20 Jan
2005
20 Jan
'05
3:50 a.m.
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,