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Closed variants, type constraints and module signature
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Date: 2010-05-17 (14:31)
From: Jacques Garrigue <garrigue@m...>
Subject: Re: [Caml-list] Closed variants, type constraints and module signature
From: Philippe Veber <>
>> Constrained types have their uses,
> which are, in brief ? Now I can't see a typical use for closed polymorphic
> variant types (I mean types of the form 'a t constraint 'a = [< ... ])

A typical use would be for objects, particularly when representing
virtual types (see the advanced part of the tutorial, in the reference
Another application is to use a record notation for type variables:

  type 'r t = .... constraint 'r = < env:'env; loc:'loc; typ:'typ; .. >
This way you can share multiple type parameters at once, and even
allow transparent addition of new parameters. This was not the
original goal of constraints, but I find it handy.

I have no immediate example with polymorphic variants, but your
arguments about scalability are valid: in some cases, constraints
allow more compact types. But at a cost.

>> but I find them often confusing as
>> the type variable you write is not really a type variable.
> why isn't it the case ? Aren't they simply type variables restricted to a
> finite number of types ?

Yes they are constrained type variables.
But the problem is that the constraint is left implicit.

For instance, if somewhere you have defined

  type 'a t constraint 'a = [< `a | `b]

and inside an interface you write

  val f : 'a t -> int

the real meaning is

  val f : [< `a | `b] t -> int

I.e., you are hiding something.

This is not the same thing as type abbreviations, because constraints
are applied from the outside, while abbreviations just have to be

Jacques Garrigue