Hey I am pretty new to Haskell.
So I want to eliminate all integers which are greater than 500 in a list.
import Data.List leng x = if(head x > 500) then leng(tail x) else [head x]++leng(tail x) I get the right output but at the end of each output is
Exception: Prelude.head: empty list
how to fix that problem?
-- You need this to stop the recursion, otherwise it would try to split the list -- when there are no elements left. leng [] = [] You could consider this as the stop condition for your method.
you could also rewrite your method as follow:
leng [] =[] leng (x:xs) | x > 500 = leng xs | otherwise = x : leng xs the first statement is often reocurring when working with lists in haskell. e.g.
last [x] = x -- the underscore means the value of the variable is not needed and can be ignored. last (_:xs) = last xs Add:
leng [] = [] before the current leng x.
But you could also do:
leng x = filter (<=500) x and even
leng = filter (<=500) 
(h:t)instead of[h]++t(so here(head x):(leng (tail x))). (:) is Haskell's cons function, adding a single item to the front of a list.