    {-# LANGUAGE DeriveFunctor #-}
    -- | This data structure holds an updateable environment which is used
    -- when compiling module loops.
    module GHC.Driver.Env.KnotVars( KnotVars(..)
                                  , emptyKnotVars
                                  , knotVarsFromModuleEnv
                                  , knotVarElems
                                  , lookupKnotVars
                                  , knotVarsWithout
                                  ) where

    import GHC.Prelude
    import GHC.Unit.Types ( Module )
    import GHC.Unit.Module.Env
    import Data.Maybe
    import GHC.Utils.Outputable

    -- See Note [Why is KnotVars not a ModuleEnv]
    -- See Note [KnotVars invariants]
    data KnotVars a = KnotVars { forall a. KnotVars a -> [Module]
    kv_domain :: [Module] -- Domain of the function , Note [KnotVars: Why store the domain?]
                               -- Invariant: kv_lookup is surjective relative to kv_domain
                               , forall a. KnotVars a -> Module -> Maybe a
    kv_lookup :: Module -> Maybe a -- Lookup function
                               }
                    | NoKnotVars
                               deriving (forall a b. (a -> b) -> KnotVars a -> KnotVars b)
    -> (forall a b. a -> KnotVars b -> KnotVars a) -> Functor KnotVars
    forall a b. a -> KnotVars b -> KnotVars a
    forall a b. (a -> b) -> KnotVars a -> KnotVars b
    forall (f :: * -> *).
    (forall a b. (a -> b) -> f a -> f b)
    -> (forall a b. a -> f b -> f a) -> Functor f
    $cfmap :: forall a b. (a -> b) -> KnotVars a -> KnotVars b
    fmap :: forall a b. (a -> b) -> KnotVars a -> KnotVars b
    $c<$ :: forall a b. a -> KnotVars b -> KnotVars a
    <$ :: forall a b. a -> KnotVars b -> KnotVars a
    Functor

    instance Outputable (KnotVars a) where
      ppr :: KnotVars a -> SDoc
    ppr KnotVars a
    NoKnotVars = String -> SDoc
    text String
    "NoKnot"
      ppr (KnotVars [Module]
    dom Module -> Maybe a
    _lookup) = String -> SDoc
    text String
    "Knotty:" SDoc -> SDoc -> SDoc
    <+> [Module] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Module]
    dom

    emptyKnotVars :: KnotVars a
    emptyKnotVars :: forall a. KnotVars a
    emptyKnotVars = KnotVars a
    forall a. KnotVars a
    NoKnotVars

    knotVarsFromModuleEnv :: ModuleEnv a -> KnotVars a
    knotVarsFromModuleEnv :: forall a. ModuleEnv a -> KnotVars a
    knotVarsFromModuleEnv ModuleEnv a
    me | ModuleEnv a -> Bool
    forall a. ModuleEnv a -> Bool
    isEmptyModuleEnv ModuleEnv a
    me = KnotVars a
    forall a. KnotVars a
    NoKnotVars
    knotVarsFromModuleEnv ModuleEnv a
    me = [Module] -> (Module -> Maybe a) -> KnotVars a
    forall a. [Module] -> (Module -> Maybe a) -> KnotVars a
    KnotVars (ModuleEnv a -> [Module]
    forall a. ModuleEnv a -> [Module]
    moduleEnvKeys ModuleEnv a
    me) (ModuleEnv a -> Module -> Maybe a
    forall a. ModuleEnv a -> Module -> Maybe a
    lookupModuleEnv ModuleEnv a
    me)

    knotVarElems :: KnotVars a -> [a]
    knotVarElems :: forall a. KnotVars a -> [a]
    knotVarElems (KnotVars [Module]
    keys Module -> Maybe a
    lookup) = (Module -> Maybe a) -> [Module] -> [a]
    forall a b. (a -> Maybe b) -> [a] -> [b]
    mapMaybe Module -> Maybe a
    lookup [Module]
    keys
    knotVarElems KnotVars a
    NoKnotVars = []

    lookupKnotVars :: KnotVars a -> Module -> Maybe a
    lookupKnotVars :: forall a. KnotVars a -> Module -> Maybe a
    lookupKnotVars (KnotVars [Module]
    _ Module -> Maybe a
    lookup) Module
    x = Module -> Maybe a
    lookup Module
    x
    lookupKnotVars KnotVars a
    NoKnotVars Module
    _ = Maybe a
    forall a. Maybe a
    Nothing

    knotVarsWithout :: Module -> KnotVars a -> KnotVars a
    knotVarsWithout :: forall a. Module -> KnotVars a -> KnotVars a
    knotVarsWithout Module
    this_mod (KnotVars [Module]
    loop_mods Module -> Maybe a
    lkup) = [Module] -> (Module -> Maybe a) -> KnotVars a
    forall a. [Module] -> (Module -> Maybe a) -> KnotVars a
    KnotVars
      ((Module -> Bool) -> [Module] -> [Module]
    forall a. (a -> Bool) -> [a] -> [a]
    filter (Module -> Module -> Bool
    forall a. Eq a => a -> a -> Bool
    /= Module
    this_mod) [Module]
    loop_mods)
      (\Module
    that_mod -> if Module
    that_mod Module -> Module -> Bool
    forall a. Eq a => a -> a -> Bool
    == Module
    this_mod then Maybe a
    forall a. Maybe a
    Nothing else Module -> Maybe a
    lkup Module
    that_mod)
    knotVarsWithout Module
    _ KnotVars a
    NoKnotVars = KnotVars a
    forall a. KnotVars a
    NoKnotVars

    {-
    Note [Why is KnotVars not a ModuleEnv]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Initially 'KnotVars' was just a 'ModuleEnv a' but there is one tricky use of
    the data structure in 'mkDsEnvs' which required this generalised structure.

    In interactive mode the TypeEnvs from all the previous statements are merged
    togethed into one big TypeEnv. 'dsLookupVar' relies on `tcIfaceVar'. The normal
    lookup functions either look in the HPT or EPS but there is no entry for the `Ghci<N>` modules
    in either, so the whole merged TypeEnv for all previous Ghci* is stored in the
    `if_rec_types` variable and then lookup checks there in the case of any interactive module.

    This is a misuse of the `if_rec_types` variable which might be fixed in future if the
    Ghci<N> modules are just placed into the HPT like normal modules with implicit imports
    between them.

    Note [KnotVars: Why store the domain?]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Normally there's a 'Module' at hand to tell us which 'TypeEnv' we want to interrogate
    at a particular time, apart from one case, when constructing the in-scope set
    when linting an unfolding. In this case the whole environemnt is needed to tell us
    everything that's in-scope at top-level in the loop because whilst we are linting unfoldings
    the top-level identifiers from modules in the cycle might not be globalised properly yet.

    This could be refactored so that the lint functions knew about 'KnotVars' and delayed
    this check until deciding whether a variable was local or not.

    Note [KnotVars invariants]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~

    There is a simple invariant which should hold for the KnotVars constructor:

    * At the end of upsweep, there should be no live KnotVars

    This invariant is difficult to test but easy to check using ghc-debug. The usage of
    NoKnotVars is intended to make this invariant easier to check.

    The most common situation where a KnotVars is retained accidently is if a HscEnv
    which contains reference to a KnotVars is used during interface file loading. The
    thunks created during this process will retain a reference to the KnotVars. In theory,
    all these references should be removed by 'maybeRehydrateAfter' as that rehydrates all
    interface files in the loop without using KnotVars.

    At the time of writing (MP: Oct 21) the invariant doesn't actually hold but also
    doesn't seem to have too much of a negative consequence on compiler residency.
    In theory it could be quite bad as each KnotVars may retain a stale reference to an entire TypeEnv.

    See #20491
    -}