[Haskell-cafe] Ultra-newbie Question

Alexander Solla ajs at 2piix.com
Sat Sep 18 17:29:40 EDT 2010

On Sep 18, 2010, at 12:51 AM, Christopher Tauss wrote:

> I am a professional programmer with 11 years experience, yet I just  
> do not seem to be able to get the hang of even simple things in  
> Haskell.  I am trying to write a function that takes a list and  
> returns the last n elements.

Note that keeping just the suffix is the same as dropping the prefix.   
Consider your data:

A finite list.
The length of the suffix to keep.
An algebraic relationship between lengths of lists,  suffixes, and  
their prefixes:

length(prefix) + length(suffix) = legnth(list)

Putting all of this together:

n_tail n list = drop (prefix_length) list where
	prefix_length = length list - n

You may be interested in how drop is carrying around some state:

drop n xs     | n <= 0 =  xs
drop _ []              =  []
drop n (_:xs)          =  drop (n-1) xs

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