Neil Mitchell
then lookup, instead of just "" as the else clause.
Thanks, all. After digesting what was on this thread as I woke up this morning, I ended up writing something rather close to this. I have a reusable wrapper around System.Console.GetOpt that adds
type Opt a = (a,String)
noArg :: a -> ArgDescr (Opt a) noArg x = NoArg (x,"")
reqArg :: a -> String -> ArgDescr (Opt a) reqArg x s = ReqArg f s where f y = (x,y)
optArg :: a -> String -> ArgDescr (Opt a) optArg x s = OptArg f s where f (Just y) = (x,y) f Nothing = (x,"")
isOption :: Eq a => a -> [Opt a] -> Bool isOption opt assoc = case lookup opt assoc of Nothing -> False Just _ -> True
getOption :: Eq a => a -> [Opt a] -> String getOption opt assoc = case lookup opt assoc of Nothing -> "" Just s -> s
Then in a project-specific module I write
data Flag = Filter | DateFormat | DocStart | DocEnd | ForceStyle | Help deriving (Eq)
defaults :: [Opt Flag] defaults = [ (Filter, "Markdown.pl") , (DateFormat, "%B %e, %Y") , (DocStart, "^\\s*{-\\s*$") , (DocEnd, "^\\s*-}\\s*$") ]
flags :: [OptDescr (Opt Flag)] flags = [ Option ['s'] ["style"] (noArg ForceStyle) "Overwrite existing style.css" , Option ['m'] ["markup"] (reqArg Filter "path") "Path to Markdown-style markup filter" , Option ['d'] ["date"] (reqArg DateFormat "format") "Unix-style modification date format" , Option ['a'] ["start"] (reqArg DocStart "string") "Documentation start string" , Option ['b'] ["end"] (reqArg DocEnd "string") "Documentation end string" , Option ['h'] ["help"] (noArg Help) "Print this help message" ]
which looks "almost" like the sample code I started with. Reading quickly, one might miss the case change from `NoArg` to `noArg`, etc. This is simple, and it works, with less option-specific boilerplate. One could imagine generating `flags` automatically from an extension of `defaults`, but I'm content to move on. The relevant code is at http://www.math.columbia.edu/~bayer/Haskell/Annote/GetOpt.html http://www.math.columbia.edu/~bayer/Haskell/Annote/Flags.html http://www.math.columbia.edu/~bayer/Haskell/Annote/Main.html