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--- a/week3.mdwn
+++ b/week3.mdwn
@@ -1,3 +1,12 @@
+[[!toc]]
+
+##More on evaluation strategies##
+
+Here are notes on [[evaluation order]] that make the choice of which
+lambda to reduce next the selection of a route through a network of
+links.
+
+
##Computing the length of a list##
How could we compute the length of a list? Without worrying yet about what lambda-calculus implementation we're using for the list, the basic idea would be to define this recursively:
@@ -211,7 +220,8 @@ Instead of writing out a long formula twice, we could write:
and the initial `(\x. x x)` is just what we earlier called the ω
combinator (lower-case omega, not the non-terminating Ω
). So the self-application of `H` can be written:
-
ω (\h \lst. (isempty lst) zero (add one ((h h) (extract-tail lst))))
+ω (\h \lst. (isempty lst) zero (add one ((h h) (extract-tail lst))))
+
and this will indeed implement the recursive function we couldn't earlier figure out how to define.
@@ -414,7 +424,7 @@ to *the tail* of the list we were evaluating its application to at the previous
##Fixed-point Combinators Are a Bit Intoxicating##
-![tatoo](/y-combinator.jpg)
+![tatoo](/y-combinator-fixed.jpg)
There's a tendency for people to say "Y-combinator" to refer to fixed-point combinators generally. We'll probably fall into that usage ourselves. Speaking correctly, though, the Y-combinator is only one of many fixed-point combinators.
@@ -493,16 +503,16 @@ As any functional programmer quickly learns, writing a recursive
function divides into two tasks: figuring out how to handle the
recursive case, and remembering to insert a base case. The
interesting and enjoyable part is figuring out the recursive pattern,
-but leaving out the base case creates a program that runs forever.
-For instance, consider computing a factorial: `n!` is `n * (n-1) *
-(n-2) * ... * 1`. The recursive case says that the factorial of a
-number `n` is `n` times the factorial of `n-1`. But if we leave out
-the base case, we get
+but the base case cannot be ignored, since leaving out the base case
+creates a program that runs forever. For instance, consider computing
+a factorial: `n!` is `n * (n-1) * (n-2) * ... * 1`. The recursive
+case says that the factorial of a number `n` is `n` times the
+factorial of `n-1`. But if we leave out the base case, we get
3! = 3 * 2! = 3 * 2 * 1! = 3 * 2 * 1 * 0! = 3 * 2 * 1 * 0 * -1! ...
-That's why it's crucial to declare that 0! = 1, and the recursive rule
-does not apply. In our terms,
+That's why it's crucial to declare that 0! = 1, in which case the
+recursive rule does not apply. In our terms,
fac = Y (\fac n. iszero n 1 (fac (predecessor n)))
@@ -514,12 +524,12 @@ case: the truth-teller paradox (and related paradoxes).
(1) This sentence is true.
-If we assume that "this sentence" can refer to (1), then the
-proposition expressed by (1) will be true just in case the thing
-referred to by "this sentence is true". Thus (1) will be true just in
-case (1) is true, and (1) is true just in case (1) is true, and so on.
-If (1) is true, then (1) is true; but if (1) is not true, then (1) is
-not true.
+If we assume that the complex demonstrative "this sentence" can refer
+to (1), then the proposition expressed by (1) will be true just in
+case the thing referred to by *this sentence* is true. Thus (1) will
+be true just in case (1) is true, and (1) is true just in case (1) is
+true, and so on. If (1) is true, then (1) is true; but if (1) is not
+true, then (1) is not true.
Without pretending to give a serious analysis of the paradox, let's
assume that sentences can have for their meaning boolean functions
@@ -529,29 +539,32 @@ is John* might denote the function `\x y. x`, our `true`.
Then (1) denotes a function from whatever the referent of *this
sentence* is to a boolean. So (1) denotes `\f. f true false`, where
the argument `f` is the referent of *this sentence*. Of course, if
-`f` is a boolean, `f true false <~~> f`, (1) denotes the identity
-function `I`.
+`f` is a boolean, `f true false <~~> f`, so for our purposes, we can
+assume that (1) denotes the identity function `I`.
If we use (1) in a context in which *this sentence* refers to the
sentence in which the demonstrative occurs, then we must find a
meaning `m` such that `I m = I`. But since in this context `m` is the
-same as the meaning `I`, we have `m = I m`, so `m` is a fixed point
-for the denotation of the sentence (when used in the appropriate context).
+same as the meaning `I`, so we have `m = I m`. In other words, `m` is
+a fixed point for the denotation of the sentence (when used in the
+appropriate context).
That means that in a context in which *this sentence* refers to the
sentence in which it occurs, the sentence denotes a fixed point for
-the identity function, `Y I`.
-
- Y I
- (\f. (\h. f (h h)) (\h. f (h h))) I
- (\h. I (h h)) (\h. I (h h)))
- (\h. (h h)) (\h. (h h)))
- ω ω
- &Omega
-
-Oh. Well. That feels right! The meaning of *This sentence is true*
+the identity function. Here's a fixed point for the identity
+function:
+
+Y I
+(\f. (\h. f (h h)) (\h. f (h h))) I
+(\h. I (h h)) (\h. I (h h)))
+(\h. (h h)) (\h. (h h)))
+ω ω
+&Omega
+
+
+Oh. Well! That feels right. The meaning of *This sentence is true*
in a context in which *this sentence* refers to the sentence in which
-it occurs is Ω, our prototypical infinite loop...
+it occurs is Ω
, our prototypical infinite loop...
What about the liar paradox?
@@ -578,3 +591,50 @@ See Barwise and Etchemendy's 1987 OUP book, [The Liar: an essay on
truth and circularity](http://tinyurl.com/2db62bk) for an approach
that is similar, but expressed in terms of non-well-founded sets
rather than recursive functions.
+
+##However...##
+
+You should be cautious about feeling too comfortable with
+these results. Thinking again of the truth-teller paradox, yes,
+Ω
is *a* fixed point for `I`, and perhaps it has
+some a privileged status among all the fixed points for `I`, being the
+one delivered by Y and all (though it is not obvious why Y should have
+any special status).
+
+But one could ask: look, literally every formula is a fixed point for
+`I`, since
+
+ X <~~> I X
+
+for any choice of X whatsoever.
+
+So the Y combinator is only guaranteed to give us one fixed point out
+of infinitely many---and not always the intuitively most useful
+one. (For instance, the squaring function has zero as a fixed point,
+since 0 * 0 = 0, and 1 as a fixed point, since 1 * 1 = 1, but `Y
+(\x. mul x x)` doesn't give us 0 or 1.) So with respect to the
+truth-teller paradox, why in the reasoning we've
+just gone through should we be reaching for just this fixed point at
+just this juncture?
+
+One obstacle to thinking this through is the fact that a sentence
+normally has only two truth values. We might consider instead a noun
+phrase such as
+
+(3) the entity that this noun phrase refers to
+
+The reference of (3) depends on the reference of the embedded noun
+phrase *this noun phrase*. It's easy to see that any object is a
+fixed point for this referential function: if this pen cap is the
+referent of *this noun phrase*, then it is the referent of (3), and so
+for any object.
+
+The chameleon nature of (3), by the way (a description that is equally
+good at describing any object), makes it particularly well suited as a
+gloss on pronouns such as *it*. In the system of
+[Jacobson 1999](http://www.springerlink.com/content/j706674r4w217jj5/),
+pronouns denote (you guessed it!) identity functions...
+
+Ultimately, in the context of this course, these paradoxes are more
+useful as a way of gaining leverage on the concepts of fixed points
+and recursion, rather than the other way around.