add/update monad code

[lambda.git] / code / reader1.hs

diff --git a/code/reader1.hs b/code/reader1.hs
new file mode 100644 (file)
index 0000000..74cb519
--- /dev/null
+++ b/code/reader1.hs
@@ -0,0 +1,31 @@
+import Control.Monad.Reader
+type Bound = Int
+type Env = Char -> Bound
+
+
+env0 = \var -> error "Not found"
+insert var value e = \sought -> if sought == var then value else e sought
+
+getint :: Char -> Reader Env Int
+getint var = asks (\e -> let x = e var in x)
+
+-- monadic versions of `x` and `y`
+getx = getint 'x'
+gety = getint 'y'
+
+-- monadic version of `y + x`
+expr1 :: Reader Env Int
+expr1 = liftM2 (+) gety getx
+
+-- monadic version of `3 + x`
+expr2 :: Reader Env Int
+expr2 = liftM2 (+) (return 3) getx
+
+letx xx body = xx >>= \x -> local (insert 'x' x) body
+
+-- monadic version of `let x = 2 in 3 + x`
+expr3 :: Reader Env Int
+expr3 = letx (return 2) expr2
+
+res = runReader expr3 env0 -- will be 5
+