error handling - How does the either function work and is used appropriately in Haskell? -

i trying learn haskell learning haskell data analysis @ moment (i read other things learn haskell... before though). there listing parse , modify csv data not work way book proposes, ie. throughs error. unfortunately, not enough able correct listing. somehow not work don't know how correct version like. did not realy found expressive examples use either function see if can derive correct version it

module learningdataanalysis02  import data.list import data.either import text.csv  -- compute average of list values average' :: fractional => [a] -> average' xs = sum xs / genericlength xs  readcolumn :: [string] -> [double] readcolumn xs = map read xs  getcolumnincsv :: csv -> string -> either string integer getcolumnincsv csv columnname = case lookupresponse of     nothing -> left "the column not exist in csv file."     x  -> right (fromintegral x)       lookupresponse = findindex (== columnname) (head csv)  applytocolumnincsv :: ([string] -> b) -> csv -> string -> either string b applytocolumnincsv func csv column = either     left     right . func . elements columnindex       columnindex = getcolumnincsv csv column     nfieldsinfile = length $ head csv     records = tail $ filter (\record -> nfieldsinfile == length record) csv     elements ci = map (\record -> genericindex record ci) records 

the error message when loading ghci is:

learningdataanalysis02.hs:22:38:     couldn't match expected type ‘either string b’                 actual type ‘a0 -> either a1 b0’     relevant bindings include       func :: [string] -> b (bound @ learningdataanalysis02.hs:22:20)       applytocolumnincsv :: ([string] -> b)                             -> csv -> string -> either string b         (bound @ learningdataanalysis02.hs:22:1)     in expression: either left right . func . elements columnindex     in equation ‘applytocolumnincsv’:         applytocolumnincsv func csv column           = either left right . func . elements columnindex                         columnindex = getcolumnincsv csv column               nfieldsinfile = length $ head csv               records                 = tail $ filter (\ record -> nfieldsinfile == length record) csv               elements ci = map (\ record -> genericindex record ci) records  learningdataanalysis02.hs:24:20:     couldn't match expected type ‘a0 -> [string]’                 actual type ‘[field]’     possible cause: ‘elements’ applied many arguments     in second argument of ‘(.)’, namely ‘elements columnindex’     in second argument of ‘(.)’, namely       ‘func . elements columnindex’ failed, modules loaded: none. 

is kind , can explain me how have modify listing make work , why not work way book suggest it?