Simon Peyton-Jones <simonpj@microsoft.com> wrote:

If, however, you can suggest some specific sentences that would help to eliminate confusion, then please do suggest them.

It is, to a certain extent, a design decision. One possibility might be to change

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.

to

Types are related by a generalization preorder (specified below); the most general type (up to the equivalence induced by that preorder) that can be assigned to a particular expression (in a given environment) is called its principal type.

Another possibility might be to leave generalization as it is:

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.

and add a sentence here:

This implies that two contexts are considered equal if and only if they imply the same set of class assertions in the class environment.

Instead of ``equal'' one might consider the somewhat less precise ``equivalent''. Wolfram