Haskell - parse error on input "=" -

i have following problem:

we can cycle list fixed amount taking given number of items off front , adding them of list. write function called rotor rotor 6 "abcdefghijklmn" produces "ghijklmnabcdef". give type definition rotor. make sure returns error message if offset number less 0 or bigger or equal length of list. 

and following haskell code:

rotor :: (ord a) => -> [a] -> [a] rotor (x:xs)    | if < 1  = error "error"   | [] = []   | otherwise = rotor a-1 xs ++ [x] 

but when compile following error:

parse error on input "="

i don't understand i'm doing wrong. ideas?

rotor :: (ord a) => -> [a] -> [a] rotor (x:xs)  

this line matches non-empty lists x:xs, only.

  | if < 1  = error "error" 

no if needed here.

  | [] = [] 

this should have been pattern: after | boolean expression expected.

an amended version of code:

rotor :: (ord a) => -> [a] -> [a] rotor _      | < 1 = error "error" rotor _ []             = [] rotor (x:xs)         = rotor a-1 xs ++ [x] 

this not correct solution, yet, since not satisfy requirements, it's step in right direction.