polymorphic variants and promotion
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Date:   (:) 
From:  Johannes Kanig <johannes.kanig@l...> 
Subject:  polymorphic variants and promotion 
Hi, I ran into the following problem using polymorphic variants. Suppose I have 2 functions f and g (the function definitions don't make much sense  they are just there for typing purposes) : # let f (`A a) (`A i) = `A i ;; val f : [< `A of 'a ] > [< `A of 'b ] > [> `A of 'b ] = <fun> # let g (`A a) = (`B a) ;; val g : [< `A of 'a ] > [> `B of 'a ] = <fun> For simplicity I assume that the type variables all instantiate to "int". Now, I want to construct a function h that takes an accumulator a and a list of `A's (for example [`A 1; `A 2; ...]) that produces either an object of type `A of int or an object of type `B of int in the following way: # let h a = function  [] > g a  xs > List.fold_left f a xs;; But this doesn't type: This expression has type [ `A of 'a ] but is here used with type [> `B of 'a ] The first variant type does not allow tag(s) `B I understand that the fold_left fixes the type of f the result of f to [`A of int] instead of [>`A of int]. The example as above can be easily repaired by replacing the second branch of the pattern matching by: let `A _ as x = List.fold_left f a xs in x which is fair enough (thanks to Romain Bardou for pointing this out). What I don't understand is why the promotion of [`A of int] to [> `A of int] is not possible in general: # `A 1;;  : [> `A of int ] = `A 1 # (`A 1 : [`A of int]) ;;  : [ `A of int ] = `A 1 # ((`A 1 : [`A of int]) : [> `A of int]) ;;  : [ `A of int ] = `A 1 Does anyone have an answer?  Johannes Kanig johannes.kanig@lri.fr