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

[Claus Reinke]

> Answering my own plea for help, I now have the following, which seems
> neater to me.

checking Maybes is best done in the Maybe Monad, or if you need specific error 
messages, using maybe. that, in turn can be abstracted out into a lookup with error
message. once the checking is done in the wrapper, there is no need to repeat it in
the generator. also, the interface to the generator is too wide for the small amount of
extra functionality it provides, so it is probably best inlined, and there seems to be no
need to commit to IO so early. i also tend to use [String], with a final unlines before
output, but that is a matter of opinion, i guess.
 
financial_output :: String -> String -> String -> 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 (error $ "no company file for " ++ company) 
                  company company_to_companyfile

  modeString  = lookupWith (error $ "no mode string for " ++ displaymode) 
                  displaymode displaymode_to_modestring

  titleEnd    = lookupWith (error $ "no title end for " ++ displaymode) 
                  displaymode displaymode_to_titleend

  lookupWith error key assocs = maybe error id $ lookup key assocs

hth,
claus