14_a_1: Add solution

1 files changed, 33 insertions, 0 deletions

diff --git a/ch14/14_a_1.hs b/ch14/14_a_1.hs
new file mode 100644
index 0000000..6768583
--- /dev/null
+++ b/ch14/14_a_1.hs
@@ -0,0 +1,33 @@
+-- Rewrite getRandom to use do notation.
+
+import Control.Monad.State
+import System.Random
+
+type RandomState a = State StdGen a
+
+getRandom :: Random a => RandomState a
+getRandom =
+  get >>= \gen ->
+  let (val, gen') = random gen in
+  put gen' >>
+  return val
+
+getRandomDo :: Random a => RandomState a
+getRandomDo = do
+  gen <- get
+  let (val, gen') = random gen
+  put gen'
+  return val
+
+
+-- ghci> :l 14_a_1.hs
+-- [1 of 1] Compiling Main             ( 14_a_1.hs, interpreted )
+-- Ok, one module loaded.
+
+-- ghci> runState getRandomDo (mkStdGen 1)
+-- (-2241774542048937483,StdGen {unStdGen = SMGen 4999253871718377453 10451216379200822465})
+
+-- ghci> (runState getRandom (mkStdGen 1)) == (runState getRandomDo (mkStdGen 234))
+-- False
+-- ghci> (runState getRandom (mkStdGen 1)) == (runState getRandomDo (mkStdGen 1))
+-- True