On Tue, May 31, 2011 at 1:40 AM, Patrick Browne <patrick.bro...@dit.ie>wrote:
> Continuing the thread on model expansion. > I have changed the example trying to focus on expanding models of M in G > Why is the operation ! ok or RHS but not visible on LHS of G? > The equation itself does not seem to suffer from the dependent type > problem of my previous post. > > class M a where > (!) :: a -> a -> a > e :: a > > > class M a => G a where > (!-) :: a -> a -> a > -- OK in G > a !- e = e ! a > This doesn't do what you think. This is equivalent to (!-) = (\a e -> e ! a) That is, "e" is lambda-bound here, not the same "e" from M. In this case you've defined (!-) as "flip (!)" When you define functions in a class declaration, you are just defining a default implementation. Generally this is used when you can implement some functions in terms of the others, such as: class Eq a where (==) :: a -> a -> a (/=) :: a -> a -> a a == b = not (a /= b) a /= b = not (a == b) Now someone making an instance of this class need only define (==) or (/=); the other one will be defined via the default instance. (This has the somewhat undesirable property that 'instance Eq X where' with no methods is a valid instance declaration but in that instance == and /= are infinite loops) Maybe this will help you think about this: what code do you expect to write for instances of this class? instance M Int where e = 0 (!) = + instance G Int where -- do I need to write any code here? It seems to me you are expecting the compiler to automatically derive the definition 'inverse = negate'; there's no general way for the compiler to do so, since it doesn't know the structure of your type. Functions in Haskell, unlike, say, Prolog, only go from left of the = to the right of the =. Thanks to referential transparency, you can go backwards from the point of view of proving properties about your code, but during evaluation term rewriting only happens from left to right. -- ryan
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