Using enum to set mupltiple flags in a field causes runtime failure.

[Hrothen]

Attempting to run the following minimal example given in the documentation for enum:

{-#LANGUAGE DeriveDataTypeable#-}

import System.Console.CmdArgs

data State = On | Off deriving (Data, Show)
data Mode = Mode {state :: [State]} deriving (Show, Data)
main = print =<< cmdArgs Mode{state = enum [[] &= ignore, [On] &= help "Turn on", [Off] &= help "Turn off"]}

results in the error message: System.Console.CmdArgs.Implicit, unexpected no available name: ? regardless of the flags passed to the executable.

I've only tested this with GHC 7.10.1

[ndmitchell]

A slightly simpler example:

data State = On | Off deriving (Data, Show, Typeable)
data Mode = Mode {state :: [State]} deriving (Show, Data, Typeable)
main = print =<< cmdArgs Mode{state = enum [[On], [Off]]}

And the smallest change that can fix it:

main = print =<< cmdArgs Mode{state = enum [[On] &= name "on", [Off] &= name "off"]}

So the problem is that enum tries to guess at flag names based on the value in each of [On] and [Off]. It can guess the value from a constructor, but not a list. Since it can't guess the name it raises an exception. The ways round this are:

• Make it guess a name by peeking inside the list. That works for my revised example, and might work for your original example or not (not sure if it looks at ignore first or not) - but it should be doable in this case. However, it's not going to be possible in all cases.
• Improve the error message, so it's clear what went wrong and how to fix it.

[Hrothen]

Tested by adding name to the non-ignored flags in my example and it does work properly without needing to name an ignored argument.