adam vogt wrote:
I think context and constraint mean the same thing. The haskell report uses the word context for http://www.haskell.org/onlinereport/decls.html for the whole list and constraint for one part of that list (Eq a). With the extension -XConstraintKinds both of those are called Constraint. In other words:
instance Context => InstanceHead
instance (Constraint, Constraint2) => InstanceHead
This is indeed more in accordance with what I believe to be a context or a constraint. But, it seems that in this page http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/type-class-extensions... the vocabulary is different: """ instance context1 => C Int a where ... -- (A) instance context2 => C a Bool where ... -- (B) instance context3 => C Int [a] where ... -- (C) instance context4 => C Int [Int] where ... -- (D) The instances (A) and (B) match the constraint C Int Bool, but (C) and (D) do not. When matching, GHC takes no account of the context of the instance declaration (context1 etc). [...] The -XOverlappingInstances flag instructs GHC to allow more than one instance to match, provided there is a most specific one. For example, the constraint C Int [Int] matches instances (A), (C) and (D), but the last is more specific, and hence is chosen. If there is no most-specific match, the program is rejected. """ So, what do I miss? Isn't the vocabulary different here? And I do not understand what is written if I take your definition (which is indeed the definition in the Haskell report). Thanks in advance, TP