Re: [Haskell-cafe] Re: Top Level etc.
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level etc.
Ben Rudiak-Gould wrote: 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 I don't get this. The second answer (the one quoted above) must be wrong... len' gets a value only in the empty '[]' case. The recursion is such that the value of '?accum' is incremented on the return of the recursively called function, therefore the value of '?accum' in the case '[]' is always zero! How on earth does this get the answer five? Keean, ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level etc.
In article [EMAIL PROTECTED], Benjamin Franksen [EMAIL PROTECTED] wrote: Please note that implicit parameters -- at least as currently implemented in GHC -- have a number of severe problems. A good summary was given by Ben Rudiak-Gould in http://www.mail-archive.com/haskell%40haskell.org/msg15595.html (although in a different context): This is mostly ambiguity due to missing type-signatures, isn't it? That's not so severe in my view. But maybe the compiler can issue a warning in such cases. -- Ashley Yakeley, Seattle WA ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level etc.
Benjamin Franksen wrote: Please note that implicit parameters -- at least as currently implemented in GHC -- have a number of severe problems. 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 Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
