Hello Lambda, Monday, August 21, 2006, 1:20:26 AM, you wrote:
> Greetings Haskellers, > I have recently found myself in a situation where I was needing to > general tuples of randomized things; in my case, two coordinates and > a custom data type, Direction. As it is always better to generalize, > I decided to try to write some code which would generalize randomR to > any type which is Ix-able: >> instance Ix i => Random i where >> random = error "No supported instance for random" >> randomR r g >> = let >> r' = range r >> (i, g') = randomR (0,length r'-1) g >> in >> (r' !! i, g') using of (!!) is very inefficient. i don't think that GHC will optimize such code to just arithmetic operations > This works splendidly; with my given example above, I could do: >> data Dir = N | NE | E | SE | S | SW | W | NW deriving (Eq, Ord, > Show, Ix) > ghci> newStdGen >>= print . take 10 . randomRs ((1,1,N),(10,10,NW)) > [(5,4,S), (3,10,SE), (6,5,N), (6,6,NW), (10,8,SE), (3,10,W), > (10,3,E), (1,6,NE), (10,10,SE), (3,3,S)] > As you can see, this results in random tuple(s) of these indexable > types. My implementation required -fallow-overlapping-instances and - > fallow-undecidable-instances for the code to compile. > A few questions: > 1) How does the compiler determine how to choose a particular > instance for those cases where they overlap? it selects most specific instance, preferring concrete types over type classes. but it don't take into account hierarchy of classes. afair, chapter 7 of GHC user's guide considers type system extensions > I noticed that if I > tried to generate a random :: Int, I was getting the error message > defined in the above instance, indicating that the compiler was > choosing my implementation over the default for Int (obviously > because Int is an instance of Ix). Is there some way to exclude > classes in the class qualifier, or to provide an instance as a > default case without resorting to -fallow-overlapping-instances? it's better to show your complete code. as one possible workaround i can suggest the following: class Ix i => Random i .... instance Random Int ... instance Random (Int,Int) -- default definitions will be used > 2) Is there a logical overlap between class Ix and class Bounded? It > seems that we could use the ranges of data types of Bounded to > generate a reasonably sane `random` implementation for all Ix, if we > can guarantee that all Ix instances are Bounded. Would adding an > additional class constraint be sufficient to catch the major cases? > i.e., instance (HasBounds i, Ix i) => Random i where ... btw, how about the following: instance (Enum i, Bounded i) => Random i where ... and then defining instances for tuples? it will be efficient and overlappings-free > 3) Is there some reason that Ix is restricted to the Int datatype? > Is there anything preventing us from moving to a genericIx,or > something along those lines which support any integral type (say > Integer), or is it merely for performance reasons that we use Int? > Is the expectation that if you are using spaces larger than what an > Int can accommodate, you should be using a different mechanism? i think that this just because Ix was introduced solely for array indexing which 1) should be fast, 2) almost don't require indexes larger than Int can hold -- Best regards, Bulat mailto:[EMAIL PROTECTED] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
