Data commit

Task/Jensens-Device/Haskell/jensens-device.hs

@@ -0,0 +1,24 @@
+import Control.Monad.ST
+import Data.STRef
+
+sum_ :: STRef s Double -> Double -> Double
+     -> ST s Double -> ST s Double
+sum_ ref lo hi term =
+   do
+     vs <- forM [lo .. hi]
+            (\k -> do { writeSTRef ref k
+                      ; term } )
+     return $ sum vs
+
+foo :: Double
+foo =
+  runST $
+  do ref <- newSTRef undefined
+          -- initial value doesn't matter
+     sum_ ref 1 100 $
+       do
+         k <- readSTRef ref
+         return $ recip k
+
+main :: IO ()
+main = print foo