On 27/11/06, Ian Lynagh
On Mon, Nov 20, 2006 at 12:05:46PM +0000, Malcolm Wallace wrote:
Prompted by recent discussion on the Hat mailing list about the problems of type-defaulting, I have added two new proposals for this issue to the Haskell-prime wiki at:
http://hackage.haskell.org/trac/haskell-prime/wiki/Defaulting
I don't see a proposal to remove defaulting defaulting altogether on that page - has that been discussed already?
Am I the only one who puts an explicit type signature in whenever my compiler warns me that it is having to do some defaulting? And probably 99% of those would be unnecessary if (^)'s second argument was an Int, with a genericPower (or whatever) function providing the current type signature (analogous to, for example, (!!) and genericIndex).
Defaulting is one wart I would be glad to be rid of.
Ack! Please, if anything, Int should be moved out of the Prelude and provided in a module as a useful, though usually unnecessary, form of hand-optimisation. I might be persuaded to put up with Integer being the second argument type for (^), but I'd *really* rather it remain polymorphic. Splitting up the generic list operations has always seemed like a wart in the Prelude to me. We have things like specialize pragmas now, so it seems like a solution to a problem which is no longer there. As for defaulting, if anything, I'd favour defaulting being generalised to the point where it was no longer a wart. The reason that it currently only works for numeric types is that those are the only ones where we presently have syntax which is ambiguous right off the bat, but people are talking about generalising the syntax for strings and such, so that may not always be so. I'm not yet totally convinced that this one-default-type-per-class thing will really work, it seems to me you probably are going to want something more general than that. However, it is moderately elegant, and otherwise the problem of intersecting multiple defaulting lists is an issue. On a tangentially related note, has anyone ever written something like: forM = flip mapM then forgotten to give it a type signature and tried using it from the ST monad with the monomorphism restriction on? Try the example module below. The first time I ran into it was a few iterations of GHC ago, and at the time I don't think the error message even mentioned the forM, and of course, it was a much more complicated case, and it took me a long time to track down what was happening. The MR is truly an insidious force of evil. :) --- MR.hs import Control.Monad import Control.Monad.ST import Data.STRef forM = flip mapM comp v = forM [1,2,3] $ \s -> do x <- readSTRef v writeSTRef v (x + s) main = do print $ runST (do v <- newSTRef 0 comp v readSTRef v)