With regards to the variable interpolation in strings problem, it's
probably worth watching
http://groups.google.de/group/fa.haskell/browse_thread/thread/34741c2a5c311a17/286dbd62748ef1c1?lnk=st&q=%22haskell+cafe%22+%22template+system%22&rnum=1&hl=en#286dbd62748ef1c1
which mentions some perl/python-like template systems in the works for haskell.
2007/4/16, 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