Add in-memory caching otherwise everything is slow

Language/Haskell/GhcMod/Caching/Types.hs

@@ -0,0 +1,52 @@
+module Language.Haskell.GhcMod.Caching.Types where
+
+import Utils
+import Data.Label
+import Data.Version
+import Distribution.Helper
+
+type CacheContents d a = Maybe ([FilePath], d, a)
+type CacheLens s d a = s :-> CacheContents d a
+
+data Cached m s d a = Cached {
+  cacheFile       :: FilePath,
+  cacheLens       :: Maybe (CacheLens s d a),
+  cachedAction    :: TimedCacheFiles
+                  -> d
+                  -> Maybe a
+                  -> m ([FilePath], a)
+
+  -- ^ @cachedAction tcf data ma@
+  --
+  -- * @tcf@: Input file timestamps. Not technically necessary, just an
+  -- optimizazion when knowing which input files changed can make updating the
+  -- cache faster
+  --
+  -- * @data@: Arbitrary static input data to cache action. Can be used to
+  -- invalidate the cache using something other than file timestamps
+  -- i.e. environment tool version numbers
+  --
+  -- * @ma@: Cached data if it existed
+  --
+  -- Returns:
+  --
+  -- * @fst@: Input files used in generating the cache
+  --
+  -- * @snd@: Cache data, will be stored alongside the static input data in the
+  --   'cacheFile'
+  --
+  -- The cached action, will only run if one of the following is true:
+  --
+  -- * 'cacheFile' doesn\'t exist yet
+  -- * 'cacheFile' exists and 'inputData' changed
+  -- * any files returned by the cached action changed
+ }
+
+data TimedCacheFiles = TimedCacheFiles {
+  tcCacheFile :: Maybe TimedFile,
+  -- ^ 'cacheFile' timestamp
+  tcFiles     :: [TimedFile]
+  -- ^ Timestamped files returned by the cached action
+ }
+
+type ChCacheData = (Programs, FilePath, FilePath, (Version, [Char]))