davidchase · August 29, 2021 12:35 · Aug 29, 2021
diff --git a/reader.elm b/reader.elm
@@ -0,0 +1,35 @@
+module Reader exposing
+  ( return
+  , ask
+  , runReader
+  , local
+  , fmap
+  , chain
+  )
+
+type Reader r a = Reader (r -> a)
+
+id x = x
+
+constant a = \_ -> a
+
+return a = Reader (constant a)
+
+ask = Reader id
+
+runReader e reader =
+  case reader of
+    Reader r -> r(e)
+
+local f reader = 
+  Reader (\c -> runReader f(c) reader)
+
+fmap f reader =
+  Reader (\c -> f(runReader c reader))
+
+chain f reader =
+  Reader (\c -> runReader c (f(runReader c reader)))
+
+-- fun to derive
+map f reader =
+  chain (\a -> pointed (f(a))) reader