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

[Thomas Hartman]

And this mess can be simplified further by something like

financial_output_wrapper company displaymode startDate endDate =
  do
    let  maybeCompanyFile = lookup company company_to_companyfile
    validate_arg "company" company maybeCompanyFile

........

validate_arg argname arg maybeTransformedArg =
    case maybeTransformedArg of
      Nothing -> error $ "no transformed " ++ argname ++ " arg for " ++ arg
      _ -> return ()

Summary: I like being able to check the validity of user input on an
arg-by-arg basis, and now I guess I can.


2007/4/13, Thomas Hartman <tphyahoo at gmail.com>:
> Answering my own plea for help, I now have the following, which seems
> neater to me.
>
> company_to_companyfile =  [("ibm","data/ibm.dat"),("cisco","data/cisco.dat")]
> displaymode_to_modestring = [("points", "using 1:2 with linespoints"),
>                    ("candles","using 1:($2+$3+$4+$5)/4:4:3 with yerrorbars")]
> displaymode_to_titleend = [("points","daily
> prices"),("candles","opening prices")]
>
> financial_output_wrapper :: String -> String -> String -> String -> IO ()
> financial_output_wrapper company displaymode startDate endDate =
>   do
>     let  maybeCompanyFile = lookup company company_to_companyfile
>     case maybeCompanyFile of Nothing -> error $ "no company file for "
> ++ company
>                              _ -> return ()
>     let  maybeModeString  = lookup displaymode displaymode_to_modestring
>     case maybeModeString of Nothing -> error $ "no mode string for "
> ++ displaymode
>                             _ -> return ()
>     let  maybeTitleEnd = lookup displaymode displaymode_to_titleend
>     case maybeTitleEnd of Nothing -> error $ "no title end for " ++ displaymode
>                           _ -> return ()
>     let maybeScript  = gen_gnuplot_financial_script company
>                                                     ( maybeCompanyFile  )
>                                                     ( maybeModeString )
>                                                     ( maybeTitleEnd )
>                                                     startDate endDate
>     case maybeScript of
>              Just script -> putStrLn script
>              _           -> error $ "bad script"
>
> gen_gnuplot_financial_script :: String -> Maybe String -> Maybe String
> -> Maybe String -> String -> String -> Maybe String
> gen_gnuplot_financial_script company (Just companyfile ) ( Just
> modestring) ( Just titleEnd ) startDate endDate
>     = Just $ gnuplot_timeseries_settings ++ "\n" ++
>                               "plot [\"" ++ startDate ++ "\":\"" ++
> endDate ++ "\"]"
>                               ++ " '" ++ companyfile ++ "'"
>                               ++ modestring
>                               ++ " title \"" ++ company ++ " " ++
> titleEnd ++ "\""
>
>
>
>
>
> 2007/4/12, Thomas Hartman <tphyahoo at gmail.com>:
> > I was translating some perl code to haskell as a learning exercise and
> > wound up with the following. (below) Simple code that accepts some
> > string arguments, and prints a string -- so, of type String -> String
> > -> String -> String -> IO ().
> >
> > I like to be concise, but I get the feeling something went awry.  What
> > seems to be costing me the most is checking whether the various
> > arguments are legitimate, and printing a helpful error message if not.
> >
> > I was able to achieve this by using Maybe and error on failed pattern
> > match, but as said, seems kind of overly complicated.
> >
> > Is there a simpler way to do the following, eg for function
> >
> >   gen_gnuplot_financial_script :: String -> String -> String -> String -> IO ()
> >
> > ?
> >
> > By the way this is being used in
> >
> > http://code.google.com/p/gnuplotwebinterface/
> >
> > **************
> >
> > module Common where
> >
> > gnuplot_png_settings = "set terminal png transparent nocrop enhanced
> > size 600,400\n" ++
> >                        "set pm3d implicit at s"
> >
> > gnuplot_math_settings =  gnuplot_png_settings ++ "\n" ++
> >                          "set border 4095 \n\
> >                          \  set xlabel \"x\" \n\
> >                          \  set ylabel \"y\""
> >
> > gnuplot_timeseries_settings = gnuplot_png_settings ++ "\n" ++
> >                               "set xdata time           # The x axis
> > data is time \n" ++
> >                               "set timefmt \"%d-%b-%y\" # The dates in
> > the file look like 10-Jun-04 \n" ++
> >                               "set format x \"%b %d\"   #On the
> > x-axis, we want tics like Jun 10"
> >
> >
> > gen_gnuplot_math_script :: String -> String -> IO ()
> > gen_gnuplot_math_script style function = let maybePlotCmd = lookup
> > style style_to_plotcmd
> >                                              style_to_plotcmd =
> > [("math-2d","plot"),("math-3d","splot")]
> >                                            in case maybePlotCmd of
> >                                                 Just plotcmd ->
> > putStrLn $ gnuplot_math_settings ++ "\n" ++ plotcmd ++ " "  ++
> > function
> >                                                 _            -> error
> > $ "bad style: " ++ style
> >
> > gen_gnuplot_financial_script :: String -> String -> String -> String -> IO ()
> > gen_gnuplot_financial_script company displaymode startDate endDate
> >     = let maybeCompanyFile = lookup company     company_to_companyfile
> >           maybeModeString  = lookup displaymode displaymode_to_modestring
> >           maybeTitleEnd    = lookup displaymode displaymode_to_titleend
> >           company_to_companyfile =
> > [("ibm","data/ibm.dat"),("cisco","data/cisco.dat")]
> >           displaymode_to_modestring = [("points", "using 1:2 with linespoints"),
> >                                        ("candles","using
> > 1:($2+$3+$4+$5)/4:4:3 with yerrorbars")]
> >           displaymode_to_titleend = [("points","daily
> > prices"),("candles","opening prices")]
> >         in case ( maybeCompanyFile,
> >                   maybeModeString,
> >                   maybeTitleEnd ) of
> >                 ( Just companyfile,
> >                   Just modestring,
> >                   Just titleEnd) -> putStrLn $
> > gnuplot_timeseries_settings ++ "\n" ++
> >                               "plot [\"" ++ startDate ++ "\":\"" ++
> > endDate ++ "\"]"
> >                               ++ " '" ++ companyfile ++ "'"
> >                               ++ modestring
> >                               ++ " title \"" ++ company ++ " " ++
> > titleEnd ++ "\""
> >                 _ -> error $ "bad lookup. " ++ company ++     " ->
> > company file: " ++ ( show maybeCompanyFile ) ++ "\n" ++
> >                              "            " ++ displaymode ++ " ->
> > displaymode: "  ++ ( show maybeModeString ) ++ "\n" ++
> >                              "            " ++ displaymode ++ " ->
> > titleEnd: "     ++ ( show maybeTitleEnd)
> >
>