#11698: GHC's tct_closed flag is not being set correctly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The `Note [Bindings with closed types]` in `TcEnv` describes when a binding is "closed", which in turn affects generalisation. (See the Note for details.) But the implementation doesn't follow the Note. Instead it uses this function in `TcEnv`: {{{ isClosedLetBndr :: Id -> TopLevelFlag -- See Note [Bindings with closed types] in TcRnTypes -- Note that we decided if a let-bound variable is closed by -- looking at its type, which is slightly more liberal, and a whole -- lot easier to implement, than looking at its free variables isClosedLetBndr id | isEmptyVarSet (tyCoVarsOfType (idType id)) = TopLevel | otherwise = NotTopLevel }}} It may be easier but it's also wrong. Consider {{{ f x = ( let g y = x+y in ... , x::Int) }}} Is `g` closed (which affects how definitions in `...` are generalised)? Well if we typecheck the second element of the tuple first, we may "know" that `x::Int` by the time we are inferring a type for `g`, conclude that `g` has no free type variables, and say that it is closed. But if we do the `x::Int` part second, so while type checking the `let` we think that `x::alpha`, then we'll say that `g` is open. This looks nasty. I think we should do it the way the `Note` says. Thanks to Facundo for pointing this out. Not urgent, but plainly wrong. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11698 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler