#15016: Referencing a do-bound variable in a rec block with ApplicativeDo results in variable not in scope during type checking -------------------------------------+------------------------------------- Reporter: rjmk | Owner: sighingnow Type: bug | Status: new Priority: normal | Milestone: 8.4.3 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by sighingnow): * owner: (none) => sighingnow Comment: I think the "not is scope" is by design. See more discussion under ticket:4148. When we use `rec` in `do` blocks, rather than `mdo` blocks, the outer variables won't be included into the scope of `rec` block. The real problem is when there's no binding for `a` inside `rec` block, the rhs `a` in `let b = a` will have the same name with the outer `a`. For code: {{{#!hs }}} The rename produces: {{{#!hs Lib.f :: MonadFix m_auv => m_auv () Lib.f = do a_auw <- return () | () <- do rec let b_aux = a_auw return () return () }}} But for code: {{{#!hs f :: MonadFix m => m () f = do a <- return () rec let b = a a = () return () }}} The rename produces: {{{#!hs Lib.f :: MonadFix m_auv => m_auv () Lib.f = do a_auw <- return () | () <- do rec let b_aux = a_auy a_auy = () return () return () }}} Indeed it's a bug. I'm optimistic to assign that to myself :) We also should note this behavior of `rec` in documentation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15016#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler