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

[Thomas Hartman]

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)