[Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args

[Dave Bayer]

Ok, I'm writing a command line tool, using System.Console.GetOpt to  
handle command line arguments. My Flags structure so far is

> data Flag
>     = Filter String
>     | DateFormat String
>     | DocStart String
>     | DocEnd String
...

and I want to write accessor functions that return the strings if  
specified, otherwise returning a default. The best I've been able to  
do is this:

> getFilter = getString f "Markdown.pl"
>     where f (Filter s) = Just s
>           f _ = Nothing
>
> getDateFormat = getString f "%B %e, %Y"
>     where f (DateFormat s) = Just s
>           f _ = Nothing
>
> getDocStart = getString f "^{-$"
>     where f (DocStart s) = Just s
>           f _ = Nothing
>
> getDocEnd = getString f "^-}$"
>     where f (DocEnd s) = Just s
>           f _ = Nothing

using a generic accessor function `getString`.

There are eight (and growing) needless lines here, where what I  
really want to do is to pass the constructors `Filter`, `DateFormat`,  
`DocStart`, or `DocEnd` to the function `getString`. ghci types each  
of these as `String -> Flag`, so one at least knows how to type such  
a `getString`, but using a constructor-passed-as-an-argument in a  
pattern match is of course a "Parse error in pattern". (I expected as  
much, but I had to try... `String -> Flag` is not enough information  
to make it clear we're passing a constructor, rather than some hairy  
arbitrary function, so such a pattern match would be undecidable in  
general.)

So what's the right idiom for avoiding this boilerplate?

Thanks,
Dave