From: jim <jim@web>
Date: Wed, 8 Apr 2015 03:59:40 +0000 (-0400)
Subject: now ++ is working in wrappers
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=b35732e3e34bb499d0b7775b81632d40d7e3876e

now ++ is working in wrappers
---

diff --git a/topics/week9_monad_transformers.mdwn b/topics/week9_monad_transformers.mdwn
index ff47d06a..6d8b5b74 100644
--- a/topics/week9_monad_transformers.mdwn
+++ b/topics/week9_monad_transformers.mdwn
@@ -151,31 +151,28 @@ When Option is on the inside, on the other hand, as in SO, failure means the who
 	Exception: Failure "bye".
 -->
 
-<!--
 Here's an example wrapping Option around List, and vice versa:
 
 	# module LO = Monad.List.T(Monad.Option);;
-	# module OL = Monad.Option.T(Monad.List);;
-        # let plus = ???;;
-	# OL.(run (plus mzero mid 20 >>= fun i -> mid (i+10)));;
+	# module OL = Monad.Option.P(Monad.List);; (* we use the `.P` transformer to hoist List's ++ operation *)
+	# OL.(run (mzero ++ mid 20 >>= fun i -> mid (i+10)));;
 	- : int OL.result = [Some 30]
 
 When List is on the inside, the failed results just got dropped and the computation proceeds without them.
 
-	# LO.(run ((++) (hoist Monad.Option.mzero) (mid 20) >>= fun i -> mid (i+10)));;
+	# LO.(run (hoist Monad.Option.mzero ++ mid 20 >>= fun i -> mid (i+10)));;
 	- : int LO.result = None
 
-On the other hand, when Option is on the inside, as in LO, failures (which we represent by `mzero`s from the Option monad, here `hoist Monad.Option.mzero`, not the List monad's own `mzero`) abort the whole computation.
+On the other hand, when Option is on the inside, as in LO, failures (which we again represent by `mzero`s from the Option monad, not the List monad's own `mzero`; but here since it's the inner monad we need to `hoist Monad.Option.mzero`) abort the whole computation. (If you instead used the List monad's `mzero`, it'd be ignored by `++` and you'd end with just `Some [30]`.)
 
 This is fun. Notice the difference it makes whether the second `++` is native to the outer `Monad.List`, or whether it's the inner `Monad.List`'s `++` hoisted into the outer wrapper:
 
 	# module LL = Monad.List.T(Monad.List);;
 
-	# LL.(run((++) (mid 1) (mid 2) >>= fun i -> (++) (mid i) (mid (10*i)) ));;
+	# LL.(run(mid 1 ++ mid 2 >>= fun x -> mid x ++ mid (10*x) ));;
 	- : int LL.result = \[[1; 10; 2; 20]]
-	# LL.(run((++) (mid 1) (mid 2) >>= fun i -> hoist L.((++) (mid i) (mid (10*i)) )));;
-	- : int LL.result = [[1; 2]; [1; 20]; [10; 2]; [10; 20]]
--->
+	#  LL.(run(mid 1 ++ mid 2 >>= fun x -> hoist Monad.List.(mid x ++ mid (10*x) )));;
+	- : int LL.result = \[[1; 2]; [1; 20]; [10; 2]; [10; 20]]
 
 <!--
 	# EL.(run((++) (throw "bye") (mid 20) >>= fun i -> mid (i+10)));;