Re: [Haskell-beginners] foldr for Nats

@Stephan thanks for the code so far. @Daniel it should look like foldrNat a b S(S(S(S(Z)))) gives a(a(a(a(b)))) The thing is the definition have to be -------- Original-Nachricht --------

Datum: Mon, 1 Feb 2010 21:56:14 +0000 Von: Stephen Tetley An: kane96@gmx.de CC: Beginners@haskell.org Betreff: Re: [Haskell-beginners] foldr for Nats

Hello

I'm suspecting this isn't homework as you've waited a week so would presumably have missed a deadline.

As Daniel Fischer wrote, one view of folds is that they replace the constructors of a data type, code follows...

data Nat = Z | S Nat deriving (Eq,Ord,Show)

-- Look at the type of foldr...

-- *GHCi> :t foldr -- foldr :: (a -> b -> b) -> b -> [a] -> b

-- It has 2 'constructor replacements': -- (a -> b -> b) & b

-- Replacing Z is easy, we can get some code to compile -- by avoiding the hard bit with a wildcard pattern "_"...

foldrNat1 :: unknown -> b -> Nat -> b foldrNat1 _ b Z = b

-- What to do about the constructor (S ..) takes a bit more -- thought or at least some experimenting. I'll do the later...

-- One thing to try, is to simply translate foldr with as few -- changes as possible:

-- foldr :: (a -> b -> b) -> b -> [a] -> b -- foldr _ z [] = z -- foldr f z (x:xs) = f x (foldr f z xs)

-- Unfortunately this leads to a problem:

foldrNat2 :: (Nat -> b -> b) -> b -> Nat -> b foldrNat2 f b Z = b -- Z case is the same as before foldrNat2 f b (S n) = f undefined (foldrNat2 f b n) -- Arggh! undefined

-- undefined is useful for prototyping, but its a real -- problem for running code!

-- Actually I had another problem as well... -- -- The difference between Nat and [a] is that List 'carries' some data -- therefore (Nat -> b -> b) on Nat is not equivalent to (a -> b -> b) -- on [a].

-- So rather than change the type signature first, get rid of the -- undefined and see what happens

foldrNat3 f b Z = b foldrNat3 f b (S n) = f (foldrNat3 f b n)

-- *GHCi> :t foldrNat3 -- > (t -> t) -> t -> Nat -> t

-- GHCi likes to call type variables t, but the signature is equal to

-- foldrNat3 :: (b -> b) -> b -> Nat -> b

-- This looks promising - it typechecks! -- So try a test:

fromNat :: Nat -> Int fromNat n = foldrNat3 (+1) 0 n

demo1 = fromNat (S (S (S Z))) -- 3 ??

-- By experimenting we seem to have a good answer, -- other people might prefer a more rigorous proof though.

-- NEU: Mit GMX DSL über 1000,- ¿ sparen! http://portal.gmx.net/de/go/dsl02