content: {-# LANGUAGE NondecreasingIndentation #-}
  -- | See <https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst>
  module Distribution.Backpack.MixLink (
      mixLink,
  ) where
  
  import Prelude ()
  import Distribution.Compat.Prelude hiding (mod)
  
  import Distribution.Backpack
  import Distribution.Backpack.UnifyM
  import Distribution.Backpack.FullUnitId
  import Distribution.Backpack.ModuleScope
  
  import qualified Distribution.Utils.UnionFind as UnionFind
  import Distribution.ModuleName
  import Distribution.Text
  import Distribution.Types.ComponentId
  
  import Text.PrettyPrint
  import Control.Monad
  import qualified Data.Map as Map
  import qualified Data.Foldable as F
  
  -----------------------------------------------------------------------
  -- Linking
  
  -- | Given to scopes of provisions and requirements, link them together.
  mixLink :: [ModuleScopeU s] -> UnifyM s (ModuleScopeU s)
  mixLink scopes = do
      let provs = Map.unionsWith (++) (map fst scopes)
          -- Invariant: any identically named holes refer to same mutable cell
          reqs  = Map.unionsWith (++) (map snd scopes)
          filled = Map.intersectionWithKey linkProvision provs reqs
      F.sequenceA_ filled
      let remaining = Map.difference reqs filled
      return (provs, remaining)
  
  -- | Link a list of possibly provided modules to a single
  -- requirement.  This applies a side-condition that all
  -- of the provided modules at the same name are *actually*
  -- the same module.
  linkProvision :: ModuleName
                -> [ModuleWithSourceU s] -- provs
                -> [ModuleWithSourceU s] -- reqs
                -> UnifyM s [ModuleWithSourceU s]
  linkProvision mod_name ret@(prov:provs) (req:reqs) = do
      -- TODO: coalesce all the non-unifying modules together
      forM_ provs $ \prov' -> do
          -- Careful: read it out BEFORE unifying, because the
          -- unification algorithm preemptively unifies modules
          mod  <- convertModuleU (unWithSource prov)
          mod' <- convertModuleU (unWithSource prov')
          r <- unify prov prov'
          case r of
              Just () -> return ()
              Nothing -> do
                  addErr $
                    text "Ambiguous module" <+> quotes (disp mod_name) $$
                    text "It could refer to" <+>
                      ( text "  " <+> (quotes (disp mod)  $$ in_scope_by (getSource prov)) $$
                        text "or" <+> (quotes (disp mod') $$ in_scope_by (getSource prov'))) $$
                    link_doc
      mod <- convertModuleU (unWithSource prov)
      req_mod <- convertModuleU (unWithSource req)
      self_cid <- fmap unify_self_cid getUnifEnv
      case mod of
        OpenModule (IndefFullUnitId cid _) _
          | cid == self_cid -> addErr $
              text "Cannot instantiate requirement" <+> quotes (disp mod_name) <+>
                  in_scope_by (getSource req) $$
              text "with locally defined module" <+> in_scope_by (getSource prov) $$
              text "as this would create a cyclic dependency, which GHC does not support." $$
              text "Try moving this module to a separate library, e.g.," $$
              text "create a new stanza: library 'sublib'."
        _ -> return ()
      r <- unify prov req
      case r of
          Just () -> return ()
          Nothing -> do
              -- TODO: Record and report WHERE the bad constraint came from
              addErr $ text "Could not instantiate requirement" <+> quotes (disp mod_name) $$
                       nest 4 (text "Expected:" <+> disp mod $$
                               text "Actual:  " <+> disp req_mod) $$
                       parens (text "This can occur if an exposed module of" <+>
                               text "a libraries shares a name with another module.") $$
                       link_doc
      return ret
    where
      unify s1 s2 = tryM $ addErrContext short_link_doc
                         $ unifyModule (unWithSource s1) (unWithSource s2)
      in_scope_by s = text "brought into scope by" <+> dispModuleSource s
      short_link_doc = text "While filling requirement" <+> quotes (disp mod_name)
      link_doc = text "While filling requirements of" <+> reqs_doc
      reqs_doc
        | null reqs = dispModuleSource (getSource req)
        | otherwise =  (       text "   " <+> dispModuleSource (getSource req)  $$
                        vcat [ text "and" <+> dispModuleSource (getSource r) | r <- reqs])
  linkProvision _ _ _ = error "linkProvision"
  
  
  
  -----------------------------------------------------------------------
  -- The unification algorithm
  
  -- This is based off of https://gist.github.com/amnn/559551517d020dbb6588
  -- which is a translation from Huet's thesis.
  
  unifyUnitId :: UnitIdU s -> UnitIdU s -> UnifyM s ()
  unifyUnitId uid1_u uid2_u
      | uid1_u == uid2_u = return ()
      | otherwise = do
          xuid1 <- liftST $ UnionFind.find uid1_u
          xuid2 <- liftST $ UnionFind.find uid2_u
          case (xuid1, xuid2) of
              (UnitIdThunkU u1, UnitIdThunkU u2)
                  | u1 == u2  -> return ()
                  | otherwise ->
                      failWith $ hang (text "Couldn't match unit IDs:") 4
                                 (text "   " <+> disp u1 $$
                                  text "and" <+> disp u2)
              (UnitIdThunkU uid1, UnitIdU _ cid2 insts2)
                  -> unifyThunkWith cid2 insts2 uid2_u uid1 uid1_u
              (UnitIdU _ cid1 insts1, UnitIdThunkU uid2)
                  -> unifyThunkWith cid1 insts1 uid1_u uid2 uid2_u
              (UnitIdU _ cid1 insts1, UnitIdU _ cid2 insts2)
                  -> unifyInner cid1 insts1 uid1_u cid2 insts2 uid2_u
  
  unifyThunkWith :: ComponentId
                 -> Map ModuleName (ModuleU s)
                 -> UnitIdU s
                 -> DefUnitId
                 -> UnitIdU s
                 -> UnifyM s ()
  unifyThunkWith cid1 insts1 uid1_u uid2 uid2_u = do
      db <- fmap unify_db getUnifEnv
      let FullUnitId cid2 insts2' = expandUnitId db uid2
      insts2 <- convertModuleSubst insts2'
      unifyInner cid1 insts1 uid1_u cid2 insts2 uid2_u
  
  unifyInner :: ComponentId
             -> Map ModuleName (ModuleU s)
             -> UnitIdU s
             -> ComponentId
             -> Map ModuleName (ModuleU s)
             -> UnitIdU s
             -> UnifyM s ()
  unifyInner cid1 insts1 uid1_u cid2 insts2 uid2_u = do
      when (cid1 /= cid2) $
          -- TODO: if we had a package identifier, could be an
          -- easier to understand error message.
          failWith $
              hang (text "Couldn't match component IDs:") 4
                   (text "   " <+> disp cid1 $$
                    text "and" <+> disp cid2)
      -- The KEY STEP which makes this a Huet-style unification
      -- algorithm.  (Also a payoff of using union-find.)
      -- We can build infinite unit IDs this way, which is necessary
      -- for support mutual recursion. NB: union keeps the SECOND
      -- descriptor, so we always arrange for a UnitIdThunkU to live
      -- there.
      liftST $ UnionFind.union uid1_u uid2_u
      F.sequenceA_ $ Map.intersectionWith unifyModule insts1 insts2
  
  -- | Imperatively unify two modules.
  unifyModule :: ModuleU s -> ModuleU s -> UnifyM s ()
  unifyModule mod1_u mod2_u
      | mod1_u == mod2_u = return ()
      | otherwise = do
          mod1 <- liftST $ UnionFind.find mod1_u
          mod2 <- liftST $ UnionFind.find mod2_u
          case (mod1, mod2) of
              (ModuleVarU _, _) -> liftST $ UnionFind.union mod1_u mod2_u
              (_, ModuleVarU _) -> liftST $ UnionFind.union mod2_u mod1_u
              (ModuleU uid1 mod_name1, ModuleU uid2 mod_name2) -> do
                  when (mod_name1 /= mod_name2) $
                      failWith $
                          hang (text "Cannot match module names") 4 $
                              text "   " <+> disp mod_name1 $$
                              text "and" <+> disp mod_name2
                  -- NB: this is not actually necessary (because we'll
                  -- detect loops eventually in 'unifyUnitId'), but it
                  -- seems harmless enough
                  liftST $ UnionFind.union mod1_u mod2_u
                  unifyUnitId uid1 uid2
  