week9 tweak

[lambda.git] / week9.mdwn

diff --git a/week9.mdwn b/week9.mdwn
index 145833d..9c886b0 100644 (file)
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -100,6 +100,8 @@ Scheme is similar. There are various sorts of reference cells available in Schem
                (set-box! ycell 3)
                (+ x (unbox ycell)))
 
+(C has explicit-style mutable variables, too, which it calls *pointers*. But simple variables in C are already mutable, in the implicit style.)
+
 When dealing with explicit-style mutation, there's a difference between the types and values of `ycell` and `!ycell` (or `(unbox ycell)`). The former has the type `int ref`: the variable `ycell` is assigned a reference cell that contains an `int`. The latter has the type `int`, and has whatever value is now stored in the relevant reference cell. In an implicit-style framework though, we only have the resources to refer to the contents of the relevant reference cell. `y` in fragment [G] or the C snippet above has the type `int`, and only ever evaluates to `int` values.
 
 
@@ -236,6 +238,14 @@ The core idea to referential transparency is that when the same value is supplie
 
 Notice that the two invocations of `f 1` yield different results, even though the same value is being supplied as an argument to the same function.
 
+Similarly, functions like these:
+
+       let f cell = !cell;;
+
+       let g cell = cell := !cell + 1; !cell;;
+
+may return different results each time they're invoked, even if they're always supplied one and the same reference cell as argument.
+
 Computer scientists also associate referential transparency with a kind of substitution principle, illustrated here:
 
        let x = 1
@@ -515,6 +525,9 @@ To get the whole process started, the complex computation so defined will need t
     Notice: h, p have same value (1), but f (h, p) and f (h, h) differ
 
 
+Fine and Pryor on "coordinated contents" (see, e.g., [Hyper-Evaluativity](http://www.jimpryor.net/research/papers/Hyper-Evaluativity.txt))
+
+
 ##Five grades of mutation involvement##
 
 -- FIXME --
@@ -578,7 +591,29 @@ To get the whole process started, the complex computation so defined will need t
        We use the `None`/`Some factorial` option type here just as a way to ensure that the contents of `fact_cell` are of the same type both at the start and the end of the block.
 
 
+##Offsite Reading##
+
+*      [[!wikipedia Declarative programming]]
+*      [[!wikipedia Functional programming]]
+*      [[!wikipedia Purely functional]]
+*      [[!wikipedia Side effect (computer science) desc="Side effects"]]
+*      [[!wikipedia Referential transparency (computer science)]]
+*      [[!wikipedia Imperative programming]]
+*      [[!wikipedia Reference (computer science) desc="References"]]
+*      [[!wikipedia Pointer (computing) desc="Pointers"]]
+*      [Pointers in OCaml](http://caml.inria.fr/resources/doc/guides/pointers.html)
+
 <!--
-Fine and Pryor on "coordinated contents" (see, e.g., [Hyper-Evaluativity](http://www.jimpryor.net/research/papers/Hyper-Evaluativity.txt))
+# General issues about variables and scope in programming languages #
+
+*      [[!wikipedia Variable (programming) desc="Variables"]]
+*      [[!wikipedia Free variables and bound variables]]
+*      [[!wikipedia Variable shadowing]]
+*      [[!wikipedia Name binding]]
+*      [[!wikipedia Name resolution]]
+*      [[!wikipedia Parameter (computer science) desc="Function parameters"]]
+*      [[!wikipedia Scope (programming) desc="Variable scope"]]
+*      [[!wikipedia Closure (computer science) desc="Closures"]]
+
 -->