fix typo, thanks Kyle

[lambda.git] / topics / week1_advanced_notes.mdwn

diff --git a/topics/week1_advanced_notes.mdwn b/topics/week1_advanced_notes.mdwn
index 560a1c4..8b6289a 100644 (file)
--- a/topics/week1_advanced_notes.mdwn
+++ b/topics/week1_advanced_notes.mdwn
@@ -102,7 +102,7 @@ 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], 'true) of
 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], 'true) of

-      [ys, _] then case ys of
+      (ys, _) then case ys of

                      x & xs then ...;
                      ...
                    end;
                      x & xs then ...;
                      ...
                    end;


@@ -112,7 +112,7 @@ 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], 'true) of
 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], 'true) of

-      [x & xs, _] then let
+      (x & xs, _) then let

                          ys match x & xs
                        in ...;
       ...
                          ys match x & xs
                        in ...;
       ...


@@ -121,7 +121,7 @@ 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], 'true) of
 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], 'true) of

-      [(x & xs) as ys, _] then ...
+      ((x & xs) as ys, _) then ...

       ...
     end
 
       ...
     end
 


@@ -197,16 +197,18 @@ All of this only works with infix operators like `-`, `&` and `+`. You can't wri
 Can you guess what our shortcut for the last function will be? It's `( + )`. That
 expresses a function that takes two arguments `(x, y)` and evaluates to `x + y`.
 
 Can you guess what our shortcut for the last function will be? It's `( + )`. That
 expresses a function that takes two arguments `(x, y)` and evaluates to `x + y`.
 

-Wait a second, you say. Isn't that just what `+` does *already*? Why am I making a distinction between `+` and `(+)`? The difference is that bare `+` without any parentheses is an *infix* operator that comes between its arguments. Whereas when we wrap it with parentheses, it loses its special infix syntax and then just behaves like a plain variable denoting a function, like `swap`. Thus whereas we write:
+Wait a second, you say. Isn't that just what `+` does *already*? Why am I making a distinction between `+` and `( + )`? The difference is that bare `+` without any parentheses is an *infix* operator that comes between its arguments. Whereas when we wrap it with parentheses, it loses its special infix syntax and then just behaves like a plain variable denoting a function, like `swap`. Thus whereas we write:

 
     x + y
 
 if we want to use `( + )`, we have to instead write:
 
 
     x + y
 
 if we want to use `( + )`, we have to instead write:
 

-    (+) (x, y)
+    ( + ) (x, y)

 
 It may not be obvious now why this would ever be useful, but sometimes it will be.
 
 
 It may not be obvious now why this would ever be useful, but sometimes it will be.
 

-Confession: actually, what I described here diverges a *tiny* bit from what OCaml and Haskell do. They wouldn't really write `(+) (x, y)` like I just did. Instead they'd write `(+) x y`. We will look at the difference between these next week.
+All of these shorthands `(10 - )`, `( & ys)` and `( + )` are called "sections". I don't know exactly why.
+
+Confession: actually, what I described here diverges *a bit* from how OCaml and Haskell treat `( + )`. They wouldn't really write `( + ) (x, y)` like I did. Instead they'd write `( + ) x y`. We will look at the difference between these next week.