Re: [Haskell-cafe] defining last using foldr

Aaron Denney wrote:

(Quoting reformatted. Try to have your responses below what you are responding to. It makes it easier to read as a conversation.)

On 2007-08-14, Alexteslin wrote:

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Aaron Denney wrote:

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Folds replace the "cons" operator (:) with the function you pass it. If you want the tail of the list, you want what is on the right hand side of every cons (unless that's []).

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Well, i have tried cons (:) operator but when it passed to foldr doesn't work because cons operator operates first character and then the list but the foldr argument takes a function (a->a->a). Maybe i am missing the point here?

I didn't say to use (:), I said foldr works by replacing (:) with some other function.

foldr also takes a function of type (a -> b -> b).

foldr f e replaces (first : (middle : (last : []))) with (first `f` (middle `f` (last `f` e)))

You want last to be kept, so f x e = x

this causes the overall pattern to reduce to (first `f` (middle `f` last))

This time you need f y last = last

This means you need to discriminate between "e" and "last".

If you make "e" the same type as last, you could accidentally compare them equal. So instead of using the same type, we want one with one more value. There is a standard one: "Maybe a", with constructors "Just a" and "Nothing". And you also need to promote last to this type with the constructor Just, because the result gets fed in on the right.

-- Aaron Denney -><-

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I am really sorry, but i still can't define the function. The chapter the exercise is in precedes algebraic types or Maybe a types. And is seems that must being easy to define. I answered some exercises on using foldr such as summing for example, but this one i am trying: myLast :: [Int] -> Int myLast (x:xs) = foldr (some function) x xs. With summing as i understood foldr goes through the list and sums up by (+) operator and one argument like 0. Like: foldr (+) 0 xs -- View this message in context: http://www.nabble.com/defining-last-using-foldr-tf4269357.html#a12169661 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.