When does the relaxed value restriction kick in in OCaml?

Question

Can someone give a concise description of when the relaxed value restriction kicks in? I\'ve had trouble finding a concise and clear description of the rules. There\'s Garr

Answer 1

I am not a type theorist, but here is my interpretation of Garrigue's explanation. You have a value V. Start with the type that would be assigned to V (in OCaml) under the usual value restriction. There will be some number (maybe 0) monomorphic type variables in the type. For each such variable that appears only in covariant position in the type (on the right sides of function arrows), you can replace it with a fully polymorphic type variable.

The argument goes as follows. Since your monomorphic variable is a variable, you can imagine replacing it with any single type. So you choose an uninhabited type U. Now since it is in covariant position only, U can in turn be replaced by any supertype. But every type is a supertype of an uninhabited type, hence it's safe to replace with a fully polymorphic variable.

So, the relaxed value restriction kicks in when you have (what would be) monomorphic variables that appear only in covariant positions.

(I hope I have this right. Certainly @gasche would do better, as octref suggests.)