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

[Thomas Hartman]

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 <mutjida at gmail.com>:
> {----
>
> 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
>