## The simply-typed lambda calculus

The untyped lambda calculus is pure. Pure in many ways: nothing but variables and lambdas, with no constants or other special symbols; also, all functions without any types. As we'll see eventually, pure also in the sense of having no side effects, no mutation, just pure computation.

But we live in an impure world. It is much more common for practical programming languages to be typed, either implicitly or explicitly. Likewise, systems used to investigate philosophical or linguistic issues are almost always typed. Types will help us reason about our computations. They will also facilitate a connection between logic and computation.

From a linguistic perspective, types are generalizations of (parts of) programs. To make this comment more concrete: types are to (e.g., lambda) terms as syntactic categories are to expressions of natural language. If so, if it makes sense to gather a class of expressions together into a set of Nouns, or Verbs, it may also make sense to gather classes of terms into a set labelled with some computational type.

To develop this analogy just a bit further, syntactic categories determine which expressions can combine with which other expressions. If a word is a member of the category of prepositions, it had better not try to combine (merge) with an expression in the category of, say, an auxilliary verb, since *under has is not a well-formed constituent in English. Likewise, types in formal languages will determine which expressions can be sensibly combined.

Now, of course it is common linguistic practice to supply an analysis of natural language both with syntactic categories and with semantic types. And there is a large degree of overlap between these type systems. However, there are mismatches in both directions: there are syntactic distinctions that do not correspond to any salient semantic difference (why can't adjectives behave syntactically like verb phrases, since they both denote properties with (extensional) type <e,t>?); and in some analyses there are semantic differences that do not correspond to any salient syntactic distinctions (as in any analysis that involves silent type-shifters, such as Herman Hendriks' theory of quantifier scope, in which expressions change their semantic type without any effect on the expressions they can combine with syntactically). We will consider again the relationship between syntactic types and semantic types later in the course.

Soon we will consider polymorphic type systems. First, however, we will consider the simply-typed lambda calculus.

[Pedantic on. Why "simply typed"? Well, the type system is particularly simple. As mentioned to us by Koji Mineshima, Church tells us that "The simple theory of types was suggested as a modification of Russell's ramified theory of types by Leon Chwistek in 1921 and 1922 and by F. P. Ramsey in 1926." This footnote appears in Church's 1940 paper A formulation of the simple theory of types. In this paper, Church writes types by simple apposition, without the ugly angle brackets and commas used by Montague. Furthermore, he omits parentheses under the convention that types associated to the left---the opposite of the modern convention. This is ok, however, because he also reverses the order, so that te is a function from objects of type e to objects of type t. Cool paper! If you ever want to see Church numerals in their native setting--but we're getting ahead of our story. Pedantic off.]

There's good news and bad news: the good news is that the simply-typed lambda calculus is strongly normalizing: every term has a normal form. We shall see that self-application is outlawed, so Ω can't even be written, let alone undergo reduction. The bad news is that fixed-point combinators are also forbidden, so recursion is neither simple nor direct.

# Types

We will have at least one ground type. For the sake of linguistic familiarity, we'll use e, the type of individuals, and t, the type of truth values.

In addition, there will be a recursively-defined class of complex types T, the smallest set such that

• ground types, including e and t, are in T

• for any types σ and τ in T, the type σ -> τ is in T.

For instance, here are some types in T:

 e
e -> t
e -> e -> t
(e -> t) -> t
(e -> t) -> e -> t


and so on.

# Typed lambda terms

Given a set of types T, we define the set of typed lambda terms Λ_T, which is the smallest set such that

• each type t has an infinite set of distinct variables, xt1, xt2, xt3, ...

• If a term M has type σ -> τ, and a term N has type σ, then the application (M N) has type τ.

• If a variable a has type σ, and term M has type τ, then the abstract λ a M has type σ -> τ.

The definitions of types and of typed terms should be highly familiar to semanticists, except that instead of writing σ -> τ, linguists write <σ, τ>. We will use the arrow notation, since it is more iconic.

Some examples (assume that x has type o):

  x            o
\x.x         o -> o
((\x.x) x)   o


Excercise: write down terms that have the following types:

               o -> o -> o
(o -> o) -> o -> o
(o -> o -> o) -> o


# A first glipse of the connection between types and logic

In the simply-typed lambda calculus, we write types like σ -> τ. This looks like logical implication. We'll take that resemblance seriously when we discuss the Curry-Howard correspondence. In the meantime, note that types respect modus ponens:

Expression    Type      Implication
-----------------------------------
fn            α -> β    α ⊃ β
arg           α         α
------        ------    --------
(fn arg)      β         β


The implication in the right-hand column is modus ponens, of course.

# Associativity of types versus terms

As we have seen many times, in the lambda calculus, function application is left associative, so that f x y z == (((f x) y) z). Types, THEREFORE, are right associative: if x, y, and z have types a, b, and c, respectively, then f has type a -> b -> c -> d == (a -> (b -> (c -> d))), where d is the type of the complete term.

It is a serious faux pas to associate to the left for types. You may as well use your salad fork to stir your tea.

# The simply-typed lambda calculus is strongly normalizing

If M is a term with type τ in Λ_T, then M has a normal form. The proof is not particularly complex, but we will not present it here; see Berendregt or Hankin.

Since Ω does not have a normal form, it follows that Ω cannot have a type in Λ_T. We can easily see why:

Ω = (\x.xx)(\x.xx)

Assume Ω has type τ, and \x.xx has type σ. Then because \x.xx takes an argument of type σ and returns something of type τ, \x.xx must also have type σ -> τ. By repeating this reasoning, \x.xx must also have type (σ -> τ) -> τ; and so on. Since variables have finite types, there is no way to choose a type for the variable x that can satisfy all of the requirements imposed on it.

In fact, we can't even type the parts of Ω, that is, &omega; \equiv \x.xx. In general, there is no way for a function to have a type that can take itself for an argument.

It follows that there is no way to define the identity function in such a way that it can take itself as an argument. Instead, there must be many different identity functions, one for each type. Some of those types can be functions, and some of those functions can be (type-restricted) identity functions; but a simply-types identity function can never apply to itself.

# Typing numerals

The Church numerals are well behaved with respect to types.
To see this, consider the first three Church numerals (starting with zero):

\s z . z
\s z . s z
\s z . s (s z)


Given the internal structure of the term we are using to represent zero, its type must have the form ρ -> σ -> σ for some ρ and σ. This type is consistent with term for one, but the structure of the definition of one is more restrictive: because the first argument (s) must apply to the second argument (z), the type of the first argument must describe a function from expressions of type σ to some result type. So we can refine ρ by replacing it with the more specific type σ -> τ. At this point, the overall type is (σ -> τ) -> σ -> σ. Note that this refined type remains compatible with the definition of zero. Finally, by examinining the definition of two, we see that expressions of type τ must be suitable to serve as arguments to functions of type σ -> τ, since the result of applying s to z serves as the argument of s. The most general way for that to be true is if τ ≡ σ. So at this point, we have the overall type of (σ -> σ) -> σ -> σ.

## Predecessor and lists are not representable in simply typed lambda-calculus

This is not because there is any difficulty typing what the functions involved do "from the outside": for instance, the predecessor function is a function from numbers to numbers, or τ -> τ, where τ is our type for Church numbers (i.e., (σ -> σ) -> σ -> σ). (Though this type will only be correct if we decide that the predecessor of zero should be a number, perhaps zero.)

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 function, based on the discussion in Pierce 2002:547:

let zero = \s z. z in
let fst = \x y. x in
let snd = \x y. y in
let pair = \x y . \f . f x y in
let succ = \n s z. s (n s z) in
let shift = \p. pair (succ (p fst)) (p fst) in
let pred = \n. n shift (pair zero zero) snd in


Note that shift takes a pair p as argument, but makes use of only the first element of the pair. 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 3
3 shift (pair zero zero) snd
(\s z.s(s(s z))) shift (pair zero zero) snd
shift (shift (shift (\f.f 0 0))) snd
shift (shift (pair (succ ((\f.f 0 0) fst)) ((\f.f 0 0) fst))) snd
shift (shift (\f.f 1 0)) snd
shift (\f. f 2 1) snd
(\f. f 3 2) snd
snd 3 2
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., N ≡ (σ -> σ) -> σ -> σ 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, pair ≡ (N -> N -> N) -> N. 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 shift function. Since n is a number, its type is (σ -> σ) -> σ -> σ. This means that the type of shift has to match σ -> σ. But we concluded above that the type of shift also had to be pair -> pair. Putting these constraints together, it appears that σ 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) -> N. Here's the difficulty: N is shorthand for a type involving σ. If σ turns out to depend on N, and N depends in turn on σ, then σ is a proper subtype of itself, which is not allowed in the simply-typed lambda calculus.

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 shift operation. And since shift had to be the sort of operation that manipulates numbers, the infinite regress is established.

Now, of course, this is only one of myriad possible implementations of the predecessor function in the lambda calculus. Could one of them possibly be simply-typeable? It turns out that this can't be done. See Oleg Kiselyov's discussion and works cited there for details: predecessor and lists can't be represented in the simply-typed lambda calculus.

Because lists are (in effect) a generalization of the Church numbers, computing the tail of a list is likewise beyond the reach of the simply-typed lambda calculus.

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

Systems based on the simply-typed lambda calculus are the bread and butter of current linguistic semantic analysis. One of the most influential modern semantic formalisms---Montague's PTQ fragment---included a simply-typed version of the Predicate Calculus with lambda abstraction.

Montague called the semantic part of his PTQ fragment Intensional Logic. Without getting too fussy about details, we'll present the popular Ty2 version of the PTQ types, roughly as proposed by Gallin (1975). [See Zimmermann, Ede. 1989. Intensional logic and two-sorted type theory. Journal of Symbolic Logic 54.1: 65--77 for a precise characterization of the correspondence between IL and two-sorted Ty2.]

We'll need three base types: e, for individuals, t, for truth values, and s for evaluation indicies (world-time pairs). The set of types is defined recursively:

the base types e, t, and s are types
if a and b are types, <a,b> is a type


So <e,<e,t>> and <s,<<s,e>,t>> are types. As we have mentioned, Montague's paper is the source for the convention in linguistics that a type of the form <a, b> corresponds to a functional type that we will write here as a -> b. So the type <a, b> is the type of a function that maps objects of type a onto objects of type b.

Montague gave rules for the types of various logical formulas. Of particular interest here, he gave the following typing rules for functional application and for lambda abstracts, which match the rules for the simply-typed lambda calculus exactly:

• If α is an expression of type <a, b>, and β is an expression of type b, then α(β) has type b.

• If α is an expression of type a, and u is a variable of type b, then λuα has type <b, a>.

When we talk about monads, we will consider Montague's treatment of intensionality in some detail. In the meantime, Montague's PTQ is responsible for making the simply-typed lambda calculus the baseline semantic analysis for linguistics.

## Wordsworth on types

Wordsworth wrote the following sonnet about the constraints of the sonnet form in 1806, but he could have been writing about strictly-typed programming languages.

### Nuns Fret Not at Their Convent's Narrow Room

Nuns fret not at their convent's narrow room And hermits are contented with their cells; And students with their pensive citadels; Maids at the wheel, the weaver at his loom, Sit blithe and happy; bees that soar for bloom, High as the highest Peak of Furness-fells, Will murmur by the hour in foxglove bells: In truth the prison, into which we doom Ourselves, no prison is: and hence for me, In sundry moods, 'twas pastime to be bound Within the Sonnet's scanty plot of ground; Pleased if some Souls (for such there needs must be) Who have felt the weight of too much liberty, Should find brief solace there, as I have found.

This sonnet has an octave (the first eight lines, with rhyme scheme abbaabba) followed by a sestet (cddccd). But the words apply to types as well as to sonnets. "The prison into which we doom / Ourselves, no prison is": as long as we get to choose the types of our expressions, we can accomplish whatever we need to. Let anyone who wants to code in the free 'verse of the untyped lambda calculus---but those of us "who have felt the weight of too much liberty" will find solace in a strictly typed language.