Re: [Caml-list] An awkwardness with type parameters

[Hendrik Boom <hendrik@topoi.pooq.com>]

On Tue, Feb 26, 2019 at 03:39:24PM +0900, Jacques Garrigue wrote:
> 
> 2019/02/26 15:23, Hendrik Boom <hendrik@topoi.pooq.com>:
> > 
> > I have a (broken) function definition starting:
> > 
> > let mixfix : 'token1 'phrase1.
> >  ('token1, 'phrase1, ('token1, 'phrase1) Phrasestream.phrasestream) grammar
> >        -> ('token1, 'phrase1) parser
> >   =  fun gram -> (
> >  ...
> > 
> > It goes o for a hundred-odd lines.
> > The type-parameterized types Phrasestream.phrasestream, grammar, and parser   
> > have been previously defined, and there are also operations defined on these
> > types.
> > 
> > The problem I'm having is that the OCaml type-checker ends up identifying
> > the type parameters 'token1 an 'phrase1 because of type errors I've made in
> > the body of the mixfix function.  Somewhere I ended up using an
> > operator that's supposed to work on values of type 'token1
> > on a value of type 'phrase1 instead, and identification of 'token1 and 
> > 'phrase1 is the natural result.
> > 
> > Is there some way of forcing the type checker to treat 'token1 and 
> > 'phrase1 as different types so that I can get meaningful type errors
> > at the point were they occur?
> 
> You can use locally abstract types, which are often used with GADTs but are not
> restricted to them:
>   let mixfix : type token1 phrase1.
>     (token1, phrase1, (token1, phrase1) Phrasestream.phrasestream) grammar
>        -> (token1, phrase1) parser
>   =  fun gram -> (

Thus *with* the type keyword, but *without* the apostrophes on token1 
and phrase1?

-- hendrik