The Haskell 98 report (also in the current revised version) includes the following in section 4.1.4:
Types are related by a generalization order (specified below); the most general type that can be assigned to a particular expression (in a given environment) is called its principal type. Haskell 's extended Hindley-Milner type system can infer the principal type of all expressions, including the proper use of overloaded class methods (although certain ambiguous overloadings could arise, as described in Section 4.3.4). Therefore, explicit typings (called type signatures) are usually optional (see Sections 3.16 and 4.4.1).
The type forall u. cx1 =>t1 is more general than the type forall w. cx2 =>t2 if and only if there is a substitution S whose domain is u such that:
t2 is identical to S(t1).
Whenever cx2 holds in the class environment, S(cx1) also holds.
The relation defined here clearly is a preorder. If it is an order, then in particular the following types are equal (via identical substitutions): (Eq a, Ord b) => (a,b) (Ord b, Eq a) => (a,b) (Eq b, Ord b, Eq a) => (a,b) (Eq [a], Ord b) => (a,b) It seems to me that this arises since the report only defines the SYNTAX of contexts (see the heading ``4.1.3 Syntax of Class Assertions and Contexts''), but never defines what a context IS, that is, the SEMANTICS of contexts. A context could be: * a SEQUENCE of class assertions (according to the syntax) * a SET of class assertions (slightly more intuitive) * a THEORY of class assertions If type generalisation is an order, then the third alternative was chosen. I suggest to include an appropriate hint in the report... (I am aware of the beginning of 4.1.4:
In this subsection, we provide informal details of the type system. (Wadler and Blott [11] and Jones [6] discuss type and constructor classes, respectively, in more detail.)
I think this kind of outsourcing of essential parts of the language definition is not desirable. ) Best regards, Wolfram