Distribution/PackageDescription/Configuration.hs

    -- -fno-warn-deprecations for use of Map.foldWithKey
    {-# OPTIONS_GHC -fno-warn-deprecations #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.PackageDescription.Configuration
    -- Copyright   :  Thomas Schilling, 2007
    -- License     :  BSD3
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- This is about the cabal configurations feature. It exports
    -- 'finalizePD' and 'flattenPackageDescription' which are
    -- functions for converting 'GenericPackageDescription's down to
    -- 'PackageDescription's. It has code for working with the tree of conditions
    -- and resolving or flattening conditions.

    module Distribution.PackageDescription.Configuration (
        finalizePD,
        finalizePackageDescription,
        flattenPackageDescription,

        -- Utils
        parseCondition,
        freeVars,
        extractCondition,
        extractConditions,
        addBuildableCondition,
        mapCondTree,
        mapTreeData,
        mapTreeConds,
        mapTreeConstrs,
        transformAllBuildInfos,
        transformAllBuildDepends,
      ) where

    import Prelude ()
    import Distribution.Compat.Prelude

    import Distribution.PackageDescription
    import Distribution.PackageDescription.Utils
    import Distribution.Version
    import Distribution.Compiler
    import Distribution.System
    import Distribution.Simple.Utils
    import Distribution.Text
    import Distribution.Compat.ReadP as ReadP hiding ( char )
    import qualified Distribution.Compat.ReadP as ReadP ( char )
    import Distribution.Types.ComponentRequestedSpec
    import Distribution.Types.ForeignLib
    import Distribution.Types.Component
    import Distribution.Types.Dependency
    import Distribution.Types.PackageName
    import Distribution.Types.UnqualComponentName
    import Distribution.Types.CondTree
    import Distribution.Types.Condition
    import Distribution.Types.DependencyMap

    import qualified Data.Map as Map
    import Data.Tree ( Tree(Node) )

    ------------------------------------------------------------------------------

    -- | Simplify a configuration condition using the OS and arch names.  Returns
    --   the names of all the flags occurring in the condition.
    simplifyWithSysParams :: OS -> Arch -> CompilerInfo -> Condition ConfVar
                          -> (Condition FlagName, [FlagName])
    simplifyWithSysParams os arch cinfo cond = (cond', flags)
      where
        (cond', flags) = simplifyCondition cond interp
        interp (OS os')    = Right $ os' == os
        interp (Arch arch') = Right $ arch' == arch
        interp (Impl comp vr)
          | matchImpl (compilerInfoId cinfo) = Right True
          | otherwise = case compilerInfoCompat cinfo of
              -- fixme: treat Nothing as unknown, rather than empty list once we
              --        support partial resolution of system parameters
              Nothing     -> Right False
              Just compat -> Right (any matchImpl compat)
              where
                matchImpl (CompilerId c v) = comp == c && v `withinRange` vr
        interp (Flag f) = Left f

    -- TODO: Add instances and check
    --
    -- prop_sC_idempotent cond a o = cond' == cond''
    --   where
    --     cond'  = simplifyCondition cond a o
    --     cond'' = simplifyCondition cond' a o
    --
    -- prop_sC_noLits cond a o = isLit res || not (hasLits res)
    --   where
    --     res = simplifyCondition cond a o
    --     hasLits (Lit _) = True
    --     hasLits (CNot c) = hasLits c
    --     hasLits (COr l r) = hasLits l || hasLits r
    --     hasLits (CAnd l r) = hasLits l || hasLits r
    --     hasLits _ = False
    --

    -- | Parse a configuration condition from a string.
    parseCondition :: ReadP r (Condition ConfVar)
    parseCondition = condOr
      where
        condOr   = sepBy1 condAnd (oper "||") >>= return . foldl1 COr
        condAnd  = sepBy1 cond (oper "&&")>>= return . foldl1 CAnd
        cond     = sp >> (boolLiteral +++ inparens condOr +++ notCond +++ osCond
                          +++ archCond +++ flagCond +++ implCond )
        inparens   = between (ReadP.char '(' >> sp) (sp >> ReadP.char ')' >> sp)
        notCond  = ReadP.char '!' >> sp >> cond >>= return . CNot
        osCond   = string "os" >> sp >> inparens osIdent >>= return . Var
        archCond = string "arch" >> sp >> inparens archIdent >>= return . Var
        flagCond = string "flag" >> sp >> inparens flagIdent >>= return . Var
        implCond = string "impl" >> sp >> inparens implIdent >>= return . Var
        boolLiteral   = fmap Lit  parse
        archIdent     = fmap Arch parse
        osIdent       = fmap OS   parse
        flagIdent     = fmap (Flag . mkFlagName . lowercase) (munch1 isIdentChar)
        isIdentChar c = isAlphaNum c || c == '_' || c == '-'
        oper s        = sp >> string s >> sp
        sp            = skipSpaces
        implIdent     = do i <- parse
                           vr <- sp >> option anyVersion parse
                           return $ Impl i vr

    ------------------------------------------------------------------------------

    -- | Result of dependency test. Isomorphic to @Maybe d@ but renamed for
    --   clarity.
    data DepTestRslt d = DepOk | MissingDeps d

    instance Semigroup d => Monoid (DepTestRslt d) where
        mempty = DepOk
        mappend = (<>)

    instance Semigroup d => Semigroup (DepTestRslt d) where
        DepOk <> x     = x
        x     <> DepOk = x
        (MissingDeps d) <> (MissingDeps d') = MissingDeps (d <> d')

    -- | Try to find a flag assignment that satisfies the constraints of all trees.
    --
    -- Returns either the missing dependencies, or a tuple containing the
    -- resulting data, the associated dependencies, and the chosen flag
    -- assignments.
    --
    -- In case of failure, the union of the dependencies that led to backtracking
    -- on all branches is returned.
    -- [TODO: Could also be specified with a function argument.]
    --
    -- TODO: The current algorithm is rather naive.  A better approach would be to:
    --
    -- * Rule out possible paths, by taking a look at the associated dependencies.
    --
    -- * Infer the required values for the conditions of these paths, and
    --   calculate the required domains for the variables used in these
    --   conditions.  Then picking a flag assignment would be linear (I guess).
    --
    -- This would require some sort of SAT solving, though, thus it's not
    -- implemented unless we really need it.
    --
    resolveWithFlags ::
         [(FlagName,[Bool])]
            -- ^ Domain for each flag name, will be tested in order.
      -> ComponentRequestedSpec
      -> OS      -- ^ OS as returned by Distribution.System.buildOS
      -> Arch    -- ^ Arch as returned by Distribution.System.buildArch
      -> CompilerInfo  -- ^ Compiler information
      -> [Dependency]  -- ^ Additional constraints
      -> [CondTree ConfVar [Dependency] PDTagged]
      -> ([Dependency] -> DepTestRslt [Dependency])  -- ^ Dependency test function.
      -> Either [Dependency] (TargetSet PDTagged, FlagAssignment)
           -- ^ Either the missing dependencies (error case), or a pair of
           -- (set of build targets with dependencies, chosen flag assignments)
    resolveWithFlags dom enabled os arch impl constrs trees checkDeps =
        either (Left . fromDepMapUnion) Right $ explore (build [] dom)
      where
        extraConstrs = toDepMap constrs

        -- simplify trees by (partially) evaluating all conditions and converting
        -- dependencies to dependency maps.
        simplifiedTrees :: [CondTree FlagName DependencyMap PDTagged]
        simplifiedTrees = map ( mapTreeConstrs toDepMap  -- convert to maps
                              . addBuildableConditionPDTagged
                              . mapTreeConds (fst . simplifyWithSysParams os arch impl))
                              trees

        -- @explore@ searches a tree of assignments, backtracking whenever a flag
        -- introduces a dependency that cannot be satisfied.  If there is no
        -- solution, @explore@ returns the union of all dependencies that caused
        -- it to backtrack.  Since the tree is constructed lazily, we avoid some
        -- computation overhead in the successful case.
        explore :: Tree FlagAssignment
                -> Either DepMapUnion (TargetSet PDTagged, FlagAssignment)
        explore (Node flags ts) =
            let targetSet = TargetSet $ flip map simplifiedTrees $
                    -- apply additional constraints to all dependencies
                    first (`constrainBy` extraConstrs) .
                    simplifyCondTree (env flags)
                deps = overallDependencies enabled targetSet
            in case checkDeps (fromDepMap deps) of
                 DepOk | null ts   -> Right (targetSet, flags)
                       | otherwise -> tryAll $ map explore ts
                 MissingDeps mds   -> Left (toDepMapUnion mds)

        -- Builds a tree of all possible flag assignments.  Internal nodes
        -- have only partial assignments.
        build :: FlagAssignment -> [(FlagName, [Bool])] -> Tree FlagAssignment
        build assigned [] = Node assigned []
        build assigned ((fn, vals) : unassigned) =
            Node assigned $ map (\v -> build ((fn, v) : assigned) unassigned) vals

        tryAll :: [Either DepMapUnion a] -> Either DepMapUnion a
        tryAll = foldr mp mz

        -- special version of `mplus' for our local purposes
        mp :: Either DepMapUnion a -> Either DepMapUnion a -> Either DepMapUnion a
        mp m@(Right _) _           = m
        mp _           m@(Right _) = m
        mp (Left xs)   (Left ys)   =
            let union = Map.foldrWithKey (Map.insertWith' combine)
                        (unDepMapUnion xs) (unDepMapUnion ys)
                combine x y = simplifyVersionRange $ unionVersionRanges x y
            in union `seq` Left (DepMapUnion union)

        -- `mzero'
        mz :: Either DepMapUnion a
        mz = Left (DepMapUnion Map.empty)

        env :: FlagAssignment -> FlagName -> Either FlagName Bool
        env flags flag = (maybe (Left flag) Right . lookup flag) flags

    -- | Transforms a 'CondTree' by putting the input under the "then" branch of a
    -- conditional that is True when Buildable is True. If 'addBuildableCondition'
    -- can determine that Buildable is always True, it returns the input unchanged.
    -- If Buildable is always False, it returns the empty 'CondTree'.
    addBuildableCondition :: (Eq v, Monoid a, Monoid c) => (a -> BuildInfo)
                          -> CondTree v c a
                          -> CondTree v c a
    addBuildableCondition getInfo t =
      case extractCondition (buildable . getInfo) t of
        Lit True  -> t
        Lit False -> CondNode mempty mempty []
        c         -> CondNode mempty mempty [condIfThen c t]

    -- | This is a special version of 'addBuildableCondition' for the 'PDTagged'
    -- type.
    --
    -- It is not simply a specialisation. It is more complicated than it
    -- ought to be because of the way the 'PDTagged' monoid instance works. The
    -- @mempty = 'PDNull'@ forgets the component type, which has the effect of
    -- completely deleting components that are not buildable.
    --
    -- See <https://github.com/haskell/cabal/pull/4094> for more details.
    --
    addBuildableConditionPDTagged :: (Eq v, Monoid c) =>
                                     CondTree v c PDTagged
                                  -> CondTree v c PDTagged
    addBuildableConditionPDTagged t =
        case extractCondition (buildable . getInfo) t of
          Lit True  -> t
          Lit False -> deleteConstraints t
          c         -> CondNode mempty mempty [condIfThenElse c t (deleteConstraints t)]
      where
        deleteConstraints = mapTreeConstrs (const mempty)

        getInfo :: PDTagged -> BuildInfo
        getInfo (Lib l) = libBuildInfo l
        getInfo (SubComp _ c) = componentBuildInfo c
        getInfo PDNull = mempty

    -- Note: extracting buildable conditions.
    -- --------------------------------------
    --
    -- If the conditions in a cond tree lead to Buildable being set to False, then
    -- none of the dependencies for this cond tree should actually be taken into
    -- account. On the other hand, some of the flags may only be decided in the
    -- solver, so we cannot necessarily make the decision whether a component is
    -- Buildable or not prior to solving.
    --
    -- What we are doing here is to partially evaluate a condition tree in order to
    -- extract the condition under which Buildable is True. The predicate determines
    -- whether data under a 'CondTree' is buildable.

    -- | Extract conditions matched by the given predicate from all cond trees in a
    -- 'GenericPackageDescription'.
    extractConditions :: (BuildInfo -> Bool) -> GenericPackageDescription
                         -> [Condition ConfVar]
    extractConditions f gpkg =
      concat [
          extractCondition (f . libBuildInfo)             <$> maybeToList (condLibrary gpkg)
        , extractCondition (f . libBuildInfo)       . snd <$> condSubLibraries   gpkg
        , extractCondition (f . buildInfo)          . snd <$> condExecutables gpkg
        , extractCondition (f . testBuildInfo)      . snd <$> condTestSuites  gpkg
        , extractCondition (f . benchmarkBuildInfo) . snd <$> condBenchmarks  gpkg
        ]

    -- | A map of dependencies that combines version ranges using 'unionVersionRanges'.
    newtype DepMapUnion = DepMapUnion { unDepMapUnion :: Map PackageName VersionRange }

    toDepMapUnion :: [Dependency] -> DepMapUnion
    toDepMapUnion ds =
      DepMapUnion $ Map.fromListWith unionVersionRanges [ (p,vr) | Dependency p vr <- ds ]

    fromDepMapUnion :: DepMapUnion -> [Dependency]
    fromDepMapUnion m = [ Dependency p vr | (p,vr) <- Map.toList (unDepMapUnion m) ]

    freeVars :: CondTree ConfVar c a  -> [FlagName]
    freeVars t = [ f | Flag f <- freeVars' t ]
      where
        freeVars' (CondNode _ _ ifs) = concatMap compfv ifs
        compfv (CondBranch c ct mct) = condfv c ++ freeVars' ct ++ maybe [] freeVars' mct
        condfv c = case c of
          Var v      -> [v]
          Lit _      -> []
          CNot c'    -> condfv c'
          COr c1 c2  -> condfv c1 ++ condfv c2
          CAnd c1 c2 -> condfv c1 ++ condfv c2

    ------------------------------------------------------------------------------

    -- | A set of targets with their package dependencies
    newtype TargetSet a = TargetSet [(DependencyMap, a)]

    -- | Combine the target-specific dependencies in a TargetSet to give the
    -- dependencies for the package as a whole.
    overallDependencies :: ComponentRequestedSpec -> TargetSet PDTagged -> DependencyMap
    overallDependencies enabled (TargetSet targets) = mconcat depss
      where
        (depss, _) = unzip $ filter (removeDisabledSections . snd) targets
        removeDisabledSections :: PDTagged -> Bool
        -- UGH. The embedded componentName in the 'Component's here is
        -- BLANK.  I don't know whose fault this is but I'll use the tag
        -- instead. -- ezyang
        removeDisabledSections (Lib _)     = componentNameRequested enabled CLibName
        removeDisabledSections (SubComp t c)
            -- Do NOT use componentName
            = componentNameRequested enabled
            $ case c of
                CLib  _ -> CSubLibName t
                CFLib _ -> CFLibName   t
                CExe  _ -> CExeName    t
                CTest _ -> CTestName   t
                CBench _ -> CBenchName t
        removeDisabledSections PDNull      = True

    -- | Collect up the targets in a TargetSet of tagged targets, storing the
    -- dependencies as we go.
    flattenTaggedTargets :: TargetSet PDTagged -> (Maybe Library, [(UnqualComponentName, Component)])
    flattenTaggedTargets (TargetSet targets) = foldr untag (Nothing, []) targets
      where
        untag (_, Lib _) (Just _, _) = userBug "Only one library expected"
        untag (_, Lib l) (Nothing, comps) = (Just l, comps)
        untag (_, SubComp n c) (mb_lib, comps)
            | any ((== n) . fst) comps =
              userBug $ "There exist several components with the same name: '" ++ unUnqualComponentName n ++ "'"

            | otherwise = (mb_lib, (n, c) : comps)

        untag (_, PDNull) x = x  -- actually this should not happen, but let's be liberal

    ------------------------------------------------------------------------------
    -- Convert GenericPackageDescription to PackageDescription
    --

    data PDTagged = Lib Library
                  | SubComp UnqualComponentName Component
                  | PDNull
                  deriving Show

    instance Monoid PDTagged where
        mempty = PDNull
        mappend = (<>)

    instance Semigroup PDTagged where
        PDNull    <> x      = x
        x         <> PDNull = x
        Lib l     <> Lib l' = Lib (l <> l')
        SubComp n x <> SubComp n' x' | n == n' = SubComp n (x <> x')
        _         <> _  = cabalBug "Cannot combine incompatible tags"

    -- | Create a package description with all configurations resolved.
    --
    -- This function takes a `GenericPackageDescription` and several environment
    -- parameters and tries to generate `PackageDescription` by finding a flag
    -- assignment that result in satisfiable dependencies.
    --
    -- It takes as inputs a not necessarily complete specifications of flags
    -- assignments, an optional package index as well as platform parameters.  If
    -- some flags are not assigned explicitly, this function will try to pick an
    -- assignment that causes this function to succeed.  The package index is
    -- optional since on some platforms we cannot determine which packages have
    -- been installed before.  When no package index is supplied, every dependency
    -- is assumed to be satisfiable, therefore all not explicitly assigned flags
    -- will get their default values.
    --
    -- This function will fail if it cannot find a flag assignment that leads to
    -- satisfiable dependencies.  (It will not try alternative assignments for
    -- explicitly specified flags.)  In case of failure it will return the missing
    -- dependencies that it encountered when trying different flag assignments.
    -- On success, it will return the package description and the full flag
    -- assignment chosen.
    --
    -- Note that this drops any stanzas which have @buildable: False@.  While
    -- this is arguably the right thing to do, it means we give bad error
    -- messages in some situations, see #3858.
    --
    finalizePD ::
         FlagAssignment  -- ^ Explicitly specified flag assignments
      -> ComponentRequestedSpec
      -> (Dependency -> Bool) -- ^ Is a given dependency satisfiable from the set of
                              -- available packages?  If this is unknown then use
                              -- True.
      -> Platform      -- ^ The 'Arch' and 'OS'
      -> CompilerInfo  -- ^ Compiler information
      -> [Dependency]  -- ^ Additional constraints
      -> GenericPackageDescription
      -> Either [Dependency]
                (PackageDescription, FlagAssignment)
                 -- ^ Either missing dependencies or the resolved package
                 -- description along with the flag assignments chosen.
    finalizePD userflags enabled satisfyDep
            (Platform arch os) impl constraints
            (GenericPackageDescription pkg flags mb_lib0 sub_libs0 flibs0 exes0 tests0 bms0) = do
      (targetSet, flagVals) <-
        resolveWithFlags flagChoices enabled os arch impl constraints condTrees check
      let
        (mb_lib, comps) = flattenTaggedTargets targetSet
        mb_lib' = fmap libFillInDefaults mb_lib
        comps' = flip map comps $ \(n,c) -> foldComponent
          (\l -> CLib   (libFillInDefaults l)   { libName = Just n
                                                , libExposed = False })
          (\l -> CFLib  (flibFillInDefaults l)  { foreignLibName = n })
          (\e -> CExe   (exeFillInDefaults e)   { exeName = n })
          (\t -> CTest  (testFillInDefaults t)  { testName = n })
          (\b -> CBench (benchFillInDefaults b) { benchmarkName = n })
          c
        (sub_libs', flibs', exes', tests', bms') = partitionComponents comps'
      return ( pkg { library = mb_lib'
                   , subLibraries = sub_libs'
                   , foreignLibs = flibs'
                   , executables = exes'
                   , testSuites = tests'
                   , benchmarks = bms'
                   , buildDepends = fromDepMap (overallDependencies enabled targetSet)
                   }
             , flagVals )
      where
        -- Combine lib, exes, and tests into one list of @CondTree@s with tagged data
        condTrees =    maybeToList (fmap (mapTreeData Lib) mb_lib0)
                    ++ map (\(name,tree) -> mapTreeData (SubComp name . CLib) tree) sub_libs0
                    ++ map (\(name,tree) -> mapTreeData (SubComp name . CFLib) tree) flibs0
                    ++ map (\(name,tree) -> mapTreeData (SubComp name . CExe) tree) exes0
                    ++ map (\(name,tree) -> mapTreeData (SubComp name . CTest) tree) tests0
                    ++ map (\(name,tree) -> mapTreeData (SubComp name . CBench) tree) bms0

        flagChoices    = map (\(MkFlag n _ d manual) -> (n, d2c manual n d)) flags
        d2c manual n b = case lookup n userflags of
                         Just val -> [val]
                         Nothing
                          | manual -> [b]
                          | otherwise -> [b, not b]
        --flagDefaults = map (\(n,x:_) -> (n,x)) flagChoices
        check ds     = let missingDeps = filter (not . satisfyDep) ds
                       in if null missingDeps
                          then DepOk
                          else MissingDeps missingDeps

    {-# DEPRECATED finalizePackageDescription "This function now always assumes tests and benchmarks are disabled; use finalizePD with ComponentRequestedSpec to specify something more specific." #-}
    finalizePackageDescription ::
         FlagAssignment  -- ^ Explicitly specified flag assignments
      -> (Dependency -> Bool) -- ^ Is a given dependency satisfiable from the set of
                              -- available packages?  If this is unknown then use
                              -- True.
      -> Platform      -- ^ The 'Arch' and 'OS'
      -> CompilerInfo  -- ^ Compiler information
      -> [Dependency]  -- ^ Additional constraints
      -> GenericPackageDescription
      -> Either [Dependency]
                (PackageDescription, FlagAssignment)
    finalizePackageDescription flags = finalizePD flags defaultComponentRequestedSpec

    {-
    let tst_p = (CondNode [1::Int] [Distribution.Package.Dependency "a" AnyVersion] [])
    let tst_p2 = (CondNode [1::Int] [Distribution.Package.Dependency "a" (EarlierVersion (Version [1,0] [])), Distribution.Package.Dependency "a" (LaterVersion (Version [2,0] []))] [])

    let p_index = Distribution.Simple.PackageIndex.fromList [Distribution.Package.PackageIdentifier "a" (Version [0,5] []), Distribution.Package.PackageIdentifier "a" (Version [2,5] [])]
    let look = not . null . Distribution.Simple.PackageIndex.lookupDependency p_index
    let looks ds = mconcat $ map (\d -> if look d then DepOk else MissingDeps [d]) ds
    resolveWithFlags [] Distribution.System.Linux Distribution.System.I386 (Distribution.Compiler.GHC,Version [6,8,2] []) [tst_p] looks   ===>  Right ...
    resolveWithFlags [] Distribution.System.Linux Distribution.System.I386 (Distribution.Compiler.GHC,Version [6,8,2] []) [tst_p2] looks  ===>  Left ...
    -}

    -- | Flatten a generic package description by ignoring all conditions and just
    -- join the field descriptors into on package description.  Note, however,
    -- that this may lead to inconsistent field values, since all values are
    -- joined into one field, which may not be possible in the original package
    -- description, due to the use of exclusive choices (if ... else ...).
    --
    -- TODO: One particularly tricky case is defaulting.  In the original package
    -- description, e.g., the source directory might either be the default or a
    -- certain, explicitly set path.  Since defaults are filled in only after the
    -- package has been resolved and when no explicit value has been set, the
    -- default path will be missing from the package description returned by this
    -- function.
    flattenPackageDescription :: GenericPackageDescription -> PackageDescription
    flattenPackageDescription
      (GenericPackageDescription pkg _ mlib0 sub_libs0 flibs0 exes0 tests0 bms0) =
        pkg { library      = mlib
            , subLibraries = reverse sub_libs
            , foreignLibs  = reverse flibs
            , executables  = reverse exes
            , testSuites   = reverse tests
            , benchmarks   = reverse bms
            , buildDepends = ldeps
                          ++ reverse sub_ldeps
                          ++ reverse pldeps
                          ++ reverse edeps
                          ++ reverse tdeps
                          ++ reverse bdeps
            }
      where
        (mlib, ldeps) = case mlib0 of
            Just lib -> let (l,ds) = ignoreConditions lib in
                        (Just ((libFillInDefaults l) { libName = Nothing }), ds)
            Nothing -> (Nothing, [])
        (sub_libs, sub_ldeps) = foldr flattenLib  ([],[]) sub_libs0
        (flibs,    pldeps)    = foldr flattenFLib ([],[]) flibs0
        (exes,     edeps)     = foldr flattenExe  ([],[]) exes0
        (tests,    tdeps)     = foldr flattenTst  ([],[]) tests0
        (bms,      bdeps)     = foldr flattenBm   ([],[]) bms0
        flattenLib (n, t) (es, ds) =
            let (e, ds') = ignoreConditions t in
            ( (libFillInDefaults $ e { libName = Just n, libExposed = False }) : es, ds' ++ ds )
        flattenFLib (n, t) (es, ds) =
            let (e, ds') = ignoreConditions t in
            ( (flibFillInDefaults $ e { foreignLibName = n }) : es, ds' ++ ds )
        flattenExe (n, t) (es, ds) =
            let (e, ds') = ignoreConditions t in
            ( (exeFillInDefaults $ e { exeName = n }) : es, ds' ++ ds )
        flattenTst (n, t) (es, ds) =
            let (e, ds') = ignoreConditions t in
            ( (testFillInDefaults $ e { testName = n }) : es, ds' ++ ds )
        flattenBm (n, t) (es, ds) =
            let (e, ds') = ignoreConditions t in
            ( (benchFillInDefaults $ e { benchmarkName = n }) : es, ds' ++ ds )

    -- This is in fact rather a hack.  The original version just overrode the
    -- default values, however, when adding conditions we had to switch to a
    -- modifier-based approach.  There, nothing is ever overwritten, but only
    -- joined together.
    --
    -- This is the cleanest way i could think of, that doesn't require
    -- changing all field parsing functions to return modifiers instead.
    libFillInDefaults :: Library -> Library
    libFillInDefaults lib@(Library { libBuildInfo = bi }) =
        lib { libBuildInfo = biFillInDefaults bi }

    flibFillInDefaults :: ForeignLib -> ForeignLib
    flibFillInDefaults flib@(ForeignLib { foreignLibBuildInfo = bi }) =
        flib { foreignLibBuildInfo = biFillInDefaults bi }

    exeFillInDefaults :: Executable -> Executable
    exeFillInDefaults exe@(Executable { buildInfo = bi }) =
        exe { buildInfo = biFillInDefaults bi }

    testFillInDefaults :: TestSuite -> TestSuite
    testFillInDefaults tst@(TestSuite { testBuildInfo = bi }) =
        tst { testBuildInfo = biFillInDefaults bi }

    benchFillInDefaults :: Benchmark -> Benchmark
    benchFillInDefaults bm@(Benchmark { benchmarkBuildInfo = bi }) =
        bm { benchmarkBuildInfo = biFillInDefaults bi }

    biFillInDefaults :: BuildInfo -> BuildInfo
    biFillInDefaults bi =
        if null (hsSourceDirs bi)
        then bi { hsSourceDirs = [currentDir] }
        else bi

    -- Walk a 'GenericPackageDescription' and apply @onBuildInfo@/@onSetupBuildInfo@
    -- to all nested 'BuildInfo'/'SetupBuildInfo' values.
    transformAllBuildInfos :: (BuildInfo -> BuildInfo)
                           -> (SetupBuildInfo -> SetupBuildInfo)
                           -> GenericPackageDescription
                           -> GenericPackageDescription
    transformAllBuildInfos onBuildInfo onSetupBuildInfo gpd = gpd'
      where
        onLibrary    lib  = lib { libBuildInfo  = onBuildInfo $ libBuildInfo  lib }
        onExecutable exe  = exe { buildInfo     = onBuildInfo $ buildInfo     exe }
        onTestSuite  tst  = tst { testBuildInfo = onBuildInfo $ testBuildInfo tst }
        onBenchmark  bmk  = bmk { benchmarkBuildInfo =
                                     onBuildInfo $ benchmarkBuildInfo bmk }

        pd = packageDescription gpd
        pd'  = pd {
          library        = fmap onLibrary        (library pd),
          subLibraries   = map  onLibrary        (subLibraries pd),
          executables    = map  onExecutable     (executables pd),
          testSuites     = map  onTestSuite      (testSuites pd),
          benchmarks     = map  onBenchmark      (benchmarks pd),
          setupBuildInfo = fmap onSetupBuildInfo (setupBuildInfo pd)
          }

        gpd' = transformAllCondTrees onLibrary onExecutable
               onTestSuite onBenchmark id
               $ gpd { packageDescription = pd' }

    -- | Walk a 'GenericPackageDescription' and apply @f@ to all nested
    -- @build-depends@ fields.
    transformAllBuildDepends :: (Dependency -> Dependency)
                             -> GenericPackageDescription
                             -> GenericPackageDescription
    transformAllBuildDepends f gpd = gpd'
      where
        onBI  bi  = bi  { targetBuildDepends = map f $ targetBuildDepends bi }
        onSBI stp = stp { setupDepends       = map f $ setupDepends stp      }
        onPD  pd  = pd  { buildDepends       = map f $ buildDepends pd       }

        pd'   = onPD $ packageDescription gpd
        gpd'  = transformAllCondTrees id id id id (map f)
                . transformAllBuildInfos onBI onSBI
                $ gpd { packageDescription = pd' }

    -- | Walk all 'CondTree's inside a 'GenericPackageDescription' and apply
    -- appropriate transformations to all nodes. Helper function used by
    -- 'transformAllBuildDepends' and 'transformAllBuildInfos'.
    transformAllCondTrees :: (Library -> Library)
                          -> (Executable -> Executable)
                          -> (TestSuite -> TestSuite)
                          -> (Benchmark -> Benchmark)
                          -> ([Dependency] -> [Dependency])
                          -> GenericPackageDescription -> GenericPackageDescription
    transformAllCondTrees onLibrary onExecutable
      onTestSuite onBenchmark onDepends gpd = gpd'
      where
        gpd'    = gpd {
          condLibrary        = condLib',
          condSubLibraries   = condSubLibs',
          condExecutables    = condExes',
          condTestSuites     = condTests',
          condBenchmarks     = condBenchs'
          }

        condLib    = condLibrary        gpd
        condSubLibs = condSubLibraries  gpd
        condExes   = condExecutables    gpd
        condTests  = condTestSuites     gpd
        condBenchs = condBenchmarks     gpd

        condLib'    = fmap (onCondTree onLibrary) condLib
        condSubLibs' = map (mapSnd $ onCondTree onLibrary)    condSubLibs
        condExes'   = map  (mapSnd $ onCondTree onExecutable) condExes
        condTests'  = map  (mapSnd $ onCondTree onTestSuite)  condTests
        condBenchs' = map  (mapSnd $ onCondTree onBenchmark)  condBenchs

        mapSnd :: (a -> b) -> (c,a) -> (c,b)
        mapSnd = fmap

        onCondTree :: (a -> b) -> CondTree v [Dependency] a
                   -> CondTree v [Dependency] b
        onCondTree g = mapCondTree g onDepends id