Hi, Here is a possible two part response. Not literate code, just using > to distinguish code from everything else. A short answer ==============
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
For starters, you could squish these down into something like
flagToString :: Flag -> Maybe String flagToString (Filter s) = Just s flagToString (DateFormat s) = Just s ... flagToString _ = Nothing
Then you would have
getFilter = getString flagToString "Markdown.pl" getDateFormat = getString flagToString "%B %e, %Y"
A long answer ============= I have noticed a lot of ways of dealing with GetOpt flags in Hakell programs and thought it might be useful to catalogue them. A lot of this could be wrong btw, for example, advantages/disadvantages. But I think the general idea might be useful, so please add to this if you see other solutions. Solution #1 Ginormous record ---------------------------- Do you happen to have some giant recordful of command line parameters? Something like
data Settings = Settings { filter :: Maybe String , dateFormat :: Maybe String , blahBlah :: Maybe Blah ... , thisIsGetting :: RatherLargeIsntIt }
emptySettings :: Settings emptySettings = Settings { filter = Nothing , dateFormat = Nothing }
toSettings :: [Flag] -> Settings toSettings fs = toSettingsH fs emptySettings
toSettingsH :: [Flag] -> Settings -> Settings toSettingsH (Filter s:fs) i = toSettingsH fs (i { filter = s }) toSettingsH (DateFormat s:fs) i = toSettingsH fs (i { dateFormat = i })
Note: You can make this a little less painful by factoring out the recursion (took me a while to realise this!).
toSettings fs = foldr ($) emptySettings (map processFlag fs)
processFlag :: Flag -> Settings -> Settings processFlag (Filter s) i = i { filter = Just s } processFlag (DateFormat s) i = i { dateFormat = s } ...
Advantages: - simple, easy to look up settings Disadvantages: boring; have to write (i) Flag type (ii) Settings record type (iii) default Settings (iv) processFlag entry (v) GetOpt entry record gets really really huge if you have a lot of flags Solution #2 List of flags (darcs) ------------------------- Don't bother keeping any records around, just pass around a big list of flags to functions that depend on settings. if the flag has any parameters, you can't just write (DateFormat `elem` fs); you'll have to write some boilerplate along the lines of
hasDateFormat :: [Flag] -> Bool hasDateFormat (DateFormat s:fs) = True hasDateFormat (_:fs) = hasDateFormat fs hasDateFormat [] = False
getDateFormat :: [Flag] -> Maybe String getDateFormat (DateFormat s:fs) = Just s getDateFormat (_:fs) = getDateFormat fs getDateFormat [] = Nothing
which again can be factored out...
fromDateFormat :: Flag -> Maybe String fromDateFormat (DateFormat x) = Just x fromDateFormat _ = Nothing
hasDateFormat fs = any (isJust.fromDateFormat) fs getDateFormat fs = listToMaybe $ mapMaybe fromDateFormat fs
Still, this is more pay-as-you-go in the sense that not all flags need to be accessed, so maybe you end up writing less boilerplate overall Advantages: simple very convenient to add flags (as a minimum, you have to write (i) flag type (ii) GetOpt entry (iii) lookup code (but pay-as-you-go) Disadvantages: still a bit boilerplatey Solution #3 No lists, just records (lhs2TeX) ---------------------------------- This one is due to Andres Löh, I think although my rendition of it may not be as nice as his. Ever considered that your Settings record could almost be your Flag type? The trick here is recognising that constructors are functions too and what GetOpt really wants is just a function, not necessarily a constructor.
type Flag a = (a -> Settings -> Settings)
options :: [OptDescr Flag] options = [ Option "f" ["filter"] (ReqArg (\x s -> s { filter = Just x }) "TYPE") "blahblah" , Option "d" ["date-format"] (ReqArg (\x s -> s { dateFormat = Just x }) "TYPE") "blahblah"
]
Advantages: very convenient/compact; have to write (i) Flag type (ii) Settings record type/GetOpt in one go (iii) default Settings easy to lookup flags Disadvantages: Not as flexible - can't group flags into blocks and have different programs that use different subsets of flags (without sharing the same Setting type) - everything must go into Settings - seems harder to say stuff like 'if flag X is set and flag Y are in the list of Flags, then parameterise flag Z this way' or 'flags X and Y are mutually exclusive' Solution #4 List of flags + existential types (GenI) --------------------------------------------- See attached code. Basically motivated by your idea that we should be able to pass constructors around like arguments. Note: attached code is written by very non-expert Eric. So be ready to consider it wrong and horrible in more ways than one can imagine. Using it looks like this: *Main> hasFlag LogFileFlag [ tf ] False *Main> hasFlag LogFileFlag [ lf, tf ] True *Main> [lf, tf] [Flag LogFileFlag "hi",Flag TimeoutFlag 3] *Main> setFlag LogFileFlag "bar" [ lf, tf ] [Flag LogFileFlag "bar",Flag TimeoutFlag 3] *Main> getFlag LogFileFlag [lf,tf] Just "bar" Advantages: - no more boilerplate only have to define (i) flag type, although ugly (ii) getopt stuff - extensible (as any list of flags approach) - mix-n-matchable (cf #3; different programs can share subset of flags) - can really just say 'getFlag FooFlag' - setFlag / deleteFlag (I'm not claiming there are more advantages; it's just that I wrote this and can remember why) Disadavantages: - can't enforce that some flags are always set (cf #1 and #4) - making things too complicated! existential types seems like overkill for GetOpt (well, I mostly did this to learn what they were) - ugly cpp macro or repetitive ata FilterFlag = FilterFlag String deriving (Eq, Show, Typeable) data TimeoutFlag = TimeoutFlag Int deriving (Eq, Show, Typeable) - ugly GetOpt wrappers reqArg :: forall f x . (Eq f, Show f, Typeable f, Eq x, Show x, Typeable x) => (x -> f) -- ^ flag -> (String -> x) -- ^ string reader for flag (probably |id| if already a String) -> String -- ^ description -> ArgDescr Flag reqArg s fn desc = ReqArg (\x -> Flag s (fn x)) desc -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.