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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

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 -> (
 …
This is the same as the type you wrote, but additionally prevents the types from being
instantiated inside the definition of mixdix.

Jacques Garrigue