Question

A bit of a neophyte haskell question, but I came across this example in Haskell’s tutorial examples. For ‘find the last element of a list’ there are some obvious versions, like

last' [x] = x last' (_:xs) = last' xs 

But I can’t make sense of an alternate version presented:

myLast' = foldr1 (const id) 

So, in trying to make sense of what the application of the id function is doing, I tried in ghci:

const id 1 2 -> gives 2 

This binds like this:

(const id) 1 2 -> gives 2 

And not like this:

 const (id 1) 2 -> gives 1  

But I’m not making sense of this. (const id) should translate to something like

`(\x y->x) (\x->x)`  

Shouldn’t this return a function that simply returns the id of its first element? Or, how is the function order making (const id) behave differently than const?

Answer 1

The definition of const is

const x = \_ -> x 

Hence, (const id) is a function which takes one argument and always returns id and

const id 1 2 = (\_ -> id) 1 2              = id 2              = 2 

The definition of foldr1 is

foldr1 f [x] = x foldr1 f (x:xs) = f x (foldr1 f xs) 

If we have

myLast' = foldr1 (const id) 

then

myLast' [x] = foldr1 (const id) [x]               {- definition of foldr1 -}             = x 

and

myLast' (x:xs) = foldr1 (const id) (x:xs)                  {- definition of foldr1 -}                = (const id) x (foldr1 (const id) xs)                  {- definition of const -}                  = (\_ -> id) x (foldr1 (const id) xs)                  {- function application -}                  = id (foldr1 (const id) xs)                  {- definition of id -}                  = foldr1 (const id) xs                  {- definition of myLast' -}                  = myLast' xs 

which agrees with the definition of last'.