manip trees: tweaks

[lambda.git] / reader_monad_for_variable_binding.mdwn

diff --git a/reader_monad_for_variable_binding.mdwn b/reader_monad_for_variable_binding.mdwn
index 9ddddc8..b9fd816 100644 (file)
--- a/reader_monad_for_variable_binding.mdwn
+++ b/reader_monad_for_variable_binding.mdwn
@@ -1,3 +1,5 @@
+[[!toc]]
+
 Substitution versus Environment-based Semantics
 -----------------------------------------------
 
@@ -155,7 +157,10 @@ Here we'll use a different monad. It's called the **Reader monad**. We define it
 
        (* here's our bind operation; how does it work? *)
        let bind (u : 'a reader) (f: 'a -> 'b reader) : 'b reader =
-               fun (e : env) -> f (u e) e
+               (* this can be written more compactly, but having it spelled out
+                  like this will be useful down the road *)
+               fun (e : env) -> let a = u e in let u' = f a in u' e
+
 
        (* we also define two special-purpose operations on our reader-monad values *)