nobsun です。

> GHCi では
> 
> Prelude> Compiling Main             ( c:/develop/mail.lhs, interpreted )
> 
> c:/develop/mail.lhs:10:
>      Illegal instance declaration for `FooArg (Int, Int)'
> 	(The instance type must be of form (T a b c)
> 	 where T is not a synonym, and a,b,c are distinct type variables)
>      In the instance declaration for `FooArg (Int, Int)'
> 
> c:/develop/mail.lhs:14:
>      Illegal instance declaration for `FooArg (Int, Int, Int)'
> 	(The instance type must be of form (T a b c)
> 	 where T is not a synonym, and a,b,c are distinct type variables)
>      In the instance declaration for `FooArg (Int, Int, Int)'
> Failed, modules loaded: none.
> Prelude> Leaving GHCi.

どうやら、Haskell98 の仕様範囲ではだめなようですね。
ghci では、-fglasgow-exts オプションを付けてください。

--nobsun

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