diff --git a/hnix-store-core/hnix-store-core.cabal b/hnix-store-core/hnix-store-core.cabal
index 520d6bb..713b4b6 100644
--- a/hnix-store-core/hnix-store-core.cabal
+++ b/hnix-store-core/hnix-store-core.cabal
@@ -18,9 +18,13 @@ cabal-version:       >=1.10
 
 library
   exposed-modules:     Crypto.Hash.Truncated
+                     , System.Nix.Build
+                     , System.Nix.Derivation
+                     , System.Nix.GC
                      , System.Nix.Nar
                      , System.Nix.Path
                      , System.Nix.Store
+                     , System.Nix.Util
   build-depends:       base >=4.10 && <4.11
                      , basement
                      , bytestring
diff --git a/hnix-store-core/src/System/Nix/Build.hs b/hnix-store-core/src/System/Nix/Build.hs
new file mode 100644
index 0000000..f77c2b0
--- /dev/null
+++ b/hnix-store-core/src/System/Nix/Build.hs
@@ -0,0 +1,50 @@
+{-# LANGUAGE RecordWildCards #-}
+{-|
+Description : Build related types
+Maintainer  : srk <srk@48.io>
+|-}
+module System.Nix.Build (
+    BuildMode(..)
+  , BuildStatus(..)
+  , BuildResult(..)
+  , buildSuccess
+  ) where
+
+import           Data.Text                 (Text)
+import           Data.HashSet              (HashSet)
+import           System.Nix.Path           (Path)
+
+data BuildMode = Normal | Repair | Check
+  deriving (Eq, Ord, Enum, Show)
+
+data BuildStatus =
+    Built
+  | Substituted
+  | AlreadyValid
+  | PermanentFailure
+  | InputRejected
+  | OutputRejected
+  | TransientFailure -- possibly transient
+  | CachedFailure    -- no longer used
+  | TimedOut
+  | MiscFailure
+  | DependencyFailed
+  | LogLimitExceeded
+  | NotDeterministic
+  deriving (Eq, Ord, Enum, Show)
+
+
+ -- | Result of the build
+data BuildResult = BuildResult
+  { -- | build status, MiscFailure should be default
+    status             :: !BuildStatus
+  , -- | possible build error message
+    error              :: !(Maybe Text)
+  , -- | How many times this build was performed
+    timesBuilt         :: !Integer
+  , -- | If timesBuilt > 1, whether some builds did not produce the same result
+    isNonDeterministic :: !Bool
+    -- XXX: | startTime stopTime time_t
+  } deriving (Eq, Ord, Show)
+
+buildSuccess BuildResult{..} = status == Built || status == Substituted || status == AlreadyValid
diff --git a/hnix-store-core/src/System/Nix/Derivation.hs b/hnix-store-core/src/System/Nix/Derivation.hs
new file mode 100644
index 0000000..56ccefc
--- /dev/null
+++ b/hnix-store-core/src/System/Nix/Derivation.hs
@@ -0,0 +1,28 @@
+{-|
+Description : Derivation types
+Maintainer  : srk <srk@48.io>
+|-}
+
+module System.Nix.Derivation (
+      BasicDerivation(..)
+    ) where
+
+
+import Data.Text (Text)
+import Data.HashMap.Strict (HashMap)
+import System.Nix.Path
+
+data BasicDerivation = BasicDerivation
+  { -- | Derivation outputs
+    outputs   ::  !(HashMap Text Path)
+  , -- | Inputs that are sources
+    inputSrcs :: !PathSet
+  , -- | Platform
+    platform  :: !Text
+  , -- | Path to builder
+    builder   :: !Path
+  , -- | Arguments
+    args      :: ![Text]
+  , -- | Environment
+    env       :: ![HashMap Text Text]
+  } deriving (Eq, Ord, Show)
diff --git a/hnix-store-core/src/System/Nix/GC.hs b/hnix-store-core/src/System/Nix/GC.hs
new file mode 100644
index 0000000..5291e62
--- /dev/null
+++ b/hnix-store-core/src/System/Nix/GC.hs
@@ -0,0 +1,47 @@
+{-|
+Description : Garbage collection actions / options
+Maintainer  : srk <srk@48.io>
+|-}
+module System.Nix.GC (
+    Action(..)
+  , Options(..)
+  , Result(..)
+  ) where
+
+import           System.Nix.Path           (PathSet)
+
+{- Garbage collector operation:
+     - ReturnLive: return the set of paths reachable from
+       (i.e. in the closure of) the roots.
+     - ReturnDead: return the set of paths not reachable from
+       the roots.
+     - DeleteDead: actually delete the latter set.
+     - DeleteSpecific: delete the paths listed in
+        `pathsToDelete', insofar as they are not reachable.
+-}
+
+data Action = ReturnLive | ReturnDead | DeleteDead | DeleteSpecific
+  deriving (Eq, Ord, Enum, Show)
+
+ -- | Garbage collector operation options
+data Options = Options
+  { -- | operation
+    operation      :: !Action
+    -- | If `ignoreLiveness' is set, then reachability from the roots is
+    -- ignored (dangerous!).  However, the paths must still be
+    -- unreferenced *within* the store (i.e., there can be no other
+    -- store paths that depend on them).
+  , ignoreLiveness :: !Bool
+    -- | For DeleteSpecific, the paths to delete
+  , pathsToDelete  :: !PathSet
+  , -- | Stop after at least `maxFreed` bytes have been freed
+    maxFreed       :: !Integer
+  } deriving (Eq, Ord, Show)
+
+data Result = Result
+ { -- | Depending on the action, the GC roots, or the paths that would be or have been deleted
+   paths      :: !PathSet
+ , -- |  For ReturnDead, DeleteDead and DeleteSpecific, the number of bytes that would be or was freed
+   bytesFreed :: !Integer
+ } deriving (Eq, Ord, Show)
+
diff --git a/hnix-store-core/src/System/Nix/Path.hs b/hnix-store-core/src/System/Nix/Path.hs
index 36307eb..197e80c 100644
--- a/hnix-store-core/src/System/Nix/Path.hs
+++ b/hnix-store-core/src/System/Nix/Path.hs
@@ -8,10 +8,13 @@ module System.Nix.Path
   ( FilePathPart(..)
   , PathHashAlgo
   , Path(..)
+  , PathSet
   , SubstitutablePathInfo(..)
+  , ValidPathInfo(..)
   , PathName(..)
   , filePathPart
   , pathName
+  , Roots
   ) where
 
 import           Crypto.Hash               (Digest)
@@ -23,6 +26,7 @@ import qualified Data.ByteString.Char8     as BSC
 import           Data.Hashable             (Hashable (..), hashPtrWithSalt)
 import           Data.HashMap.Strict       (HashMap)
 import           Data.HashSet              (HashSet)
+import           Data.Map.Strict           (Map)
 import           Data.Text                 (Text)
 import qualified Data.Text                 as T
 import           System.IO.Unsafe          (unsafeDupablePerformIO)
@@ -53,6 +57,7 @@ type PathHashAlgo = Truncated SHA256 20
 
 -- | A path in a store.
 data Path = Path !(Digest PathHashAlgo) !PathName
+  deriving (Eq, Ord, Show)
 
 -- | Wrapper to defined a 'Hashable' instance for 'Digest'.
 newtype HashableDigest a = HashableDigest (Digest a)
@@ -67,18 +72,51 @@ instance Hashable Path where
     (HashableDigest digest) `hashWithSalt` name
 
 
+type PathSet = HashSet Path
+
 -- | Information about substitutes for a 'Path'.
 data SubstitutablePathInfo = SubstitutablePathInfo
   { -- | The .drv which led to this 'Path'.
     deriver      :: !(Maybe Path)
   , -- | The references of the 'Path'
-    references   :: !(HashSet Path)
+    references   :: !PathSet
   , -- | The (likely compressed) size of the download of this 'Path'.
     downloadSize :: !Integer
   , -- | The size of the uncompressed NAR serialization of this
     -- 'Path'.
     narSize      :: !Integer
-  }
+  } deriving (Eq, Ord, Show)
+
+ -- | Information about 'Path'.
+data ValidPathInfo = ValidPathInfo
+  { -- | Path itself
+    path             :: !Path
+  , -- | The .drv which led to this 'Path'.
+    deriverVP        :: !(Maybe Path)
+  , -- | NAR hash
+    narHash          :: !Text
+  , -- | The references of the 'Path'
+    referencesVP     :: !PathSet
+  , -- | Registration time should be time_t
+    registrationTime :: !Integer
+  , -- | The size of the uncompressed NAR serialization of this
+    -- 'Path'.
+    narSizeVP        :: !Integer
+  , -- | Whether the path is ultimately trusted, that is, it's a
+    -- derivation output that was built locally.
+    ultimate         :: !Bool
+  , -- | Signatures
+    sigs             :: ![Text]
+  , -- | Content-addressed
+    -- Store path is computed from a cryptographic hash
+    -- of the contents of the path, plus some other bits of data like
+    -- the "name" part of the path.
+    --
+    -- ‘ca’ has one of the following forms:
+    -- * ‘text:sha256:<sha256 hash of file contents>’ (paths by makeTextPath() / addTextToStore())
+    -- * ‘fixed:<r?>:<ht>:<h>’ (paths by makeFixedOutputPath() / addToStore())
+    ca               :: !Text
+  } deriving (Eq, Ord, Show)
 
 -- | A valid filename or directory name
 newtype FilePathPart = FilePathPart { unFilePathPart :: BSC.ByteString }
@@ -90,3 +128,5 @@ filePathPart :: BSC.ByteString -> Maybe FilePathPart
 filePathPart p = case BSC.any (`elem` ['/', '\NUL']) p of
   False -> Just $ FilePathPart p
   True  -> Nothing
+
+type Roots = Map Path Path
diff --git a/hnix-store-core/src/System/Nix/Util.hs b/hnix-store-core/src/System/Nix/Util.hs
new file mode 100644
index 0000000..62a17ab
--- /dev/null
+++ b/hnix-store-core/src/System/Nix/Util.hs
@@ -0,0 +1,48 @@
+{-|
+Description : Utilities for packing stuff
+Maintainer  : srk <srk@48.io>
+|-}
+module System.Nix.Util where
+
+import           Control.Monad
+import           Data.Binary.Get
+import           Data.Binary.Put
+import qualified Data.ByteString.Lazy as LBS
+
+putInt :: Integral a => a -> Put
+putInt = putWord64le . fromIntegral
+
+getInt :: Integral a => Get a
+getInt = fromIntegral <$> getWord64le
+
+-- length prefixed string packing with padding to 8 bytes
+putByteStringLen :: LBS.ByteString -> Put
+putByteStringLen x = do
+  putInt $ fromIntegral $ len
+  putLazyByteString x
+  when (len `mod` 8 /= 0) $
+    pad $ fromIntegral $ 8 - (len `mod` 8)
+  where len = LBS.length x
+        pad x = forM_ (take x $ cycle [0]) putWord8
+
+putByteStrings :: Foldable t => t LBS.ByteString -> Put
+putByteStrings xs = do
+  putInt $ fromIntegral $ length xs
+  mapM_ putByteStringLen xs
+
+getByteStringLen :: Get LBS.ByteString
+getByteStringLen = do
+  len <- getInt
+  st <- getLazyByteString len
+  when (len `mod` 8 /= 0) $ do
+    pads <- unpad $ fromIntegral $ 8 - (len `mod` 8)
+    unless (all (==0) pads) $ fail $ "No zeroes" ++ show (st, len, pads)
+  return st
+  where unpad x = sequence $ replicate x getWord8
+
+getByteStrings :: Get [LBS.ByteString]
+getByteStrings = do
+  count <- getInt
+  res <- sequence $ replicate count getByteStringLen
+  return res
+