Re: [Haskell-beginners] Given a list of lists, how to drop the last item in each (sub)list.

Perhaps a glance at SICP will be helpful. As David and Peter have rightly pointed out, higher order functions make it easier to abstract common patterns. A glance at the SICP chapter on this will be useful, me thinks. It helped me a lot. http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#%_sec_1.3 On Thu, Jan 2, 2014 at 5:44 PM, David Flicker wrote:

The operation you are trying to is abstracted in Haskell using the higher-order function *map. *Commonly, we want to take a list or other sequence of data and perform an operation (function) on each element of the list. In non-functional languages, we would need to explicitly write a for-loop through each element (ie for (int i = 0; i < array.length; i++) { array[i] = doSomething(array[i]); }

Of course in Haskell and other functional languages use of mutable state and explicit iteration aren't used in most circumstances so new functional programmers go "Iteration is the same as recursion so I'll write a recursive function instead something like: recursiveIteration :: [a] -> [a] recursiveIteration [] = [] recursiveIteration x:xs = $$ Lots of operations using x $$ : recursiveIteration xs

Of course, the lazy intermediate functional programmer after writing this function a few times realizes that "I'm using a functional language, why not abstract the operations using x using a function and pass the function as a parameter. Something like this: recursiveIterationAbstractOperations :: (a -> b) -> [a] -> [b] recursiveIterationsAbstractOperations _ [] = [] recursiveIterationsAbstractOperations func x:xs = func x : recursiveIterationsAbstractOperations func xs

This is exactly the definition of *map* in the GHC source codehttp://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#map !

I would write it as a map over the outer list and then process each inner element. Something like this:

discardparitybyte :: [[Bit]] -> [[Bit]] discardparitybyte = map (take 8)

Learning to use higher-order functions like map, foldl, and filter making functional programming great. In my mind as an intermediate Haskell programmer, when I start using direct recursion especially when using lists, I stop myself and think about a better way to structure my code so that I can write it as a combination of these and other higher order functions because they allow for more modulatrity in the code.

-David

On Thu, Jan 2, 2014 at 10:38 PM, Angus Comber wrote:

...

I have a list like this:

[[1,0,0,0,1,1,1,0,0],[1,1,1,0,1,1,1,0,0],[1,0,1,0,0,1,1,0,0],[0,1,0,0,1,1,1,0,0],[0,0,1,0,1,1,1,0,0],[1,0,0,1,1,1,1,0,1]]

The 'inner' list is a list of 9 items. I want to process the list so that a list of lists is returned but the 9th element in each inner list is dropped.

So the function type would be [[a]] -> [[a]]

So to get started I wrote a function like this:

discardparitybyte :: [[Bit]] -> [[Bit]]

But then not sure how to transform the inner list.

I know I can access the first inner element using take 1 list. But how do I then access/manipulate this inner list?

discardparitybyte (x:xs) = take 9 ??? (take 1 xs) : discardparitybyte ???

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