Hello, On 10/14/06, Bulat Ziganshin wrote:

Hello haskell-prime,

first is the monomorphism restriction. why isn't it possible to check _kind_ of parameter-less equation and apply monomorphism restrictions only to values of kind '*'? so, this:

sum = foldr1 (*)

will become polymorphic because its kind is '*->*' while this

Kinds are use to classify _types_ and not _expressions_. In this case, the expression has the type "a -> a", with the constraint that "a" is a numeric type. This type is of kind *. In general, all the executable parts of a Haskell program have types that are of kind *. The things that have kinds like "* -> *" are type constructors. On the other hand, you might wonder why not use the _type_ of an expression in the monomorphism restriction (MR) and only monomorphize expressions of a non-functional type. The reason this would not work is that the MR is used to avoid repeated evaluation of overloaded expressions when they are instantiated to the same types, but the type of an expression does not tell us if the expression is already evaluated. Remember that in a functional language functions are values like any other, and so we might have to do computation to evaluate them. This is why the MR uses a syntactic heuristic. Having said that, many people find the MR weird because it is kind of implementation specific. Even without the MR, a compiler could perform an optimization that notices that a value is being instantiated with the same dictionary multiple times and avoid the repeated evaluation. The MR is especially weird if we consider the fact that the Haskell report does not even require implementations to have sharing (e.g., call by name semantics are OK), so implementation may duplicate evaluation at will... -Iavor