On Aug 25, 11:22 pm, Dan Doel
On Wednesday 25 August 2010 5:05:11 pm DavidA wrote:
The code below defines a type synonym family:
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-} {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
[snip]
The problem with mult is that k is not specified unambiguously. You either need v to determine k (which is probably not what you want, at a guess), mult to take a dummy argument that determines what k is: [...] or, to make Tensor a data family instead of a type family. What is the difference making it work?
However, it would make more sense to have it be a type family, without the overhead of data (both in space and in typing). Is there a non- hacky approach, without dummies and without making Tensor a data family without a semantic need? I am having a similar problem with the vector package, with the Data.Vector.Generic.Mutable module, because of the PrimState type family. How am I expected to best solve this? This function is easy to write: fillVector n elems = do arr <- VectorGM.new n VectorG.copy arr $ VectorG.fromList elems return arr However, what I have below is not easy to write for me - some specific type annotations are needed. Interestingly, I needed to annotate a statement rather than the return value, because unifying PrimState IO and PrimState m fails, rather than producing m = IO. While I managed to make them work, it looks like type inference is not helping much - the code seems more complicated to properly type-annotate than it would be in a language with just typechecking. V1, not generic - it works for arrays of integers: fillSortVector n elems = do arr <- VectorGM.new n :: IO (VectorU.MVector (PrimState IO) Int) -- I can't annotate just arr, I need to annotate the IO action! -- arr :: (VectorU.MVector (PrimState IO) Int) <- VectorGM.new n -- doesn't compile! let arrI :: VectorU.Vector Int = VectorG.fromList elems VectorG.copy arr arrI (timing, _) <- timed $ VectorI.sort arr return timing V2, almost fully generic - but not in the array types (that was even more complicated IIRC): fillSortVector :: forall a. (Ord a, VectorG.Vector VectorU.Vector a, VectorGM.MVector VectorU.MVector a) => Int -> [a] -> IO Double fillSortVector n elems = do arr <- VectorGM.new n :: IO (VectorU.MVector (PrimState IO) a) -- I can't annotate just arr, I need to annotate the IO action! -- arr :: (VectorU.MVector (PrimState IO) a) <- VectorGM.new n -- doesn't compile! let arrI :: VectorU.Vector a = VectorG.fromList elems VectorG.copy arr arrI (timing, _) <- timed $ VectorI.sort arr return timing Best regards Paolo G. Giarrusso -- PhD student