diff --git a/Cabal-syntax/src/Distribution/Fields/ConfVar.hs b/Cabal-syntax/src/Distribution/Fields/ConfVar.hs
index 68601cd53e9..b045c3ef172 100644
--- a/Cabal-syntax/src/Distribution/Fields/ConfVar.hs
+++ b/Cabal-syntax/src/Distribution/Fields/ConfVar.hs
@@ -1,14 +1,15 @@
 {-# LANGUAGE OverloadedStrings #-}
-module Distribution.Fields.ConfVar (parseConditionConfVar) where
+module Distribution.Fields.ConfVar (parseConditionConfVar, parseConditionConfVarFromClause) where
 
 import Distribution.Compat.CharParsing     (char, integral)
 import Distribution.Compat.Prelude
-import Distribution.Fields.Field           (SectionArg (..))
+import Distribution.Fields.Field           (SectionArg (..), Field(..))
 import Distribution.Fields.ParseResult
 import Distribution.Parsec                 (Parsec (..), Position (..), runParsecParser)
 import Distribution.Parsec.FieldLineStream (fieldLineStreamFromBS)
 import Distribution.Types.Condition
 import Distribution.Types.ConfVar          (ConfVar (..))
+import Distribution.Fields.Parser          (readFields)
 import Distribution.Version
        (anyVersion, earlierVersion, intersectVersionRanges, laterVersion, majorBoundVersion,
        mkVersion, noVersion, orEarlierVersion, orLaterVersion, thisVersion, unionVersionRanges,
@@ -16,7 +17,14 @@ import Distribution.Version
 import Prelude ()
 
 import qualified Text.Parsec       as P
+import qualified Text.Parsec.Pos   as P
 import qualified Text.Parsec.Error as P
+import qualified Data.ByteString.Char8          as B8
+
+parseConditionConfVarFromClause :: B8.ByteString -> Either P.ParseError (Condition ConfVar)
+parseConditionConfVarFromClause x = readFields x >>= \r -> case r of
+                                       (Section _ xs _ : _ ) -> P.runParser (parser <* P.eof) () "<condition>" xs
+                                       _ -> Left $ P.newErrorMessage (P.Message "No fields in clause") (P.initialPos "<condition>")
 
 -- | Parse @'Condition' 'ConfVar'@ from section arguments provided by parsec
 -- based outline parser.
diff --git a/Cabal-syntax/src/Distribution/PackageDescription/Configuration.hs b/Cabal-syntax/src/Distribution/PackageDescription/Configuration.hs
index 3a52ca783db..b9bb9dcb555 100644
--- a/Cabal-syntax/src/Distribution/PackageDescription/Configuration.hs
+++ b/Cabal-syntax/src/Distribution/PackageDescription/Configuration.hs
@@ -32,6 +32,7 @@ module Distribution.PackageDescription.Configuration (
     transformAllBuildInfos,
     transformAllBuildDepends,
     transformAllBuildDependsN,
+    simplifyWithSysParams
   ) where
 
 import Distribution.Compat.Prelude
diff --git a/Cabal-syntax/src/Distribution/Types/CondTree.hs b/Cabal-syntax/src/Distribution/Types/CondTree.hs
index 8fd233658f5..8f1d17a8163 100644
--- a/Cabal-syntax/src/Distribution/Types/CondTree.hs
+++ b/Cabal-syntax/src/Distribution/Types/CondTree.hs
@@ -68,6 +68,13 @@ instance (Binary v, Binary c, Binary a) => Binary (CondTree v c a)
 instance (Structured v, Structured c, Structured a) => Structured (CondTree v c a)
 instance (NFData v, NFData c, NFData a) => NFData (CondTree v c a) where rnf = genericRnf
 
+instance (Semigroup a, Semigroup c) => Semigroup (CondTree v c a) where
+  (CondNode a c bs) <> (CondNode a' c' bs') = CondNode (a <> a') (c <> c') (bs <> bs')
+
+instance (Semigroup a, Semigroup c, Monoid a, Monoid c) => Monoid (CondTree v c a) where
+   mappend = (<>)
+   mempty = CondNode mempty mempty mempty
+
 -- | A 'CondBranch' represents a conditional branch, e.g., @if
 -- flag(foo)@ on some syntax @a@.  It also has an optional false
 -- branch.
@@ -191,4 +198,4 @@ foldCondTree e u mergeInclusive mergeExclusive = goTree
     goTree :: CondTree v c a -> b
     goTree (CondNode a c ifs) = u (c, a) `mergeInclusive` foldl goBranch e ifs
     goBranch :: b -> CondBranch v c a -> b
-    goBranch acc (CondBranch _ t mt) = mergeInclusive acc (maybe (goTree t) (mergeExclusive (goTree t) . goTree) mt)
+    goBranch acc (CondBranch _ t mt) = mergeInclusive acc (maybe (goTree t) (mergeExclusive (goTree t) . goTree) mt)
\ No newline at end of file
diff --git a/cabal-install/src/Distribution/Client/CmdConfigure.hs b/cabal-install/src/Distribution/Client/CmdConfigure.hs
index e75f25d0486..a17d7cd2f7d 100644
--- a/cabal-install/src/Distribution/Client/CmdConfigure.hs
+++ b/cabal-install/src/Distribution/Client/CmdConfigure.hs
@@ -30,12 +30,17 @@ import Distribution.Verbosity
 import Distribution.Simple.Command
          ( CommandUI(..), usageAlternatives )
 import Distribution.Simple.Utils
-         ( wrapText, notice )
+         ( wrapText, notice, die' )
 
 import Distribution.Client.DistDirLayout
          ( DistDirLayout(..) )
 import Distribution.Client.RebuildMonad (runRebuild)
 import Distribution.Client.ProjectConfig.Types
+import Distribution.Client.HttpUtils
+import Distribution.Utils.NubList
+         ( fromNubList )
+import Distribution.Types.CondTree
+         ( CondTree (..) )
 
 configureCommand :: CommandUI (NixStyleFlags ())
 configureCommand = CommandUI {
@@ -126,8 +131,12 @@ configureAction' flags@NixStyleFlags {..} _extraArgs globalFlags = do
          -- If the flag @configAppend@ is set to true, append and do not overwrite
         if exists && appends
           then do
-            conf <- runRebuild (distProjectRootDirectory . distDirLayout $ baseCtx) $
-              readProjectLocalExtraConfig v (distDirLayout baseCtx)
+            httpTransport <- configureTransport v
+                     (fromNubList . projectConfigProgPathExtra $ projectConfigShared cliConfig)
+                     (flagToMaybe . projectConfigHttpTransport $ projectConfigBuildOnly cliConfig)
+            (CondNode conf imps bs)  <- runRebuild (distProjectRootDirectory . distDirLayout $ baseCtx) $
+              readProjectLocalExtraConfig v httpTransport (distDirLayout baseCtx)
+            when (not (null imps && null bs)) $ die' v "local project file has conditional and/or import logic, unable to perform and automatic in-place update"
             return (baseCtx, conf <> cliConfig)
           else
             return (baseCtx, cliConfig)
diff --git a/cabal-install/src/Distribution/Client/CmdOutdated.hs b/cabal-install/src/Distribution/Client/CmdOutdated.hs
index d03d759bebc..c511c53c1c6 100644
--- a/cabal-install/src/Distribution/Client/CmdOutdated.hs
+++ b/cabal-install/src/Distribution/Client/CmdOutdated.hs
@@ -29,9 +29,8 @@ import Distribution.Client.DistDirLayout
     ( defaultDistDirLayout
     , DistDirLayout(distProjectRootDirectory, distProjectFile) )
 import Distribution.Client.ProjectConfig
-    ( ProjectConfig(projectConfigShared),
-      ProjectConfigShared(projectConfigConstraints), findProjectRoot,
-      readProjectLocalFreezeConfig )
+import Distribution.Client.ProjectConfig.Legacy
+    ( instantiateProjectConfigSkeleton )
 import Distribution.Client.ProjectFlags
     ( projectFlagsOptions, ProjectFlags(..), defaultProjectFlags
     , removeIgnoreProjectOption )
@@ -40,8 +39,6 @@ import Distribution.Client.RebuildMonad
 import Distribution.Client.Sandbox
     ( loadConfigOrSandboxConfig )
 import Distribution.Client.Setup
-    ( withRepoContext, GlobalFlags, configCompilerAux'
-    , ConfigExFlags(configExConstraints) )
 import Distribution.Client.Targets
     ( userToPackageConstraint, UserConstraint )
 import Distribution.Client.Types.SourcePackageDb as SourcePackageDb
@@ -65,7 +62,7 @@ import Distribution.Simple.Setup
 import Distribution.Simple.Utils
     ( die', notice, debug, tryFindPackageDesc )
 import Distribution.System
-    ( Platform )
+    ( Platform (..) )
 import Distribution.Types.ComponentRequestedSpec
     ( ComponentRequestedSpec(..) )
 import Distribution.Types.Dependency
@@ -86,6 +83,9 @@ import Distribution.Simple.PackageDescription
 import qualified Distribution.Compat.CharParsing as P
 import Distribution.ReadE
     ( parsecToReadE )
+import Distribution.Client.HttpUtils
+import Distribution.Utils.NubList
+         ( fromNubList )
 
 import qualified Data.Set as S
 import System.Directory
@@ -220,18 +220,23 @@ outdatedAction (ProjectFlags{flagProjectFileName}, OutdatedFlags{..}) _targetStr
   config <- loadConfigOrSandboxConfig verbosity globalFlags
   let globalFlags' = savedGlobalFlags config `mappend` globalFlags
       configFlags  = savedConfigureFlags config
-  (comp, platform, _progdb) <- configCompilerAux' configFlags
   withRepoContext verbosity globalFlags' $ \repoContext -> do
     when (not newFreezeFile && isJust mprojectFile) $
       die' verbosity $
         "--project-file must only be used with --v2-freeze-file."
 
     sourcePkgDb <- IndexUtils.getSourcePackages verbosity repoContext
+    (comp, platform, _progdb) <- configCompilerAux' configFlags
     deps <- if freezeFile
             then depsFromFreezeFile verbosity
             else if newFreezeFile
-                then depsFromNewFreezeFile verbosity mprojectFile
-                else depsFromPkgDesc verbosity comp platform
+                then do
+                       httpTransport <- configureTransport verbosity
+                         (fromNubList . globalProgPathExtra $ globalFlags)
+                         (flagToMaybe . globalHttpTransport $ globalFlags)
+                       depsFromNewFreezeFile verbosity httpTransport comp platform mprojectFile
+                else do
+                  depsFromPkgDesc verbosity comp platform
     debug verbosity $ "Dependencies loaded: "
       ++ intercalate ", " (map prettyShow deps)
     let outdatedDeps = listOutdated deps sourcePkgDb
@@ -293,14 +298,15 @@ depsFromFreezeFile verbosity = do
   return deps
 
 -- | Read the list of dependencies from the new-style freeze file.
-depsFromNewFreezeFile :: Verbosity -> Maybe FilePath -> IO [PackageVersionConstraint]
-depsFromNewFreezeFile verbosity mprojectFile = do
+depsFromNewFreezeFile :: Verbosity -> HttpTransport -> Compiler -> Platform -> Maybe FilePath -> IO [PackageVersionConstraint]
+depsFromNewFreezeFile verbosity httpTransport compiler (Platform arch os) mprojectFile = do
   projectRoot <- either throwIO return =<<
                  findProjectRoot Nothing mprojectFile
   let distDirLayout = defaultDistDirLayout projectRoot
                       {- TODO: Support dist dir override -} Nothing
-  projectConfig  <- runRebuild (distProjectRootDirectory distDirLayout) $
-                    readProjectLocalFreezeConfig verbosity distDirLayout
+  projectConfig <- runRebuild (distProjectRootDirectory distDirLayout) $ do
+                      pcs <- readProjectLocalFreezeConfig verbosity httpTransport distDirLayout
+                      pure $ instantiateProjectConfigSkeleton os arch (compilerInfo compiler) mempty pcs
   let ucnstrs = map fst . projectConfigConstraints . projectConfigShared
                 $ projectConfig
       deps    = userConstraintsToDependencies ucnstrs
diff --git a/cabal-install/src/Distribution/Client/ParseUtils.hs b/cabal-install/src/Distribution/Client/ParseUtils.hs
index bf71fb6b00b..0b8e45c5641 100644
--- a/cabal-install/src/Distribution/Client/ParseUtils.hs
+++ b/cabal-install/src/Distribution/Client/ParseUtils.hs
@@ -369,4 +369,3 @@ parseConfig fieldDescrs sectionDescrs fgSectionDescrs empty str =
 --
 showConfig :: [FieldDescr a] -> [SectionDescr a] -> [FGSectionDescr FG.PrettyFieldGrammar a] -> a -> Disp.Doc
 showConfig = ppFieldsAndSections
-
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig.hs b/cabal-install/src/Distribution/Client/ProjectConfig.hs
index 2680707f411..9ca62bd63e7 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig.hs
@@ -29,7 +29,6 @@ module Distribution.Client.ProjectConfig (
     readGlobalConfig,
     readProjectLocalExtraConfig,
     readProjectLocalFreezeConfig,
-    parseProjectConfig,
     reportParseResult,
     showProjectConfig,
     withProjectOrGlobalConfig,
@@ -504,14 +503,15 @@ withProjectOrGlobalConfig' verbosity globalConfigFlag with without = do
 -- file if any, plus other global config.
 --
 readProjectConfig :: Verbosity
+                  -> HttpTransport
                   -> Flag FilePath
                   -> DistDirLayout
-                  -> Rebuild ProjectConfig
-readProjectConfig verbosity configFileFlag distDirLayout = do
-    global <- readGlobalConfig                verbosity configFileFlag
-    local  <- readProjectLocalConfigOrDefault verbosity distDirLayout
-    freeze <- readProjectLocalFreezeConfig    verbosity distDirLayout
-    extra  <- readProjectLocalExtraConfig     verbosity distDirLayout
+                  -> Rebuild ProjectConfigSkeleton
+readProjectConfig verbosity httpTransport configFileFlag distDirLayout = do
+    global <- singletonProjectConfigSkeleton <$> readGlobalConfig verbosity configFileFlag
+    local  <- readProjectLocalConfigOrDefault verbosity httpTransport distDirLayout
+    freeze <- readProjectLocalFreezeConfig    verbosity httpTransport distDirLayout
+    extra  <- readProjectLocalExtraConfig     verbosity httpTransport distDirLayout
     return (global <> local <> freeze <> extra)
 
 
@@ -519,16 +519,17 @@ readProjectConfig verbosity configFileFlag distDirLayout = do
 -- or returns the default project config for an implicitly defined project.
 --
 readProjectLocalConfigOrDefault :: Verbosity
+                                -> HttpTransport
                                 -> DistDirLayout
-                                -> Rebuild ProjectConfig
-readProjectLocalConfigOrDefault verbosity distDirLayout = do
+                                -> Rebuild ProjectConfigSkeleton
+readProjectLocalConfigOrDefault verbosity httpTransport distDirLayout = do
   usesExplicitProjectRoot <- liftIO $ doesFileExist projectFile
   if usesExplicitProjectRoot
     then do
-      readProjectFile verbosity distDirLayout "" "project file"
+      readProjectFileSkeleton verbosity httpTransport distDirLayout "" "project file"
     else do
       monitorFiles [monitorNonExistentFile projectFile]
-      return defaultImplicitProjectConfig
+      return (singletonProjectConfigSkeleton defaultImplicitProjectConfig)
 
   where
     projectFile :: FilePath
@@ -547,66 +548,43 @@ readProjectLocalConfigOrDefault verbosity distDirLayout = do
 -- or returns empty. This file gets written by @cabal configure@, or in
 -- principle can be edited manually or by other tools.
 --
-readProjectLocalExtraConfig :: Verbosity -> DistDirLayout
-                            -> Rebuild ProjectConfig
-readProjectLocalExtraConfig verbosity distDirLayout =
-    readProjectFile verbosity distDirLayout "local"
+readProjectLocalExtraConfig :: Verbosity -> HttpTransport -> DistDirLayout
+                            -> Rebuild ProjectConfigSkeleton
+readProjectLocalExtraConfig verbosity httpTransport distDirLayout =
+    readProjectFileSkeleton verbosity httpTransport distDirLayout "local"
                              "project local configuration file"
 
 -- | Reads a @cabal.project.freeze@ file in the given project root dir,
 -- or returns empty. This file gets written by @cabal freeze@, or in
 -- principle can be edited manually or by other tools.
 --
-readProjectLocalFreezeConfig :: Verbosity -> DistDirLayout
-                             -> Rebuild ProjectConfig
-readProjectLocalFreezeConfig verbosity distDirLayout =
-    readProjectFile verbosity distDirLayout "freeze"
+readProjectLocalFreezeConfig :: Verbosity -> HttpTransport ->DistDirLayout
+                             -> Rebuild ProjectConfigSkeleton
+readProjectLocalFreezeConfig verbosity httpTransport distDirLayout =
+    readProjectFileSkeleton verbosity httpTransport distDirLayout "freeze"
                              "project freeze file"
 
--- | Reads a named config file in the given project root dir, or returns empty.
+-- | Reads a named extended (with imports and conditionals) config file in the given project root dir, or returns empty.
 --
-readProjectFile :: Verbosity
-                -> DistDirLayout
-                -> String
-                -> String
-                -> Rebuild ProjectConfig
-readProjectFile verbosity DistDirLayout{distProjectFile}
+readProjectFileSkeleton :: Verbosity -> HttpTransport -> DistDirLayout -> String -> String -> Rebuild ProjectConfigSkeleton
+readProjectFileSkeleton verbosity httpTransport DistDirLayout{distProjectFile, distDownloadSrcDirectory}
                          extensionName extensionDescription = do
     exists <- liftIO $ doesFileExist extensionFile
     if exists
       then do monitorFiles [monitorFileHashed extensionFile]
-              addProjectFileProvenance <$> liftIO readExtensionFile
+              pcs <- liftIO readExtensionFile
+              monitorFiles $ map monitorFileHashed (projectSkeletonImports pcs)
+              pure pcs
       else do monitorFiles [monitorNonExistentFile extensionFile]
               return mempty
   where
-    extensionFile :: FilePath
     extensionFile = distProjectFile extensionName
 
-    readExtensionFile :: IO ProjectConfig
     readExtensionFile =
           reportParseResult verbosity extensionDescription extensionFile
-        . (parseProjectConfig extensionFile)
+      =<< parseProjectSkeleton distDownloadSrcDirectory httpTransport verbosity [] extensionFile
       =<< BS.readFile extensionFile
 
-    addProjectFileProvenance :: ProjectConfig -> ProjectConfig
-    addProjectFileProvenance config =
-      config {
-        projectConfigProvenance =
-          Set.insert (Explicit extensionFile) (projectConfigProvenance config)
-      }
-
-
--- | Parse the 'ProjectConfig' format.
---
--- For the moment this is implemented in terms of parsers for legacy
--- configuration types, plus a conversion.
---
-parseProjectConfig :: FilePath -> BS.ByteString -> OldParser.ParseResult ProjectConfig
-parseProjectConfig source content =
-    convertLegacyProjectConfig <$>
-      (parseLegacyProjectConfig source content)
-
-
 -- | Render the 'ProjectConfig' format.
 --
 -- For the moment this is implemented in terms of a pretty printer for the
@@ -647,12 +625,12 @@ readGlobalConfig verbosity configFileFlag = do
     monitorFiles [monitorFileHashed configFile]
     return (convertLegacyGlobalConfig config)
 
-reportParseResult :: Verbosity -> String -> FilePath -> OldParser.ParseResult a -> IO a
+reportParseResult :: Verbosity -> String -> FilePath -> OldParser.ParseResult ProjectConfigSkeleton -> IO ProjectConfigSkeleton
 reportParseResult verbosity _filetype filename (OldParser.ParseOk warnings x) = do
-    unless (null warnings) $
-      let msg = unlines (map (OldParser.showPWarning filename) warnings)
+   unless (null warnings) $
+      let msg = unlines (map (OldParser.showPWarning (intercalate ", " $ filename : projectSkeletonImports x)) warnings)
        in warn verbosity msg
-    return x
+   return x
 reportParseResult verbosity filetype filename (OldParser.ParseFailed err) =
     let (line, msg) = OldParser.locatedErrorMsg err
      in die' verbosity $ "Error parsing " ++ filetype ++ " " ++ filename
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
index dcf12ec9815..2ed03cc6888 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
@@ -1,9 +1,16 @@
-{-# LANGUAGE RecordWildCards, NamedFieldPuns, DeriveGeneric, ConstraintKinds #-}
+{-# LANGUAGE RecordWildCards, NamedFieldPuns, DeriveGeneric, ConstraintKinds, FlexibleInstances #-}
 
 -- | Project configuration, implementation in terms of legacy types.
 --
 module Distribution.Client.ProjectConfig.Legacy (
 
+   -- Project config skeletons
+    ProjectConfigSkeleton,
+    parseProjectSkeleton,
+    instantiateProjectConfigSkeleton,
+    singletonProjectConfigSkeleton,
+    projectSkeletonImports,
+
     -- * Project config in terms of legacy types
     LegacyProjectConfig,
     parseLegacyProjectConfig,
@@ -17,13 +24,12 @@ module Distribution.Client.ProjectConfig.Legacy (
 
     -- * Internals, just for tests
     parsePackageLocationTokenQ,
-    renderPackageLocationToken,
+    renderPackageLocationToken
   ) where
 
-import Prelude ()
 import Distribution.Client.Compat.Prelude
 
-import Distribution.Types.Flag (parsecFlagAssignment)
+import Distribution.Types.Flag (parsecFlagAssignment, FlagName)
 
 import Distribution.Client.ProjectConfig.Types
 import Distribution.Client.Types.RepoName (RepoName (..), unRepoName)
@@ -38,18 +44,23 @@ import Distribution.Client.CmdInstall.ClientInstallFlags
          ( ClientInstallFlags(..), defaultClientInstallFlags
          , clientInstallOptions )
 
+import Distribution.Compat.Lens (view)
+
 import Distribution.Solver.Types.ConstraintSource
 
 import Distribution.FieldGrammar
 import Distribution.Package
 import Distribution.Types.SourceRepo (RepoType)
+import Distribution.Types.CondTree
+         ( CondTree (..), CondBranch (..), mapTreeConds, traverseCondTreeC )
 import Distribution.PackageDescription
-         ( dispFlagAssignment )
+         ( dispFlagAssignment, Condition (..), ConfVar (..), FlagAssignment )
+import Distribution.PackageDescription.Configuration (simplifyWithSysParams)
 import Distribution.Simple.Compiler
-         ( OptimisationLevel(..), DebugInfoLevel(..) )
+         ( OptimisationLevel(..), DebugInfoLevel(..), CompilerInfo(..) )
 import Distribution.Simple.InstallDirs ( CopyDest (NoCopyDest) )
 import Distribution.Simple.Setup
-         ( Flag(Flag), toFlag, fromFlagOrDefault
+         ( Flag(..), toFlag, fromFlagOrDefault
          , ConfigFlags(..), configureOptions
          , HaddockFlags(..), haddockOptions, defaultHaddockFlags
          , TestFlags(..), testOptions', defaultTestFlags
@@ -85,7 +96,7 @@ import Distribution.Deprecated.ParseUtils
          ( ParseResult(..), PError(..), syntaxError, PWarning(..)
          , commaNewLineListFieldParsec, newLineListField, parseTokenQ
          , parseHaskellString, showToken
-         , simpleFieldParsec
+         , simpleFieldParsec, parseFail
          )
 import Distribution.Client.ParseUtils
 import Distribution.Simple.Command
@@ -94,11 +105,130 @@ import Distribution.Simple.Command
 import Distribution.Types.PackageVersionConstraint
          ( PackageVersionConstraint )
 import Distribution.Parsec (ParsecParser, parsecToken)
+import Distribution.System (OS, Arch)
 
 import qualified Data.Map as Map
-import qualified Data.ByteString as BS
+import qualified Data.Set as Set
+import qualified Data.ByteString.Char8 as BS
+
+import Network.URI (URI (..), parseURI)
+
+import Distribution.Fields.ConfVar (parseConditionConfVarFromClause)
+
+import Distribution.Client.HttpUtils
+import System.FilePath ((</>), isPathSeparator, makeValid)
+import System.Directory (createDirectoryIfMissing)
+
+
+
+
+------------------------------------------------------------------
+-- Handle extended project config files with conditionals and imports.
+--
+
+-- | ProjectConfigSkeleton is a tree of conditional blocks and imports wrapping a config. It can be finalized by providing the conditional resolution info
+-- and then resolving and downloading the imports
+type ProjectConfigSkeleton = CondTree ConfVar [ProjectConfigImport] ProjectConfig
+type ProjectConfigImport = String
+
+
+singletonProjectConfigSkeleton :: ProjectConfig -> ProjectConfigSkeleton
+singletonProjectConfigSkeleton x = CondNode x mempty mempty
+
+instantiateProjectConfigSkeleton :: OS -> Arch -> CompilerInfo -> FlagAssignment -> ProjectConfigSkeleton -> ProjectConfig
+instantiateProjectConfigSkeleton os arch impl _flags skel = go $ mapTreeConds (fst . simplifyWithSysParams os arch impl) skel
+    where
+        go :: CondTree
+               FlagName
+               [ProjectConfigImport]
+               ProjectConfig
+             -> ProjectConfig
+        go (CondNode l _imps ts) =
+           let branches = concatMap processBranch ts
+           in l <> mconcat branches
+        processBranch (CondBranch cnd t mf) = case cnd of
+           (Lit True) ->  [go t]
+           (Lit False) -> maybe ([]) ((:[]) . go) mf
+           _ -> error $ "unable to process condition: " ++ show cnd -- TODO it would be nice if there were a pretty printer
+
+projectSkeletonImports :: ProjectConfigSkeleton -> [ProjectConfigImport]
+projectSkeletonImports = view traverseCondTreeC
+
+parseProjectSkeleton :: FilePath -> HttpTransport -> Verbosity -> [ProjectConfigImport] -> FilePath -> BS.ByteString -> IO (ParseResult ProjectConfigSkeleton)
+parseProjectSkeleton cacheDir httpTransport verbosity seenImports source bs = (sanityWalkPCS False =<<) <$> liftPR (go []) (ParseUtils.readFields bs)
+  where
+    go :: [ParseUtils.Field] -> [ParseUtils.Field] -> IO (ParseResult ProjectConfigSkeleton)
+    go acc (x:xs) = case x of
+         (ParseUtils.F l "import" importLoc) ->
+            if importLoc `elem` seenImports
+              then pure . parseFail $ ParseUtils.FromString ("cyclical import of " ++ importLoc) (Just l)
+              else do
+                let fs = fmap (\z -> CondNode z [importLoc] mempty) $ fieldsToConfig (reverse acc)
+                res <- parseProjectSkeleton cacheDir httpTransport verbosity (importLoc : seenImports) importLoc =<< fetchImportConfig importLoc
+                rest <- go [] xs
+                pure . fmap mconcat . sequence $ [fs, res, rest]
+         (ParseUtils.Section l "if" p xs') -> do
+                subpcs <- go [] xs'
+                let fs = fmap singletonProjectConfigSkeleton $ fieldsToConfig (reverse acc)
+                (elseClauses, rest) <- parseElseClauses xs
+                let condNode =  (\c pcs e -> CondNode mempty mempty [CondBranch c pcs e]) <$>
+                      -- we rewrap as as a section so the readFields lexer of the conditional parser doesn't get confused
+                      adaptParseError l (parseConditionConfVarFromClause . BS.pack $ "if(" <> p <> ")") <*>
+                      subpcs <*>
+                      elseClauses
+                pure . fmap mconcat . sequence $ [fs, condNode, rest]
+         _ -> go (x:acc) xs
+    go acc [] = pure . fmap singletonProjectConfigSkeleton . fieldsToConfig $ reverse acc
+
+    parseElseClauses :: [ParseUtils.Field] -> IO (ParseResult (Maybe ProjectConfigSkeleton), ParseResult ProjectConfigSkeleton)
+    parseElseClauses x = case x of
+         (ParseUtils.Section _l "else" _p xs':xs) -> do
+               subpcs <- go [] xs'
+               rest <- go [] xs
+               pure (Just <$> subpcs, rest)
+         (ParseUtils.Section l "elif" p xs':xs) -> do
+               subpcs <- go [] xs'
+               (elseClauses, rest) <- parseElseClauses xs
+               let condNode = (\c pcs e -> CondNode mempty mempty [CondBranch c pcs e]) <$>
+                                  adaptParseError l (parseConditionConfVarFromClause . BS.pack $ "else("<> p <> ")") <*>
+                                  subpcs <*>
+                                  elseClauses
+               pure (Just <$> condNode, rest)
+         _ -> (\r -> (pure Nothing,r)) <$> go [] x
+
+    fieldsToConfig xs = fmap (addProvenance . convertLegacyProjectConfig) $ parseLegacyProjectConfigFields source xs
+    addProvenance x = x {projectConfigProvenance = Set.singleton (Explicit source)}
+
+    adaptParseError _ (Right x) = pure x
+    adaptParseError l (Left e) = parseFail $ ParseUtils.FromString (show e) (Just l)
+
+    liftPR :: (a -> IO (ParseResult b)) -> ParseResult a -> IO (ParseResult b)
+    liftPR f (ParseOk ws x) = addWarnings <$> f x
+       where addWarnings (ParseOk ws' x') = ParseOk (ws' ++ ws) x'
+             addWarnings x' = x'
+    liftPR _ (ParseFailed e) = pure $ ParseFailed e
+
+    fetchImportConfig :: ProjectConfigImport -> IO BS.ByteString
+    fetchImportConfig pci = case parseURI pci of
+         Just uri -> do
+            let fp = cacheDir </> map (\x -> if isPathSeparator x then '_' else x) (makeValid $ show uri)
+            createDirectoryIfMissing True cacheDir
+            _ <- downloadURI httpTransport verbosity uri fp
+            BS.readFile fp
+         Nothing -> BS.readFile pci
+
+    modifiesCompiler :: ProjectConfig -> Bool
+    modifiesCompiler pc = isSet projectConfigHcFlavor || isSet projectConfigHcPath || isSet projectConfigHcPkg
+      where
+         isSet f = f (projectConfigShared pc) /= NoFlag
 
-import Network.URI (URI (..))
+    sanityWalkPCS :: Bool -> ProjectConfigSkeleton -> ParseResult ProjectConfigSkeleton
+    sanityWalkPCS underConditional t@(CondNode d _c comps)
+           | underConditional && modifiesCompiler d = parseFail $ ParseUtils.FromString "Cannot set compiler in a conditional clause of a cabal project file" Nothing
+           | otherwise = mapM_ sanityWalkBranch comps >> pure t
+
+    sanityWalkBranch:: CondBranch ConfVar [ProjectConfigImport] ProjectConfig -> ParseResult ()
+    sanityWalkBranch (CondBranch _c t f) = traverse (sanityWalkPCS True) f >> sanityWalkPCS True t >> pure ()
 
 ------------------------------------------------------------------
 -- Representing the project config file in terms of legacy types
@@ -851,15 +981,18 @@ convertToLegacyPerPackageConfig PackageConfig {..} =
 -- Parsing and showing the project config file
 --
 
-parseLegacyProjectConfig :: FilePath -> BS.ByteString -> ParseResult LegacyProjectConfig
-parseLegacyProjectConfig source =
-    parseConfig (legacyProjectConfigFieldDescrs constraintSrc)
+parseLegacyProjectConfigFields :: FilePath -> [ParseUtils.Field] -> ParseResult LegacyProjectConfig
+parseLegacyProjectConfigFields source =
+    parseFieldsAndSections (legacyProjectConfigFieldDescrs constraintSrc)
                 legacyPackageConfigSectionDescrs
                 legacyPackageConfigFGSectionDescrs
                 mempty
   where
     constraintSrc = ConstraintSourceProjectConfig source
 
+parseLegacyProjectConfig :: FilePath -> BS.ByteString -> ParseResult LegacyProjectConfig
+parseLegacyProjectConfig source bs = parseLegacyProjectConfigFields source =<< ParseUtils.readFields bs
+
 showLegacyProjectConfig :: LegacyProjectConfig -> String
 showLegacyProjectConfig config =
     Disp.render $
diff --git a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
index d4de014a7de..d7933b6c2a9 100644
--- a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
+++ b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
@@ -139,6 +139,7 @@ import qualified Distribution.Client.BuildReports.Storage as BuildReports
          ( storeLocal )
 
 import           Distribution.Client.Config (getCabalDir)
+import           Distribution.Client.HttpUtils
 import           Distribution.Client.Setup hiding (packageName)
 import           Distribution.Compiler
                    ( CompilerFlavor(GHC) )
@@ -170,7 +171,8 @@ import           Distribution.Version
 import           Distribution.Simple.Compiler
                    ( compilerCompatVersion, showCompilerId, compilerId, compilerInfo
                    , OptimisationLevel(..))
-
+import           Distribution.Utils.NubList
+                   ( fromNubList )
 import           Distribution.System
                    ( Platform(Platform) )
 
@@ -210,7 +212,7 @@ establishProjectBaseContext verbosity cliConfig currentCommand = do
     establishProjectBaseContextWithRoot verbosity cliConfig projectRoot currentCommand
   where
     mprojectFile   = Setup.flagToMaybe projectConfigProjectFile
-    ProjectConfigShared { projectConfigProjectFile } = projectConfigShared cliConfig
+    ProjectConfigShared { projectConfigProjectFile} = projectConfigShared cliConfig
 
 -- | Like 'establishProjectBaseContext' but doesn't search for project root.
 establishProjectBaseContextWithRoot
@@ -224,8 +226,13 @@ establishProjectBaseContextWithRoot verbosity cliConfig projectRoot currentComma
 
     let distDirLayout  = defaultDistDirLayout projectRoot mdistDirectory
 
+    httpTransport <- configureTransport verbosity
+                     (fromNubList . projectConfigProgPathExtra $ projectConfigShared cliConfig)
+                     (flagToMaybe . projectConfigHttpTransport $ projectConfigBuildOnly cliConfig)
+
     (projectConfig, localPackages) <-
       rebuildProjectConfig verbosity
+                           httpTransport
                            distDirLayout
                            cliConfig
 
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 6ed918bb9c7..7d4ec4186f7 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -40,6 +40,7 @@ module Distribution.Client.ProjectPlanning (
     -- * Utils required for building
     pkgHasEphemeralBuildTargets,
     elabBuildTargetWholeComponents,
+    configureCompiler,
 
     -- * Setup.hs CLI flags for building
     setupHsScriptOptions,
@@ -71,11 +72,13 @@ import Prelude ()
 import Distribution.Client.Compat.Prelude
 
 import           Distribution.Client.HashValue
+import           Distribution.Client.HttpUtils
 import           Distribution.Client.ProjectPlanning.Types as Ty
 import           Distribution.Client.PackageHash
 import           Distribution.Client.RebuildMonad
 import           Distribution.Client.Store
 import           Distribution.Client.ProjectConfig
+import           Distribution.Client.ProjectConfig.Legacy
 import           Distribution.Client.ProjectPlanOutput
 
 import           Distribution.Client.Types
@@ -300,11 +303,13 @@ sanityCheckElaboratedPackage ElaboratedConfiguredPackage{..}
 -- packages within the project.
 --
 rebuildProjectConfig :: Verbosity
+                     -> HttpTransport
                      -> DistDirLayout
                      -> ProjectConfig
                      -> IO ( ProjectConfig
                            , [PackageSpecifier UnresolvedSourcePackage] )
 rebuildProjectConfig verbosity
+                     httpTransport
                      distDirLayout@DistDirLayout {
                        distProjectRootDirectory,
                        distDirectory,
@@ -322,10 +327,14 @@ rebuildProjectConfig verbosity
       runRebuild distProjectRootDirectory
       $ rerunIfChanged verbosity
                        fileMonitorProjectConfig
-                       fileMonitorProjectConfigKey
+                       fileMonitorProjectConfigKey -- todo check deps too?
       $ do
           liftIO $ info verbosity "Project settings changed, reconfiguring..."
-          projectConfig <- phaseReadProjectConfig
+          liftIO $ createDirectoryIfMissingVerbose verbosity True distProjectCacheDirectory
+          projectConfigSkeleton <- phaseReadProjectConfig
+          -- have to create the cache directory before configuring the compiler
+          (compiler, Platform arch os, _) <- configureCompiler verbosity distDirLayout ((fst $ PD.ignoreConditions projectConfigSkeleton) <> cliConfig)
+          let projectConfig = instantiateProjectConfigSkeleton os arch (compilerInfo compiler) mempty projectConfigSkeleton
           localPackages <- phaseReadLocalPackages (projectConfig <> cliConfig)
           return (projectConfig, localPackages)
 
@@ -353,9 +362,9 @@ rebuildProjectConfig verbosity
     -- Read the cabal.project (or implicit config) and combine it with
     -- arguments from the command line
     --
-    phaseReadProjectConfig :: Rebuild ProjectConfig
+    phaseReadProjectConfig :: Rebuild ProjectConfigSkeleton
     phaseReadProjectConfig = do
-      readProjectConfig verbosity projectConfigConfigFile distDirLayout
+      readProjectConfig verbosity httpTransport projectConfigConfigFile distDirLayout
 
     -- Look for all the cabal packages in the project
     -- some of which may be local src dirs, tarballs etc
@@ -380,6 +389,60 @@ rebuildProjectConfig verbosity
                                  pkgLocations
 
 
+configureCompiler :: Verbosity ->
+                     DistDirLayout ->
+                     ProjectConfig ->
+                     Rebuild (Compiler, Platform, ProgramDb)
+configureCompiler verbosity
+                  DistDirLayout {
+                     distProjectCacheFile
+                   }
+                  ProjectConfig {
+                             projectConfigShared = ProjectConfigShared {
+                               projectConfigHcFlavor,
+                               projectConfigHcPath,
+                               projectConfigHcPkg
+                             },
+                             projectConfigLocalPackages = PackageConfig {
+                               packageConfigProgramPaths,
+                               packageConfigProgramPathExtra
+                             }
+                           } = do
+        let fileMonitorCompiler       = newFileMonitor . distProjectCacheFile $ "compiler"
+
+        progsearchpath <- liftIO $ getSystemSearchPath
+        rerunIfChanged verbosity fileMonitorCompiler
+                       (hcFlavor, hcPath, hcPkg, progsearchpath,
+                        packageConfigProgramPaths,
+                        packageConfigProgramPathExtra) $ do
+
+          liftIO $ info verbosity "Compiler settings changed, reconfiguring..."
+          result@(_, _, progdb') <- liftIO $
+            Cabal.configCompilerEx
+              hcFlavor hcPath hcPkg
+              progdb verbosity
+
+        -- Note that we added the user-supplied program locations and args
+        -- for /all/ programs, not just those for the compiler prog and
+        -- compiler-related utils. In principle we don't know which programs
+        -- the compiler will configure (and it does vary between compilers).
+        -- We do know however that the compiler will only configure the
+        -- programs it cares about, and those are the ones we monitor here.
+          monitorFiles (programsMonitorFiles progdb')
+
+          return result
+      where
+        hcFlavor = flagToMaybe projectConfigHcFlavor
+        hcPath   = flagToMaybe projectConfigHcPath
+        hcPkg    = flagToMaybe projectConfigHcPkg
+        progdb   =
+            userSpecifyPaths (Map.toList (getMapLast packageConfigProgramPaths))
+          . modifyProgramSearchPath
+              (++ [ ProgramSearchPathDir dir
+                  | dir <- fromNubList packageConfigProgramPathExtra ])
+          $ defaultProgramDb
+
+
 -- | Return an up-to-date elaborated install plan.
 --
 -- Two variants of the install plan are returned: with and without packages
@@ -451,7 +514,6 @@ rebuildInstallPlan verbosity
       return (improvedPlan, elaboratedPlan, elaboratedShared, totalIndexState, activeRepos)
 
   where
-    fileMonitorCompiler       = newFileMonitorInCacheDir "compiler"
     fileMonitorSolverPlan     = newFileMonitorInCacheDir "solver-plan"
     fileMonitorSourceHashes   = newFileMonitorInCacheDir "source-hashes"
     fileMonitorElaboratedPlan = newFileMonitorInCacheDir "elaborated-plan"
@@ -468,49 +530,7 @@ rebuildInstallPlan verbosity
     --
     phaseConfigureCompiler :: ProjectConfig
                            -> Rebuild (Compiler, Platform, ProgramDb)
-    phaseConfigureCompiler ProjectConfig {
-                             projectConfigShared = ProjectConfigShared {
-                               projectConfigHcFlavor,
-                               projectConfigHcPath,
-                               projectConfigHcPkg
-                             },
-                             projectConfigLocalPackages = PackageConfig {
-                               packageConfigProgramPaths,
-                               packageConfigProgramPathExtra
-                             }
-                           } = do
-        progsearchpath <- liftIO $ getSystemSearchPath
-        rerunIfChanged verbosity fileMonitorCompiler
-                       (hcFlavor, hcPath, hcPkg, progsearchpath,
-                        packageConfigProgramPaths,
-                        packageConfigProgramPathExtra) $ do
-
-          liftIO $ info verbosity "Compiler settings changed, reconfiguring..."
-          result@(_, _, progdb') <- liftIO $
-            Cabal.configCompilerEx
-              hcFlavor hcPath hcPkg
-              progdb verbosity
-
-        -- Note that we added the user-supplied program locations and args
-        -- for /all/ programs, not just those for the compiler prog and
-        -- compiler-related utils. In principle we don't know which programs
-        -- the compiler will configure (and it does vary between compilers).
-        -- We do know however that the compiler will only configure the
-        -- programs it cares about, and those are the ones we monitor here.
-          monitorFiles (programsMonitorFiles progdb')
-
-          return result
-      where
-        hcFlavor = flagToMaybe projectConfigHcFlavor
-        hcPath   = flagToMaybe projectConfigHcPath
-        hcPkg    = flagToMaybe projectConfigHcPkg
-        progdb   =
-            userSpecifyPaths (Map.toList (getMapLast packageConfigProgramPaths))
-          . modifyProgramSearchPath
-              (++ [ ProgramSearchPathDir dir
-                  | dir <- fromNubList packageConfigProgramPathExtra ])
-          $ defaultProgramDb
-
+    phaseConfigureCompiler = configureCompiler verbosity distDirLayout
 
     -- Configuring other programs.
     --
diff --git a/cabal-install/src/Distribution/Client/ScriptUtils.hs b/cabal-install/src/Distribution/Client/ScriptUtils.hs
index 9f012380256..6555b92ef7c 100644
--- a/cabal-install/src/Distribution/Client/ScriptUtils.hs
+++ b/cabal-install/src/Distribution/Client/ScriptUtils.hs
@@ -27,13 +27,21 @@ import Distribution.Client.DistDirLayout
     ( DistDirLayout(..) )
 import Distribution.Client.HashValue
     ( hashValue, showHashValue )
+import Distribution.Client.HttpUtils
+         ( HttpTransport, configureTransport )
 import Distribution.Client.NixStyleOptions
     ( NixStyleFlags (..) )
 import Distribution.Client.ProjectConfig
     ( ProjectConfig(..), ProjectConfigShared(..)
-    , parseProjectConfig, reportParseResult, withProjectOrGlobalConfig )
+    , reportParseResult, withProjectOrGlobalConfig
+    , projectConfigHttpTransport )
+import Distribution.Client.ProjectConfig.Legacy
+    ( ProjectConfigSkeleton
+    , parseProjectSkeleton, instantiateProjectConfigSkeleton )
 import Distribution.Client.ProjectFlags
     ( flagIgnoreProject )
+import Distribution.Client.RebuildMonad
+    ( runRebuild )
 import Distribution.Client.Setup
     ( ConfigFlags(..), GlobalFlags(..) )
 import Distribution.Client.TargetSelector
@@ -44,6 +52,8 @@ import Distribution.FieldGrammar
     ( parseFieldGrammar, takeFields )
 import Distribution.Fields
     ( ParseResult, parseFatalFailure, readFields )
+import Distribution.PackageDescription
+    ( ignoreConditions )
 import Distribution.PackageDescription.FieldGrammar
     ( executableFieldGrammar )
 import Distribution.PackageDescription.PrettyPrint
@@ -51,16 +61,20 @@ import Distribution.PackageDescription.PrettyPrint
 import Distribution.Parsec
     ( Position(..) )
 import Distribution.Simple.Flag
-    ( fromFlagOrDefault )
+    ( fromFlagOrDefault, flagToMaybe )
 import Distribution.Simple.PackageDescription
     ( parseString )
 import Distribution.Simple.Setup
     ( Flag(..) )
+import Distribution.Simple.Compiler
+    ( compilerInfo )
 import Distribution.Simple.Utils
     ( createDirectoryIfMissingVerbose, createTempDirectory, die', handleDoesNotExist, readUTF8File, warn, writeUTF8File )
 import qualified Distribution.SPDX.License as SPDX
 import Distribution.Solver.Types.SourcePackage as SP
     ( SourcePackage(..) )
+import Distribution.System
+    ( Platform(..) )
 import Distribution.Types.BuildInfo
     ( BuildInfo(..) )
 import Distribution.Types.CondTree
@@ -73,6 +87,10 @@ import Distribution.Types.PackageDescription
     ( PackageDescription(..), emptyPackageDescription )
 import Distribution.Types.PackageName.Magic
     ( fakePackageCabalFileName, fakePackageId )
+import Distribution.Utils.NubList
+    ( fromNubList )
+import Distribution.Client.ProjectPlanning
+    ( configureCompiler )
 import Distribution.Verbosity
     ( normal )
 import Language.Haskell.Extension
@@ -217,7 +235,17 @@ withContextAndSelectors noTargets kind flags@NixStyleFlags {..} targetStrings gl
 
         scriptContents <- BS.readFile script
         executable     <- readExecutableBlockFromScript verbosity scriptContents
-        projectCfg     <- readProjectBlockFromScript verbosity (takeFileName script) scriptContents
+
+
+        httpTransport <- configureTransport verbosity
+                     (fromNubList . projectConfigProgPathExtra $ projectConfigShared cliConfig)
+                     (flagToMaybe . projectConfigHttpTransport $ projectConfigBuildOnly cliConfig)
+
+        projectCfgSkeleton <- readProjectBlockFromScript verbosity httpTransport (distDirLayout ctx) (takeFileName script) scriptContents
+
+        (compiler, Platform arch os, _) <- runRebuild (distProjectRootDirectory . distDirLayout $ ctx) $ configureCompiler verbosity (distDirLayout ctx) ((fst $ ignoreConditions projectCfgSkeleton) <> projectConfig ctx)
+
+        let projectCfg = instantiateProjectConfigSkeleton os arch (compilerInfo compiler) mempty projectCfgSkeleton :: ProjectConfig
 
         let executable' = executable & L.buildInfo . L.defaultLanguage %~ maybe (Just Haskell2010) Just
             ctx'        = ctx & lProjectConfig %~ (<> projectCfg)
@@ -312,12 +340,12 @@ readExecutableBlockFromScript verbosity str = do
 -- * @-}@
 --
 -- Return the metadata.
-readProjectBlockFromScript :: Verbosity -> String -> BS.ByteString -> IO ProjectConfig
-readProjectBlockFromScript verbosity scriptName str = do
+readProjectBlockFromScript :: Verbosity -> HttpTransport -> DistDirLayout -> String -> BS.ByteString -> IO ProjectConfigSkeleton
+readProjectBlockFromScript verbosity httpTransport DistDirLayout{distDownloadSrcDirectory} scriptName str = do
     case extractScriptBlock "project" str of
         Left  _ -> return mempty
-        Right x -> reportParseResult verbosity "script" scriptName
-                 $ parseProjectConfig scriptName x
+        Right x ->    reportParseResult verbosity "script" scriptName
+                  =<< parseProjectSkeleton distDownloadSrcDirectory httpTransport verbosity [] scriptName x
 
 -- | Extract the first encountered script metadata block started end
 -- terminated by the tokens
diff --git a/cabal-install/src/Distribution/Deprecated/ParseUtils.hs b/cabal-install/src/Distribution/Deprecated/ParseUtils.hs
index 78f57b58d78..6ac62a6e82d 100644
--- a/cabal-install/src/Distribution/Deprecated/ParseUtils.hs
+++ b/cabal-install/src/Distribution/Deprecated/ParseUtils.hs
@@ -105,13 +105,21 @@ instance Monad ParseResult where
         ParseOk ws x >>= f = case f x of
                                ParseFailed err -> ParseFailed err
                                ParseOk ws' x' -> ParseOk (ws'++ws) x'
-
 #if !(MIN_VERSION_base(4,9,0))
         fail = parseResultFail
 #elif !(MIN_VERSION_base(4,13,0))
         fail = Fail.fail
 #endif
 
+instance Foldable ParseResult where
+  foldMap _ (ParseFailed _ ) = mempty
+  foldMap f (ParseOk _ x) = f x
+
+instance Traversable ParseResult where
+  traverse _ (ParseFailed err) = pure (ParseFailed err)
+  traverse f (ParseOk ws x) = ParseOk ws <$> f x
+
+
 instance Fail.MonadFail ParseResult where
         fail = parseResultFail
 
diff --git a/cabal-install/tests/IntegrationTests2.hs b/cabal-install/tests/IntegrationTests2.hs
index 57b21bc3b10..94fbac61578 100644
--- a/cabal-install/tests/IntegrationTests2.hs
+++ b/cabal-install/tests/IntegrationTests2.hs
@@ -16,6 +16,7 @@ import Prelude ()
 import Distribution.Client.DistDirLayout
 import Distribution.Client.ProjectConfig
 import Distribution.Client.Config (getCabalDir)
+import Distribution.Client.HttpUtils
 import Distribution.Client.TargetSelector hiding (DirActions(..))
 import qualified Distribution.Client.TargetSelector as TS (DirActions(..))
 import Distribution.Client.ProjectPlanning
@@ -1690,8 +1691,11 @@ configureProject testdir cliConfig = do
     -- ended in an exception (as we leave the files to help with debugging).
     cleanProject testdir
 
+    httpTransport <- configureTransport verbosity [] Nothing
+
     (projectConfig, localPackages) <-
       rebuildProjectConfig verbosity
+                           httpTransport
                            distDirLayout
                            cliConfig
 
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/Foo.hs b/cabal-testsuite/PackageTests/ConditionalAndImport/Foo.hs
new file mode 100644
index 00000000000..efbf93bbde8
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/Foo.hs
@@ -0,0 +1 @@
+module Foo where
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-bad-conditional.project b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-bad-conditional.project
new file mode 100644
index 00000000000..1cd137e6de7
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-bad-conditional.project
@@ -0,0 +1,4 @@
+packages: .
+
+if(True)
+  compiler: ghc
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-cyclical.project b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-cyclical.project
new file mode 100644
index 00000000000..db226311abc
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal-cyclical.project
@@ -0,0 +1,3 @@
+packages: .
+
+import: cabal-cyclical.project
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.out b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.out
new file mode 100644
index 00000000000..e125635e0eb
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.out
@@ -0,0 +1,18 @@
+# cabal v2-update
+Downloading the latest package list from test-local-repo
+# cabal v2-run
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - some-exe-0.0.1.0 (exe:some-exe) (requires build)
+Configuring some-exe-0.0.1.0...
+Preprocessing executable 'some-exe' for some-exe-0.0.1.0..
+Building executable 'some-exe' for some-exe-0.0.1.0..
+Installing executable some-exe in <PATH>
+Warning: The directory <ROOT>/cabal.dist/home/.cabal/store/ghc-<GHCVER>/incoming/new-<RAND><ROOT>/cabal.dist/home/.cabal/store/ghc-<GHCVER>/<PACKAGE>-<HASH>/bin is not in the system search path.
+# cabal v2-build
+Error: cabal: Error parsing project file <ROOT>/cabal-cyclical.project:3:
+cyclical import of cabal-cyclical.project
+# cabal v2-build
+Error: cabal: Error parsing project file <ROOT>/cabal-bad-conditional.project:
+Cannot set compiler in a conditional clause of a cabal project file
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.project b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.project
new file mode 100644
index 00000000000..0aeed82370c
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.project
@@ -0,0 +1,3 @@
+packages: .
+
+import: extra.project
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
new file mode 100644
index 00000000000..0791050f66d
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
@@ -0,0 +1,6 @@
+import Test.Cabal.Prelude
+main = cabalTest $
+  withRepo "repo" $ do
+     cabal "v2-run" [ "some-exe" ]
+     fails $ cabal "v2-build" [ "--project=cabal-cyclical.project" ]
+     fails $ cabal "v2-build" [ "--project=cabal-bad-conditional.project" ]
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/extra.project b/cabal-testsuite/PackageTests/ConditionalAndImport/extra.project
new file mode 100644
index 00000000000..52a1f847fbb
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/extra.project
@@ -0,0 +1,8 @@
+if(os(NoSuchOs) || False)
+  extra-packages: no-such-package
+elif(os(Whoops))
+  extra-packages: no-can-do
+elif(True)
+  extra-packages: some-exe
+else
+  extra-packages: nope
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/my.cabal b/cabal-testsuite/PackageTests/ConditionalAndImport/my.cabal
new file mode 100644
index 00000000000..b1b36c1e620
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/my.cabal
@@ -0,0 +1,9 @@
+name:           my
+version:        0.1
+license:        BSD3
+cabal-version:  >= 1.2
+build-type:     Simple
+
+library
+    exposed-modules:    Foo
+    build-depends:      base
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/Main.hs b/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/Main.hs
new file mode 100644
index 00000000000..33581fa8421
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/Main.hs
@@ -0,0 +1,4 @@
+module Main where
+
+main :: IO ()
+main = putStrLn "hello world"
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/some-exe.cabal b/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/some-exe.cabal
new file mode 100644
index 00000000000..3a2e620d96e
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/repo/some-exe-0.0.1.0/some-exe.cabal
@@ -0,0 +1,9 @@
+name:           some-exe
+version:        0.0.1.0
+license:        BSD3
+cabal-version:  >= 1.2
+build-type:     Simple
+
+Executable some-exe
+  main-is: Main.hs
+  build-depends: base
diff --git a/changelog.d/pr-7783 b/changelog.d/pr-7783
new file mode 100644
index 00000000000..bbf8cda6f8d
--- /dev/null
+++ b/changelog.d/pr-7783
@@ -0,0 +1,11 @@
+synopsis: Conditionals and imports in cabal.project files
+packages: cabal-install
+prs: #7783
+issues: #7556
+significance: significant
+
+description: {
+
+Cabal.project files now allow conditional logic on compiler version, arch, etc. as well as imports of other local or remote project of freeze files (both old and new style).
+
+}
diff --git a/doc/cabal-package.rst b/doc/cabal-package.rst
index 2cad579ea04..8dc0568a4e8 100644
--- a/doc/cabal-package.rst
+++ b/doc/cabal-package.rst
@@ -2581,6 +2581,8 @@ Since Cabal 2.2 conditional blocks support ``elif`` construct.
       else
            property-descriptions-or-conditionals
 
+.. _conditions:
+
 Conditions
 """"""""""
 
diff --git a/doc/cabal-project.rst b/doc/cabal-project.rst
index 988572e0a3d..52322a04b51 100644
--- a/doc/cabal-project.rst
+++ b/doc/cabal-project.rst
@@ -35,6 +35,32 @@ options):
 Any call to ``cabal build`` will consider ``cabal.project*`` files from parent 
 directories when there is none in the current directory.
 
+Conditionals and imports
+------------------------
+
+As of ``cabal-install`` version 3.8, cabal supports conditional logic
+and imports in ``cabal.project`` files. :ref:`conditions` in cabal
+may case on operating system, architecture, and
+compiler (i.e. there is no support for a notion of custom flags in
+project files). Imports may specify local filepaths or remote urls,
+and may reference either cabal.project files or v1-style cabal.config
+freeze files. As a usage example:
+
+::
+
+    if(os(darwin))
+      optimization: False
+    elif(os(freebsd))
+      packages: freebsd/*.cabal
+    else
+      optimization: True
+
+    import: https://some.remote.source/subdir/cabal.config
+
+    import: relativepath/extra-project.project
+
+    import: /absolutepath/some-project.project
+
 Specifying the local packages
 -----------------------------