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Editorial Team
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Asked: 2026-05-21T04:29:55+00:00

I have the following snippet, which compiles fine, but when I try to run

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I have the following snippet, which compiles fine, but when I try to run it I get an error from cmdargs. *** Exception: System.Console.CmdArgs.Implicit, unexpected flag (edit): Main.Sel. It works if I change the Sel to Int, but in my real use case I’d like to have type enforcement that differentiates selections from Ints.

I can fix it by using Int with cmdargs and after checking the arguments casting it to Sel, but I think this is suboptimal solution.

{-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving #-}
import System.Console.CmdArgs
import System.Console.CmdArgs.Default

newtype Sel = Sel Int deriving (Num, Real, Integral, Show, Read, Data, Typeable, Enum, Eq, Ord, Default)

data Foo = Foo {
    edit :: Sel
  } deriving (Show, Read, Data, Typeable)

foo = Foo {
  edit = def
  }
main = do
  a <- cmdArgs foo
  print a
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1 Answer

  1. Editorial Team

    It is suboptimal indeed, and I’ve raised a bug to track the addition of this feature: http://code.google.com/p/ndmitchell/issues/detail?id=422

    I might have time to work on it this weekend, it shouldn’t be too hard (but depends how much further than newtypes I want to go). (For context, I wrote cmdargs)

    Update: I just released cmdargs 0.6.9 on Hackage which supports newtypes as requested in this question.

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