Feature Requests item #1124080, was opened at 2005-02-16 17:01 Message generated for change (Comment added) made by simonmar You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=358032&aid=1124080&group_id=8032 Category: None Group: None Status: Open
Priority: 3 Submitted By: Nobody/Anonymous (nobody) Assigned to: Nobody/Anonymous (nobody) Summary: Implicit Parameters and monomorphism
Initial Comment: http://www.haskell.org/pipermail/haskell-cafe/2005-January/008571.html Notes some oddness with recursive binding of implicit parameters. Roughly, giving a type signature to a function with implicit params causes its bindings to act recursively, despite what section 7.4.5.2 of the user's guide says. ----------------------------------------------------------------------
Comment By: Simon Marlow (simonmar) Date: 2005-03-08 10:35
Message: Logged In: YES user_id=48280 This "bug" is just to record the strange interaction between implicit parameters and monomorphism. [adding text of original message] Jim Apple wrote:
Does anyone have examples of these? This one scares the foo out of me:
* It's not even safe in general to add a signature giving the same type that the compiler would infer anyway
Here's an example: > len :: [a] -> Int > > len xs = let ?accum = 0 in len' xs > > len' [] = ?accum > len' (x:xs) = let ?accum = ?accum + (1::Int) in len' xs *Main> :t len' len' :: forall a. (?accum :: Int) => [a] -> Int *Main> len "hello" 0 > len :: [a] -> Int > > len xs = let ?accum = 0 in len' xs > > len' :: forall a. (?accum :: Int) => [a] -> Int > > len' [] = ?accum > len' (x:xs) = let ?accum = ?accum + (1::Int) in len' xs *Main> :t len' len' :: forall a. (?accum :: Int) => [a] -> Int *Main> len "hello" 5 This happens as a side effect of the way that type inference currently works on recursive binding groups. It happens with typeclass dictionaries too, but it isn't observable because they can't be rebound in a local scope. -- Ben ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=358032&aid=1124080&group_id=8032