Re: [Caml-list] How to compute variance of a type

[rossberg@ps.uni-sb.de]

Tom Primožiè wrote:
>
> But I have no idea on how to compute variance of complex types, like
>
>    type ('a, 'b, 'c) long_funct = ('a -> 'b) -> 'c
>
> since the whole ('a -> 'b) is in contravariant position, so 'a is twice
contravariant,
> and b is first contra-, and the covariant.
>
> Any resources?

Off-hand I don't have any easy resources I could point to, but the basic
rule is very simple: whenever you descent into a type argument where the
respective type constructor has negative polarity (like the left argument
of "->"), you just negate the variances. Thus, in your example, long_funct
is covariant in 'a and 'c, and contravariant in 'b. It is straightforward
to formulate that as a recursive algorithm.

- Andreas