Simon Peyton-Jones wrote:
In Haskell today, you can at least tell what value is bound to each identifier in the program, *without* first doing type checking.
I'm afraid I'm confused. In the following code
data Z data S a
class Card c where c2int:: c -> Int
instance Card Z where c2int _ = 0 instance (Card c) => Card (S c) where c2int _ = 1 + c2int (undefined::c)
foo = c2int (undefined::(S (S (S (S Z)))))
how can one tell the value of foo without first doing the typechecking? Without typechecking, we can't use c2int and can't even construct any meaningful value of class Card. For c2int, the type is the ``value.'' Overlapping instances, polymorphic recursion -- all seem to make the value determination even more uncertain. Andre Pang wrote:
1) now I have to manually declare a class definition for every single function, and I have to declare it in advance before any module defines that function (most serious problem; see below),
However, declaring the instance first requires declaring the type class itself, and that _is_ a problem, because that's exactly what I'm trying to work around. Without 20/20 hindsight, you cannot say with certainty what type signatures a "generic" function (like 'phase' or even 'add') can support, because it's not a generic function,
But in the solution posted previously, for each ad hoc overloadable function, the corresponding class *always* has the same signature: class HasAdd a b | a->b where add:: a->b Therefore, we don't need clairvoyance to define an overloadable function that way. If I need an overloadable function add, I can go ahead and define the above class, and then add an instance. I can do that without knowing all possible overloadable instances of add, present or future. What if somebody else did the same in some other module? If that somebody else followed the conventions, he would introduce exactly the same class declaration. If GHC or its developers could somehow be persuaded to overlook _exact_ duplicate class declarations, then that part of the problem can be solved. The class declaration itself could perhaps be generated by Template Haskell. The discussed solution is quite related to some of the Records proposals (which have been discussed here half a year ago). There too we have the inconvenience of choosing unique names for field labels.
data Person = Person {_pname:: String, _paddress:: String} data Computer = Computer {_cname::[String], _caddress:: Int}
class HasName a b | a->b where name:: a->b class HasAddress a b | a->b where address:: a->b
instance HasName Person String where name = _pname instance HasAddress Person String where address = _paddress instance HasName Computer [String] where name = _cname
instance (HasName n r) => HasName (Maybe n) (Maybe r) where name = fmap name
-- Alas, the following will break the dependency... --instance (Num a) => HasName a String where name = show
-- But the following works: overlapping instances at work instance (HasAddress a b) => HasName a b where name = address
instance HasAddress Int (Int->String) where address x y = show (x+y)
newtype W a = W a instance (Num a) => HasAddress (W a) String where address (W a) = show a
test2 = let p = Person "Anonymous" "N/A" c = Computer ["FQDN","localhost"] 10 in "person named " ++ (name p) ++ " at a computer " ++ (head (name c)) ++ " and another " ++ (show$ name p1) where p1 = (Nothing::Maybe Person)
The first few lines of the code is boilerplate and could be automatically generated. As you can see, we can even handle a limited form of polymorphism, and even do a "hand off". For example, if some thing doesn't have a name but has an address, we can use the address as its name. Alas this ``backtracking'' isn't as general as we might wish. At some point we have to introduce wrappers (like W a above) to hand over the dispatch to another class. It's possible to do the dispatch on a class, but it's a bit too painful. OTH, the wrappers such as 'W' may be considered as an 'alternative view' of an object. By wrapping an object, we can switch its behavior from the main one to an alternative without any run-time penalty.