Hi, Not sure if this will help avoid the boilerplate, but I've always liked the approach at http://leiffrenzel.de/papers/commandline-options-in-haskell.html (particularly the section "Towards a higher level") for being able to specify defaults. It's the best resource I've found on command line options in Haskell so far. Levi
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
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