From: Jim <jim.pryor@nyu.edu>
Date: Sun, 1 Feb 2015 18:21:18 +0000 (-0500)
Subject: tweak week1 advanced
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=536b3ca117f2fbd3fecc229e62a63a41294428d3;ds=sidebyside

tweak week1 advanced
---

diff --git a/topics/week1_advanced_notes.mdwn b/topics/week1_advanced_notes.mdwn
index 1f58f61a..2742a7fc 100644
--- a/topics/week1_advanced_notes.mdwn
+++ b/topics/week1_advanced_notes.mdwn
@@ -101,7 +101,7 @@ If we get to the `y & ys` line in the pattern list, and the pattern-match succee
 
 Sometimes it's useful to bind variables against overlapping parts of a structure. For instance, suppose I'm writing a pattern that is to be matched against multivalues like `([10, 20], 'true)`. And suppose I want to end up with `ys` bound to `[10, 20]`, `x` bound to `10`, and `xs` bound to `[20]`. Using the techniques introduced so far, I have two options. First, I could bind `ys` against `[10, 20]`, and then initiate a second pattern-match to break that up into `10` and `[20]`. Like this:
 
-    case [10, 20] of
+    case ([10, 20], 'true) of
       [ys, _] then case ys of
                      x & xs then ...;
                      ...
@@ -111,7 +111,7 @@ Sometimes it's useful to bind variables against overlapping parts of a structure
 
 Alternatively, I could directly bind `x` against `10` and `xs` against `[20]`. But then I would have to re-cons them together again to get `ys`. Like this:
 
-    case [10, 20] of
+    case ([10, 20], 'true) of
       [x & xs, _] then let
                          ys match x & xs
                        in ...;
@@ -120,7 +120,7 @@ Alternatively, I could directly bind `x` against `10` and `xs` against `[20]`. B
 
 Both of these strategies work. But they are a bit inefficient. I said you didn't really need to worry about efficiency in this seminar. But these are also a bit cumbersome to write. There's a special syntax that enables us to bind all three of `ys`, `x`, and `xs` in the desired way, despite the fact that they will be matching against overlapping, rather than discrete, parts of the value `[10, 20]`. The special syntax looks like this:
 
-    case [10, 20] of
+    case ([10, 20], 'true) of
       [(x & xs) as ys, _] then ...
       ...
     end