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From: Jacques Garrigue <>
To: Hendrik Boom <>
Cc: Mailing List OCaml <>
Subject: Re: [Caml-list] An awkwardness with type parameters
Date: Tue, 26 Feb 2019 15:39:24 +0900
Message-ID: <> (raw)
In-Reply-To: <>

2019/02/26 15:23, Hendrik Boom <>:
> 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

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-26  6:24 Hendrik Boom
2019-02-26  6:39 ` Jacques Garrigue [this message]
2019-02-27  2:00   ` Hendrik Boom
2019-02-27  3:46     ` Jacques Garrigue
2019-02-27 18:21   ` Hendrik Boom

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