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
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- This is about the cabal configurations feature. It exports
    -- 'finalizePackageDescription' 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.

    {- All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are
    met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Isaac Jones nor the names of other
          contributors may be used to endorse or promote products derived
          from this software without specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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    module Distribution.PackageDescription.Configuration (
        finalizePackageDescription,
        flattenPackageDescription,

        -- Utils
        parseCondition,
        freeVars,
        mapCondTree,
        mapTreeData,
        mapTreeConds,
        mapTreeConstrs,
      ) where

    import Distribution.Package
             ( PackageName, Dependency(..) )
    import Distribution.PackageDescription
             ( GenericPackageDescription(..), PackageDescription(..)
             , Library(..), Executable(..), BuildInfo(..)
             , Flag(..), FlagName(..), FlagAssignment
             , Benchmark(..), CondTree(..), ConfVar(..), Condition(..)
             , TestSuite(..) )
    import Distribution.PackageDescription.Utils
             ( cabalBug, userBug )
    import Distribution.Version
             ( VersionRange, anyVersion, intersectVersionRanges, withinRange )
    import Distribution.Compiler
             ( CompilerId(CompilerId) )
    import Distribution.System
             ( Platform(..), OS, Arch )
    import Distribution.Simple.Utils
             ( currentDir, lowercase )

    import Distribution.Text
             ( Text(parse) )
    import Distribution.Compat.ReadP as ReadP hiding ( char )
    import Control.Arrow (first)
    import qualified Distribution.Compat.ReadP as ReadP ( char )

    import Data.Char ( isAlphaNum )
    import Data.Maybe ( catMaybes, maybeToList )
    import Data.Map ( Map, fromListWith, toList )
    import qualified Data.Map as Map
    import Data.Monoid

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

    -- | Simplify the condition and return its free variables.
    simplifyCondition :: Condition c
                      -> (c -> Either d Bool)   -- ^ (partial) variable assignment
                      -> (Condition d, [d])
    simplifyCondition cond i = fv . walk $ cond
      where
        walk cnd = case cnd of
          Var v   -> either Var Lit (i v)
          Lit b   -> Lit b
          CNot c  -> case walk c of
                       Lit True -> Lit False
                       Lit False -> Lit True
                       c' -> CNot c'
          COr c d -> case (walk c, walk d) of
                       (Lit False, d') -> d'
                       (Lit True, _)   -> Lit True
                       (c', Lit False) -> c'
                       (_, Lit True)   -> Lit True
                       (c',d')         -> COr c' d'
          CAnd c d -> case (walk c, walk d) of
                        (Lit False, _) -> Lit False
                        (Lit True, d') -> d'
                        (_, Lit False) -> Lit False
                        (c', Lit True) -> c'
                        (c',d')        -> CAnd c' d'
        -- gather free vars
        fv c = (c, fv' c)
        fv' c = case c of
          Var v     -> [v]
          Lit _      -> []
          CNot c'    -> fv' c'
          COr c1 c2  -> fv' c1 ++ fv' c2
          CAnd c1 c2 -> fv' c1 ++ fv' c2

    -- | Simplify a configuration condition using the os and arch names.  Returns
    --   the names of all the flags occurring in the condition.
    simplifyWithSysParams :: OS -> Arch -> CompilerId -> Condition ConfVar
                          -> (Condition FlagName, [FlagName])
    simplifyWithSysParams os arch (CompilerId comp compVer) 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) = Right $ comp' == comp
                                      && compVer `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 . FlagName . 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

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

    mapCondTree :: (a -> b) -> (c -> d) -> (Condition v -> Condition w)
                -> CondTree v c a -> CondTree w d b
    mapCondTree fa fc fcnd (CondNode a c ifs) =
        CondNode (fa a) (fc c) (map g ifs)
      where
        g (cnd, t, me) = (fcnd cnd, mapCondTree fa fc fcnd t,
                               fmap (mapCondTree fa fc fcnd) me)

    mapTreeConstrs :: (c -> d) -> CondTree v c a -> CondTree v d a
    mapTreeConstrs f = mapCondTree id f id

    mapTreeConds :: (Condition v -> Condition w) -> CondTree v c a -> CondTree w c a
    mapTreeConds f = mapCondTree id id f

    mapTreeData :: (a -> b) -> CondTree v c a -> CondTree v c b
    mapTreeData f = mapCondTree f id id

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

    instance Monoid d => Monoid (DepTestRslt d) where
        mempty = DepOk
        mappend DepOk x = x
        mappend x DepOk = x
        mappend (MissingDeps d) (MissingDeps d') = MissingDeps (d `mappend` d')

    data BT a = BTN a | BTB (BT a) (BT a)  -- very simple binary tree

    -- | 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 _smallest_ number of of missing dependencies 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.
      -> OS      -- ^ OS as returned by Distribution.System.buildOS
      -> Arch    -- ^ Arch as returned by Distribution.System.buildArch
      -> CompilerId -- ^ Compiler flavour + version
      -> [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 os arch impl constrs trees checkDeps =
        case try dom [] of
          Right r -> Right r
          Left dbt -> Left $ findShortest dbt
      where
        extraConstrs = toDepMap constrs

        -- simplify trees by (partially) evaluating all conditions and converting
        -- dependencies to dependency maps.
        simplifiedTrees = map ( mapTreeConstrs toDepMap  -- convert to maps
                              . mapTreeConds (fst . simplifyWithSysParams os arch impl))
                              trees

        -- @try@ recursively tries all possible flag assignments in the domain and
        -- either succeeds or returns a binary tree with the missing dependencies
        -- encountered in each run.  Since the tree is constructed lazily, we
        -- avoid some computation overhead in the successful case.
        try [] flags =
            let targetSet = TargetSet $ flip map simplifiedTrees $
                    -- apply additional constraints to all dependencies
                    first (`constrainBy` extraConstrs) .
                    simplifyCondTree (env flags)
                deps = overallDependencies targetSet
            in case checkDeps (fromDepMap deps) of
                 DepOk           -> Right (targetSet, flags)
                 MissingDeps mds -> Left (BTN mds)

        try ((n, vals):rest) flags =
            tryAll $ map (\v -> try rest ((n, v):flags)) vals

        tryAll = foldr mp mz

        -- special version of `mplus' for our local purposes
        mp (Left xs)   (Left ys)   = (Left (BTB xs ys))
        mp (Left _)    m@(Right _) = m
        mp m@(Right _) _           = m

        -- `mzero'
        mz = Left (BTN [])

        env flags flag = (maybe (Left flag) Right . lookup flag) flags

        -- for the error case we inspect our lazy tree of missing dependencies and
        -- pick the shortest list of missing dependencies
        findShortest (BTN x) = x
        findShortest (BTB lt rt) =
            let l = findShortest lt
                r = findShortest rt
            in case (l,r) of
                 ([], xs) -> xs  -- [] is too short
                 (xs, []) -> xs
                 ([x], _) -> [x] -- single elem is optimum
                 (_, [x]) -> [x]
                 (xs, ys) -> if lazyLengthCmp xs ys
                             then xs else ys
        -- lazy variant of @\xs ys -> length xs <= length ys@
        lazyLengthCmp [] _ = True
        lazyLengthCmp _ [] = False
        lazyLengthCmp (_:xs) (_:ys) = lazyLengthCmp xs ys

    -- | A map of dependencies.  Newtyped since the default monoid instance is not
    --   appropriate.  The monoid instance uses 'intersectVersionRanges'.
    newtype DependencyMap = DependencyMap { unDependencyMap :: Map PackageName VersionRange }
      deriving (Show, Read)

    instance Monoid DependencyMap where
        mempty = DependencyMap Map.empty
        (DependencyMap a) `mappend` (DependencyMap b) =
            DependencyMap (Map.unionWith intersectVersionRanges a b)

    toDepMap :: [Dependency] -> DependencyMap
    toDepMap ds =
      DependencyMap $ fromListWith intersectVersionRanges [ (p,vr) | Dependency p vr <- ds ]

    fromDepMap :: DependencyMap -> [Dependency]
    fromDepMap m = [ Dependency p vr | (p,vr) <- toList (unDependencyMap m) ]

    simplifyCondTree :: (Monoid a, Monoid d) =>
                        (v -> Either v Bool)
                     -> CondTree v d a
                     -> (d, a)
    simplifyCondTree env (CondNode a d ifs) =
        mconcat $ (d, a) : catMaybes (map simplifyIf ifs)
      where
        simplifyIf (cnd, t, me) =
            case simplifyCondition cnd env of
              (Lit True, _) -> Just $ simplifyCondTree env t
              (Lit False, _) -> fmap (simplifyCondTree env) me
              _ -> error $ "Environment not defined for all free vars"

    -- | Flatten a CondTree.  This will resolve the CondTree by taking all
    --  possible paths into account.  Note that since branches represent exclusive
    --  choices this may not result in a \"sane\" result.
    ignoreConditions :: (Monoid a, Monoid c) => CondTree v c a -> (a, c)
    ignoreConditions (CondNode a c ifs) = (a, c) `mappend` mconcat (concatMap f ifs)
      where f (_, t, me) = ignoreConditions t
                           : maybeToList (fmap ignoreConditions me)

    freeVars :: CondTree ConfVar c a  -> [FlagName]
    freeVars t = [ f | Flag f <- freeVars' t ]
      where
        freeVars' (CondNode _ _ ifs) = concatMap compfv ifs
        compfv (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 :: TargetSet PDTagged -> DependencyMap
    overallDependencies (TargetSet targets) = mconcat depss
      where
        (depss, _) = unzip $ filter (removeDisabledSections . snd) targets
        removeDisabledSections :: PDTagged -> Bool
        removeDisabledSections (Lib _) = True
        removeDisabledSections (Exe _ _) = True
        removeDisabledSections (Test _ t) = testEnabled t
        removeDisabledSections (Bench _ b) = benchmarkEnabled b
        removeDisabledSections PDNull = True

    -- Apply extra constraints to a dependency map.
    -- Combines dependencies where the result will only contain keys from the left
    -- (first) map.  If a key also exists in the right map, both constraints will
    -- be intersected.
    constrainBy :: DependencyMap  -- ^ Input map
                -> DependencyMap  -- ^ Extra constraints
                -> DependencyMap
    constrainBy left extra =
        DependencyMap $
          Map.foldWithKey tightenConstraint (unDependencyMap left)
                                            (unDependencyMap extra)
      where tightenConstraint n c l =
                case Map.lookup n l of
                  Nothing -> l
                  Just vr -> Map.insert n (intersectVersionRanges vr c) l

    -- | Collect up the targets in a TargetSet of tagged targets, storing the
    -- dependencies as we go.
    flattenTaggedTargets :: TargetSet PDTagged ->
            (Maybe Library, [(String, Executable)], [(String, TestSuite)]
            , [(String, Benchmark)])
    flattenTaggedTargets (TargetSet targets) = foldr untag (Nothing, [], [], []) targets
      where
        untag (_, Lib _) (Just _, _, _, _) = userBug "Only one library expected"
        untag (deps, Lib l) (Nothing, exes, tests, bms) =
            (Just l', exes, tests, bms)
          where
            l' = l {
                    libBuildInfo = (libBuildInfo l) { targetBuildDepends = fromDepMap deps }
                }
        untag (deps, Exe n e) (mlib, exes, tests, bms)
            | any ((== n) . fst) exes =
              userBug $ "There exist several exes with the same name: '" ++ n ++ "'"
            | any ((== n) . fst) tests =
              userBug $ "There exists a test with the same name as an exe: '" ++ n ++ "'"
            | any ((== n) . fst) bms =
              userBug $ "There exists a benchmark with the same name as an exe: '" ++ n ++ "'"
            | otherwise = (mlib, (n, e'):exes, tests, bms)
          where
            e' = e {
                    buildInfo = (buildInfo e) { targetBuildDepends = fromDepMap deps }
                }
        untag (deps, Test n t) (mlib, exes, tests, bms)
            | any ((== n) . fst) tests =
              userBug $ "There exist several tests with the same name: '" ++ n ++ "'"
            | any ((== n) . fst) exes =
              userBug $ "There exists an exe with the same name as the test: '" ++ n ++ "'"
            | any ((== n) . fst) bms =
              userBug $ "There exists a benchmark with the same name as the test: '" ++ n ++ "'"
            | otherwise = (mlib, exes, (n, t'):tests, bms)
          where
            t' = t {
                testBuildInfo = (testBuildInfo t)
                    { targetBuildDepends = fromDepMap deps }
                }
        untag (deps, Bench n b) (mlib, exes, tests, bms)
            | any ((== n) . fst) bms =
              userBug $ "There exist several benchmarks with the same name: '" ++ n ++ "'"
            | any ((== n) . fst) exes =
              userBug $ "There exists an exe with the same name as the benchmark: '" ++ n ++ "'"
            | any ((== n) . fst) tests =
              userBug $ "There exists a test with the same name as the benchmark: '" ++ n ++ "'"
            | otherwise = (mlib, exes, tests, (n, b'):bms)
          where
            b' = b {
                benchmarkBuildInfo = (benchmarkBuildInfo b)
                    { targetBuildDepends = fromDepMap deps }
                }
        untag (_, PDNull) x = x  -- actually this should not happen, but let's be liberal

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

    data PDTagged = Lib Library
                  | Exe String Executable
                  | Test String TestSuite
                  | Bench String Benchmark
                  | PDNull
                  deriving Show

    instance Monoid PDTagged where
        mempty = PDNull
        PDNull `mappend` x = x
        x `mappend` PDNull = x
        Lib l `mappend` Lib l' = Lib (l `mappend` l')
        Exe n e `mappend` Exe n' e' | n == n' = Exe n (e `mappend` e')
        Test n t `mappend` Test n' t' | n == n' = Test n (t `mappend` t')
        Bench n b `mappend` Bench n' b' | n == n' = Bench n (b `mappend` b')
        _ `mappend` _ = 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 a /minimum/
    -- number of dependencies that could not be satisfied.  On success, it will
    -- return the package description and the full flag assignment chosen.
    --
    finalizePackageDescription ::
         FlagAssignment  -- ^ Explicitly specified flag assignments
      -> (Dependency -> Bool) -- ^ Is a given depenency satisfiable from the set of available packages?
                              -- If this is unknown then use True.
      -> Platform      -- ^ The 'Arch' and 'OS'
      -> CompilerId    -- ^ Compiler + Version
      -> [Dependency]  -- ^ Additional constraints
      -> GenericPackageDescription
      -> Either [Dependency]
                (PackageDescription, FlagAssignment)
                 -- ^ Either missing dependencies or the resolved package
                 -- description along with the flag assignments chosen.
    finalizePackageDescription userflags satisfyDep (Platform arch os) impl constraints
            (GenericPackageDescription pkg flags mlib0 exes0 tests0 bms0) =
        case resolveFlags of
          Right ((mlib, exes', tests', bms'), targetSet, flagVals) ->
            Right ( pkg { library = mlib
                        , executables = exes'
                        , testSuites = tests'
                        , benchmarks = bms'
                        , buildDepends = fromDepMap (overallDependencies targetSet)
                          --TODO: we need to find a way to avoid pulling in deps
                          -- for non-buildable components. However cannot simply
                          -- filter at this stage, since if the package were not
                          -- available we would have failed already.
                        }
                  , flagVals )

          Left missing -> Left missing
      where
        -- Combine lib, exes, and tests into one list of @CondTree@s with tagged data
        condTrees = maybeToList (fmap (mapTreeData Lib) mlib0 )
                    ++ map (\(name,tree) -> mapTreeData (Exe name) tree) exes0
                    ++ map (\(name,tree) -> mapTreeData (Test name) tree) tests0
                    ++ map (\(name,tree) -> mapTreeData (Bench name) tree) bms0

        resolveFlags =
            case resolveWithFlags flagChoices os arch impl constraints condTrees check of
              Right (targetSet, fs) ->
                  let (mlib, exes, tests, bms) = flattenTaggedTargets targetSet in
                  Right ( (fmap libFillInDefaults mlib,
                           map (\(n,e) -> (exeFillInDefaults e) { exeName = n }) exes,
                           map (\(n,t) -> (testFillInDefaults t) { testName = n }) tests,
                           map (\(n,b) -> (benchFillInDefaults b) { benchmarkName = n }) bms),
                         targetSet, fs)
              Left missing      -> Left missing

        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     = if all satisfyDep ds
                       then DepOk
                       else MissingDeps $ filter (not . satisfyDep) ds

    {-
    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 exes0 tests0 bms0) =
        pkg { library = mlib
            , executables = reverse exes
            , testSuites = reverse tests
            , benchmarks = reverse bms
            , buildDepends = ldeps ++ reverse edeps ++ reverse tdeps ++ reverse bdeps
            }
      where
        (mlib, ldeps) = case mlib0 of
            Just lib -> let (l,ds) = ignoreConditions lib in
                        (Just (libFillInDefaults l), ds)
            Nothing -> (Nothing, [])
        (exes, edeps) = foldr flattenExe ([],[]) exes0
        (tests, tdeps) = foldr flattenTst ([],[]) tests0
        (bms, bdeps) = foldr flattenBm ([],[]) bms0
        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 }

    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