{---- Hello, Here is a variation on Claus' code which returns an Either type rather than fails with error. This could be further generalized to use any instance of MonadError, rather than Either. -Jeff ----} import Control.Monad.Error financial_output :: String -> String -> String -> String -> Either String String financial_output company displaymode startDate endDate = financial_script where financial_script = gnuplot_timeseries_settings <++> "\n" <++> "plot [\"" <++> startDate <++> "\":\"" <++> endDate <++> "\"]" <++> " '" <++> companyFile <++> "'" <++> modeString <++> " title \"" <++> company <++> " " <++> titleEnd <++> "\"" companyFile = lookupWith ("no company file for " ++ company) company company_to_companyfile modeString = lookupWith ("no mode string for " ++ displaymode) displaymode displaymode_to_modestring titleEnd = lookupWith ("no title end for " ++ displaymode) displaymode displaymode_to_titleend lookupWith :: (Eq a) => String -> a -> [(a,String)] -> Either String String lookupWith error key assocs = maybe (Left error) Right $ lookup key assocs class MyString a where mystr :: a -> Either String String instance MyString (Either String String) where mystr = id instance MyString String where mystr = Right x <++> y = do xv <- mystr x yv <- mystr y return $ xv ++ yv