.read with
> a char stream.
>
>
> So what is the *correct* way of providing this parametrization,
> so after specializing the module to char, I can call
> its functions knowing that Test.token is char, and the type check
> will succeed?
>
> Or should I be using some entirely different mechanism to
> accomplish my purpose?
>
> -- hendrik
>
>
>
>
>
>
> ------------------------------------
> Posted by: Hendrik Boom <[email protected]>
> ------------------------------------
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