
week2.mdwn  24 +++++++
1 file changed, 7 insertions(+), 17 deletions()
diff git a/week2.mdwn b/week2.mdwn
index 0aa23fc0..13d1d449 100644
 a/week2.mdwn
+++ b/week2.mdwn
@@ 17,7 +17,7 @@ One can't just rename variables freely. (a) and (b) are different than what's ex
Substituting `y` into the body of `(\x. \z. z x)` is unproblematic:
+Substituting `y` into the body of (a) `(\x. \z. z x)` is unproblematic:
(\x. \z. z x) y ~~> \z. z y
@@ 27,11 +27,11 @@ To reduce (b), then, we need to be careful to that no free variables in what we'
In practical terms, you'd just replace (b) with (a) and do the unproblematic substitution into (a).
What attitude should we have to this?
+How should we think about the explanation and justification for that practical procedure?
One way to think of it is to identify expressions of the lambda calculus with particular alphabetic sequences. Then (a) and (b) would be distinct expressions, and we'd have to explicitly articulate a rule permitting you to do the kind of variablerenaming that would take you from (a) to (b) (or vice versa). This kind of renaming is called "alphaconversion."
+One way to think about things here is to identify expressions of the lambda calculus with *particular alphabetic sequences*. Then (a) and (b) would be distinct expressions, and we'd have to have an explicit rule permitting us to do the kind of variablerenaming that takes us from (a) to (b) (or vice versa). This kind of renaming is called "alphaconversion." Look in the standard treatments of the lambda calculus for detailed discussion of this.
Another way to think of it is to identify expressions not with particular alphabetic sequences, but rather with classes of alphabetic sequences, which stand to each other in the way that (a) and (b) do. That's the way we'll talk. We say that (a) and (b) are just typographically different notations for a *single* lambda formula. As we'll say, the lambda formula written with (a) and the lambda formula written with (b) are literally syntactically identical.
+Another way to think of it is to identify expressions not with particular alphabetic sequences, but rather with *classes* of alphabetic sequences, which stand to each other in the way that (a) and (b) do. That's the way we'll talk. We say that (a) and (b) are just typographically different notations for a *single* lambda formula. As we'll say, the lambda formula written with (a) and the lambda formula written with (b) are literally syntactically identical.
A third way to think is to identify the lambda formula not with classes of alphabetic sequences, but rather with abstract structures that we might draw like this:
@@ 45,10 +45,11 @@ Here there are no bound variables, but there are *bound positions*. We can regar
A version of this last approach is known as **de Bruijn notation** for the lambda calculus.
It doesn't matter which of these approaches one takes; the logical properties of the systems are exactly the same. It just affects the particulars of how one states the rules for substitution, and so on. And whether one talks about expressions being literally "syntactically identical," or whether one instead counts them as "equivalent modulu alphaconversion."
+It doesn't seem to matter which of these approaches one takes; the logical properties of the systems are exactly the same. It just affects the particulars of how one states the rules for substitution, and so on. And whether one talks about expressions being literally "syntactically identical," or whether one instead counts them as "equivalent modulu alphaconversion."
(In a bit, we'll discuss other systems that lack variables. Those systems will not just lack variables in the sense that de Bruijn notation does; they will furthermore lack any notion of a bound position.)
+(Linguistic trivia: however, some linguistic discussions do suppose that alphabetic variance has important linguistic consequences; see Ivan Sag's dissertation.)
+In a bit, we'll discuss other systems that lack variables. Those systems will not just lack variables in the sense that de Bruijn notation does; they will furthermore lack any notion of a bound position.
Syntactic equality, reduction, convertibility
@@ 59,17 +60,6 @@ Define T to be `(\x. x y) z`. Then T and `(\x. x y) z` are syntactically equal,
T ≡ (\x. x y) z ≡ (\z. z y) z
[Fussy note: the justification for counting `(\x. x y) z` as
equivalent to `(\z. z y) z` is that when a lambda binds a set of
occurrences, it doesn't matter which variable serves to carry out the
binding. Either way, the function does the same thing and means the
same thing.
Linguistic trivia: some linguistic discussions suppose that alphabetic variance
has important linguistic consequences (notably Ivan Sag's dissertation).
Look in the standard treatments for discussions of alpha
equivalence for more detail. Also, as mentioned below, one of the intriguing
properties of Combinatory Logic is that alpha equivalence is not an issue.]

This:
T ~~> z y

2.11.0