On Thu, 20 Dec 2007, Stefan O'Rear wrote: > On Thu, Dec 20, 2007 at 11:39:42PM -0500, Ronald Guida wrote: > > > data PZero = PZero deriving (Show) > > > data PSucc a = PSucc a deriving (Show) > > > > > > type P1 = PSucc PZero > > > type P2 = PSucc P1 > > > type P3 = PSucc P2 > > > -- etc > > ... > > > Now here's the puzzle. I want to create a function "vecLength" that > > accepts a vector and returns its length. The catch is that I want to > > calculate the length based on the /type/ of the vector, without > > looking at the number of elements in the list. > > > > So I started by defining a class that allows me to convert a Peano > > number to an integer. I couldn't figure out how to define a function > > that converts the type directly to an integer, so I am using a > > two-step process. Given a Peano type /t/, I would use the expression > > "pToInt (pGetValue :: t)". > > > > > class Peano t where > > > pGetValue :: t > > > pToInt :: t -> Int > > > > > > instance Peano PZero where > > > pGetValue = PZero > > > pToInt _ = 0 > > > > > > instance (Peano t) => Peano (PSucc t) where > > > pGetValue = PSucc pGetValue > > > pToInt (PSucc a) = 1 + pToInt a
Why the two methods pGetValue and pToInt? I thought that pGetValue should have signature pGetValue :: t -> Peano and that pToInt can convert the result from pGetValue to Int. > > Finally, I tried to define vecLength, but I am getting an error. > > > > > vecLength :: (Peano s) => Vec s t -> Int > > > vecLength _ = pToInt (pGetValue :: s) > > 1. pGetValue is unneccessary, undefined :: s will work just as well. > This is a fairly standard approach; the precision values in Floating, > bitSize :: Bits a => a -> Int, and sizeOf :: Storable a => a -> Int > all work this way. What about functions of type bitSize :: Bits a => (a, Int) This would also make explicit, that you don't have to provide a value of type 'a'. 'sizeOf' might still return different size depending on the value, right? E.g. when converting a sum data type to a (C++) object. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
