[Haskell-cafe] Re: Translating perl -> haskell, string "fill ins" with an error on invalid inputseems awfullycomplex. Is there a way to simplify?

[jeff p]

{----

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