Sorry this was my complete confustion (List.\\) is not same as List.(\\). Ahn, Ki Yung 쓴 글:
Running ghci without any option or scripts to load, I've experienced some inconsistencies prelude List.\\ and List.(\\) . The former is in scope, but the latter is not. This seem strange. My expectation is that neither of them should be in scope unless we load the List module with with the :m command.
I'm using Debian distribution of GHC 6.10.4, and here is an example run of ghci to show this issue.
kyagrd@kyagrd:~/tmp$ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> [1,2,3,4] List.\\ [1,3] [2,4] Prelude> List.(\\) [1,2,3,4] [1,3]
<interactive>:1:0: Not in scope: data constructor `List'
<interactive>:1:5: Not in scope: `\\' Prelude> :q Leaving GHCi. kyagrd@kyagrd:~/tmp$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.4 kyagrd@kyagrd:~/tmp$ uname -a Linux kyagrd 2.6.30-2-686 #1 SMP Sat Sep 26 01:16:22 UTC 2009 i686 GNU/Linux