'a (doc request)

david baelde
 David MENTRE
 Olivier Andrieu
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Date:  20050907 (13:00) 
From:  David MENTRE <david.mentre@g...> 
Subject:  Re: [Camllist] 'a (doc request) 
Hello David, 2005/9/7, david baelde <david.baelde@gmail.com>: > I just saw in cairo.mli a type declaration which I don't understand: > type 'a surface > What's the difference with 'a ? See http://caml.inria.fr/pub/docs/manualocaml/manual016.html#s:typedefs [the copy/paste is missing greek letters, look at the web page] The type variables appearing as type parameters can optionally be prefixed by + or  to indicate that the type constructor is covariant or contravariant with respect to this parameter. This variance information is used to decide subtyping relations when checking the validity of >: coercions (see section 6.7.5). For instance, type +'a t declares t as an abstract type that is covariant in its parameter; this means that if the type t is a subtype of the type s, then t t is a subtype of s t. Similarly, type 'a t declares that the abstract type t is contravariant in its parameter: if t is subtype of s, then s t is subtype of t t. If no + or  variance annotation is given, the type constructor is assumed invariant in the corresponding parameter. For instance, the abstract type declaration type 'a t means that t t is neither a subtype nor a supertype of s t if t is subtype of s. The variance indicated by the + and  annotations on parameters are required only for abstract types. For abbreviations, variant types or record types, the variance properties of the type constructor are inferred from its definition, and the variance annotations are only checked for conformance with the definition. Yours, d.