#11577: GHCi accepts invalid programs when recompiling -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.10.2-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This ticket may or may not be a duplicate of #9729, but I ran into it again in a different context, so I'm reporting it. My GHC version is 7.10.2.20151030. The example is as minimal as I could make it. File Bar.hs {{{ {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-} module Bar where class Bar a b where (*^) :: a -> b -> b }}} File Foo.hs {{{ {-# LANGUAGE MultiParamTypeClasses #-} module Foo where import Bar import GHC.Prim newtype Foo fp = Foo [fp] instance {-# OVERLAPS #-} Bar (Foo fp) [fp] }}} File Main.hs {{{ {-# LANGUAGE MultiParamTypeClasses #-} import Foo import Bar newtype LW a = LW [a] instance Bar (Foo fp) (LW fp) where r *^ (LW xs) = LW $ r *^ xs }}} If I start GHCi with `ghci Main`, everything compiles as it should. Then I remove the `{-# OVERLAPS #-}` pragma in Foo.hs, which should break Main.hs. But GHCi happily recompiles with `:r`. A couple of notes: *with* the pragma, `:i Bar` gives the instances {{{ instance Bar (Foo fp) (LW fp) -- Defined at Main.hs:8:10 instance Bar a b => Bar a [b] -- Defined at Bar.hs:8:10 instance [overlap ok] Bar (Foo fp) [fp] -- Defined at Foo.hs:22:3 }}} while after (successfully, but incorrectly) recompiling without the pragma, I get the instances {{{ instance Bar (Foo fp) (LW fp) -- Defined at Main.hs:8:10 instance Bar a b => Bar a [b] -- Defined at Bar.hs:8:10 instance Bar (Foo fp) [fp] -- Defined at Foo.hs:22:3 }}} (i.e., without the `[overlap ok]`). Another strange thing is that the bug is only triggered if I import specific (superfluous) modules in Foo.hs. For example, if I replace GHC.Prim with Control.Monad or Data.Maybe, the bug is not triggered (i.e., GHCi correctly detects when I remove the pragma). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11577 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler