Getting error "Parse error in pattern: length Failed, modules loaded: none"

Question

 --To check the list is in ascending order or not
 ascending::[Int]->Bool
 ascending [] =True
 ascending ((length l) == 1) =True 
 ascending l  =((head l)<=l !! 1) && ascending(tail l)

This is my Haskell code and when I try to run this code in my GHCI interpreter I get the following error:

  ascending.hs(File_name):4:13: Parse error in pattern: length
  Failed, modules loaded: none.

Could anyone tell me where the bug is?

Answer 1

It looks like you're trying to add a guard. You need to bind the input list to l and separate the condition with |:

ascending :: [Int]->Bool
ascending [] =True
ascending l | ((length l) == 1) =True 
ascending l = ((head l)<=l !! 1) && ascending(tail l)

You can replace your use of guards, head and tail with pattern matching instead:

ascending :: [Int]->Bool
ascending [] = True
ascending [_] = True 
ascending (x:y:xs) = (x <= y) && ascending (y:xs)