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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 }


  • simple, easy to look up settings


  • boring; have to write
    1. Flag type
    2. Settings record type
    3. default Settings
    4. processFlag entry
    5. 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


  • simple
  • very convenient to add flags (as a minimum, you have to write
    1. flag type
    2. GetOpt entry
    3. lookup code (but pay-as-you-go)


  • still a bit boilerplatey

Solution #3 No lists, just records (lhs2TeX)

This one is due to Andres Löh (maybe), 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")
  , Option "d" ["date-format"]
      (ReqArg (\x s -> s { dateFormat = Just x }) "TYPE")



  • very convenient/compact; have to write
    1. Flag type
    2. Settings record type/GetOpt in one go
    3. default Settings
  • easy to lookup flags


  • 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 ]
*Main> hasFlag LogFileFlag [ lf, tf ]

*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"


  • no more boilerplate only have to define
    1. flag type, although ugly
    2. 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)


  • 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
    data 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