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

18 Apr 2007


      Without the flag:

thartman@linodehaskell:~/websites/gnuplotwebinterface/cgi-bin/gnuplot-scripts>./financial2.hs
cisco candles 31-May-04 11-Jun-04

./financial2.hs:58:0:
    Illegal instance declaration for `MyString (Either String String)'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `MyString (Either String String)'

./financial2.hs:61:0:
    Illegal instance declaration for `MyString String'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `MyString String'

thartman@linodehaskell:~/websites/gnuplotwebinterface/cgi-bin/gnuplot-scripts>cat
--number financial2.hs | tail
    58  instance MyString (Either String String)
    59     where mystr = id
    60
    61  instance MyString String
    62     where mystr = Right
    63
    64  x <++> y = do xv <- mystr x
    65                yv <- mystr y
    66                return $ xv ++ yv
    67


2007/4/18, Thomas Hartman :
...
Looks like this needs to be run with
#!/usr/lib/ghc-6.6/bin/runghc
{-# OPTIONS_GHC -fglasgow-exts #-}
to get
instance MyString (Either String String)
   where mystr = id
instance MyString String
   where mystr = Right
to work.
I'm curious if there is a community feeling on whether glasgow-exts is
sort of a de-facto standard now? Or is it common to try to get things
to work without this, for maximum portability?
Is there an easy way to get the above to work without the compiler flag?
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