src/Tools/Haskell/Haskell.thy

             err_lines = drop n lines
           in return $ Process_Result.make rc out_lines err_lines timing
         _ -> err
 \<close>
 
+generate_file "Isabelle/Cache.hs" = \<open>
+{-  Title:      Isabelle/Cache.hs
+    Author:     Makarius
+    LICENSE:    BSD 3-clause (Isabelle)
+
+Cache for slow computations.
+-}
+
+module Isabelle.Cache (
+  T, init, apply, prune
+)
+where
+
+import Prelude hiding (init)
+import Control.Exception (evaluate)
+import Data.IORef
+import Data.Map.Strict (Map)
+import qualified Data.Map.Strict as Map
+import qualified Data.List as List
+
+import Isabelle.Time (Time)
+import qualified Isabelle.Time as Time
+
+
+data Entry v = Entry {_value :: v, _access :: Time, _timing :: Time}
+
+newtype T k v = Cache (IORef (Map k (Entry v)))
+
+init :: IO (T k v)
+init = Cache <$> newIORef Map.empty
+
+commit :: Ord k => T k v -> k -> Entry v -> IO v
+commit (Cache ref) x e = do
+  atomicModifyIORef' ref (\entries ->
+    let
+      entry =
+        case Map.lookup x entries of
+          Just e' | _access e' > _access e -> e'
+          _ -> e
+    in (Map.insert x entry entries, _value entry))
+
+apply :: Ord k => T k v -> (k -> v) -> k -> IO v
+apply cache@(Cache ref) f x = do
+  start <- Time.now
+  entries <- readIORef ref
+  case Map.lookup x entries of
+    Just entry -> do
+      commit cache x (entry {_access = start})
+    Nothing -> do
+      y <- evaluate $ f x
+      stop <- Time.now
+      commit cache x (Entry y start (stop - start))
+
+prune :: Ord k => T k v -> Int -> Time -> IO ()
+prune (Cache ref) max_size min_timing = do
+  atomicModifyIORef' ref (\entries ->
+    let
+      trim x e = if _timing e < min_timing then Map.delete x else id
+      sort = List.sortBy (\(_, e1) (_, e2) -> compare (_access e2) (_access e1))
+      entries1 = Map.foldrWithKey trim entries entries
+      entries2 =
+        if Map.size entries1 <= max_size then entries1
+        else Map.fromList $ List.take max_size $ sort $ Map.toList entries1
+    in (entries2, ()))
+\<close>
+
 export_generated_files _
 
 end