#9748: Disambiguate IO actions in GHCi with :set +t -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: GHCi | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 | Type of failure: hour) | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Currently GHCi's `:set +t` displays the same type for the following: {{{#!hs % ghci -ignore-dot-ghci GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :set +t Prelude> 'a' 'a' it :: Char Prelude> return 'a' 'a' it :: Char Prelude> return 'a' :: IO Char 'a' it :: Char }}} A user at `#haskell-beginners` ran into a problem where they mistook an expression that defaulted to IO for a pure expression (`Id . even <$> pure 5`), `:set +t` was suggested but ended up showing the incorrect type: {{{#!hs ghci> Id . even <$> pure 5 Id False it :: Id Bool }}} This is bad for pedagogical reasons, it would be an improvement to display the following: {{{#!hs Prelude> :set +t Prelude> 'a' 'a' it :: Char Prelude> return 'a' 'a' it :: Char <- return 'a' :: IO Char Prelude> return 'a' :: IO Char 'a' it :: Char <- return 'a' :: IO Char }}} mirroring existing syntax: {{{#!hs Prelude> :set -XScopedTypeVariables Prelude> a :: Char <- return 'a' :: IO Char a :: Char }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9748 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler