Rather, the problem is that the definition of the function requires
subterms that can't be simply-typed. We'll illustrate with our
-implementation of the predecessor, sightly modified in inessential
-ways to suit present purposes:
+implementation of the predecessor function, based on the discussion in
+Pierce 2002:547:
let zero = \s z. z in
let snd = \a b. b in
let pair = \a b. \v. v a b in
let succ = \n s z. s (n s z) in
- let collect = \p. p (\a b. pair (succ a) a)
- let pred = \n. n collect (pair zero zero) snd in
+ let shift = \p. p (\a b. pair (succ a) a)
+ let pred = \n. n shift (pair zero zero) snd in
+
+Note that `shift` applies its argument p ("p" for "pair") to a
+function that ignores its second argument---why does it do that? In
+order to understand what this code is doing, it is helpful to go
+through a sample computation, the predecessor of 3:
+
+ pred (\s z.s(s(s z)))
+ (\s z.s(s(s z))) (\n.n shift (\f.f 0 0) snd)
+ shift (shift (shift (\f.f 0 0))) snd
+ shift (shift ((\f.f 0 0) (\a b.pair(succ a) a))) snd
+ shift (shift (\f.f 1 0)) snd
+ shift (\f. f 2 1) snd
+ (\f. f 3 2) snd
+ 2
+
+At each stage, `shift` sees an ordered pair that contains two numbers
+related by the successor function. It can safely discard the second
+element without losing any information. The reason we carry around
+the second element at all is that when it comes time to complete the
+computation---that is, when we finally apply the top-level ordered
+pair to `snd`---it's the second element of the pair that will serve as
+the final result.
Let's see how far we can get typing these terms. `zero` is the Church
encoding of zero. Using `N` as the type for Church numbers (i.e.,
-<code>N ≡ (σ -> σ) -> σ -> σ</code> for some
-σ, `zero` has type `N`. `snd` takes two numbers, and returns
-the second, so `snd` has type `N -> N -> N`. Then the type of `pair`
-is `N -> N -> (type(snd)) -> N`, that is, `N -> N -> (N -> N -> N) ->
-N`. Likewise, `succ` has type `N -> N`, and `collect` has type `pair
--> pair`, where `pair` is the type of an ordered pair of numbers,
-namely, <code>pair ≡ (N -> N -> N) -> N</code>. So far so good.
+<code>N ≡ (σ -> σ) -> σ -> σ</code> for
+some σ, `zero` has type `N`. `snd` takes two numbers, and
+returns the second, so `snd` has type `N -> N -> N`. Then the type of
+`pair` is `N -> N -> (type(snd)) -> N`, that is, `N -> N -> (N -> N ->
+N) -> N`. Likewise, `succ` has type `N -> N`, and `shift` has type
+`pair -> pair`, where `pair` is the type of an ordered pair of
+numbers, namely, <code>pair ≡ (N -> N -> N) -> N</code>. So far
+so good.
The problem is the way in which `pred` puts these parts together. In
particular, `pred` applies its argument, the number `n`, to the
-`collect` function. Since `n` is a number, its type is <code>(σ
+`shift` function. Since `n` is a number, its type is <code>(σ
-> σ) -> σ -> σ</code>. This means that the type of
-`collect` has to match <code>σ -> σ</code>. But we
-concluded above that the type of `collect` also had to be `pair ->
+`shift` has to match <code>σ -> σ</code>. But we
+concluded above that the type of `shift` also had to be `pair ->
pair`. Putting these constraints together, it appears that
<code>σ</code> must be the type of a pair of numbers. But we
already decided that the type of a pair of numbers is `(N -> N -> N)
The way we got here is that the `pred` function relies on the built-in
right-fold structure of the Church numbers to recursively walk down
the spine of its argument. In order to do that, the argument had to
-apply to the `collect` operation. And since `collect` had to be the
+apply to the `shift` operation. And since `shift` had to be the
sort of operation that manipulates numbers, the infinite regress is
established.
computing the tail of a list is likewise beyond the reach of the
simply-typed lambda calculus.
-This result is surprising. It illustrates how recursion is built into
-the structure of the Church numbers (and lists). Most importantly for
-the discussion of the simply-typed lambda calculus, it demonstrates
-that even fairly basic recursive computations are beyond the reach of
-a simply-typed system.
+This result is not obvious, to say the least. It illustrates how
+recursion is built into the structure of the Church numbers (and
+lists). Most importantly for the discussion of the simply-typed
+lambda calculus, it demonstrates that even fairly basic recursive
+computations are beyond the reach of a simply-typed system.
## Montague grammar is based on a simply-typed lambda calculus