`zero ≡ \s z. s`^{0} z ≡ \s z. z
@@ -297,17 +305,19 @@ Multiplication is even more elegant. Consider that applying an arbitrary functio
\s z. m (\z. n s z) z
-which, doing an eta-reduction and permitting ourselves the `∘` notation, is:
+which can be eta-reduced to:
-`\s z. (m ∘ n) s z`

+ \s. m (n s)
-and some more eta-reduction gives us:
+and we might abbreviate that as:
`m ∘ n`

Isn't that nice?
+And if we *apply* `m` to `n` instead of composing it, we get a implementation of exponentiation.
+
However, at this point the elegance gives out. The predecessor function is substantially more difficult to construct on this implementation. As with all of these operations, there are several ways to do it, but they all take at least a bit of ingenuity. If you're only first learning programming right now, it would be unreasonable to expect you to be able to figure out how to do it.
However, if on the other hand you do have some experience programming, consider how you might construct a predecessor function for numbers implemented in this way. Using only the resources we've so far discussed. (So you have no general facility for performing recursion, for instance.)
@@ -316,11 +326,11 @@ However, if on the other hand you do have some experience programming, consider
Lists, version 3
----------------
-It's possible to follow the same design for implementing lists, too. To see this, let's first step back and consider some of the more complex things you might do with a list. We don't need to think specifically inside the confines of the lambda calculus right now. These are generanl reflections.
+It's possible to follow the same design for implementing lists, too. To see this, let's first step back and consider some of the more complex things you might do with a list. We don't need to think specifically inside the confines of the lambda calculus right now. These are general reflections.
Assume you have a list of five integers, which I'll write using the OCaml notation: `[1; 2; 3; 4; 5]`.
-Now one thing you might want to do with the list is to double every member. Another thing you might want to do is to increment every number. More generally, given an arbitrary function `f`, uou might want to get the list which is `[f 1; f 2; f 3; f 4; f 5]`. Computer scientists call this **mapping** the function `f` over the list `[1; 2; 3; 4; 5]`.
+Now one thing you might want to do with the list is to double every member. Another thing you might want to do is to increment every number. More generally, given an arbitrary function `f`, you might want to get the list which is `[f 1; f 2; f 3; f 4; f 5]`. Computer scientists call this **mapping** the function `f` over the list `[1; 2; 3; 4; 5]`.
Another thing you might want to do with the list is to retrieve every member which is even. Or every member which is prime. Or, given an arbitrary function f, you might want to **filter** the original list to a shorter list containing only those elements `x` for which `f x` evaluates to true.