Hi cafe, I'm trying to implement a Perceptron in Haskell and I found one in: http://jpmoresmau.blogspot.com/2007/05/perceptron-in-haskell.html (Thanks JP Moresmau) but there is one line I don't understand, I was wondering if someone could explain it to me. I know the theory behind a perceptron, my question is more about the Haskell syntax in that line I don't understand. epoch :: [([Float],Float)] -> -- ^ Test Cases and Expected Values for each test case [Float] -> -- ^ weights ([Float],Float) -- ^ New weights, delta epoch allInputs weights= let f w (inputs,expected) = step inputs w expected -- I don't understand this line newWeights = foldl f weights allInputs -- Neither this one delta = (foldl (+) 0 (map abs (zipWith (-) newWeights weights))) / (fromIntegral $ length weights) in (newWeights,delta) What is f and what is w? I really don't get it, Is like it is defining a function f which calls step unziping the input, taking one of the elements from the fst and it's corresponding snd and invoking step with that, along with w (which seems to be a list according to step's signature but I don't know where it comes from), and then applying fold to the weights and all the Inputs using that f function... But I don't get it! Maybe if someone could rewrite that redefining f as an separate function and calling fold with that function I'll get it. The input for epoch would be something like this: epoch [([0,0],0),([0,1],0),([1,0],0),([1,1],1)] [-0,413,0.135] and the output for that examples is: ([0.0,412.9],3.333537e-2) Thanks a lot, Hector Guilarte