## 3 - LF and Judgments-as-Types

< 2 - A First Theory

LF is a logical framework based on the dependently typed lambda calculus with two universes, type and kind. If you know what this means, you can safely skip this section.

A thorough description of both LF’s abstract formalization as well as how to use it in practice can be found in Part V here.

### LF’s lambda calculus

As seen in the previous section, LF declares a symbol type, which is a universe. LF has a second universe kind. Their behaviour is governed by the following rules:

• kind is a universe. That means: kind is inhabitable (may occur as a type of constants) and every symbol c with type kind is also inhabitable.
• type is a universe as well.
• type:kind, i.e. type has type kind.

We can mostly ignore kind, since it rarely comes into play. For most purposes, we are only interested in types.

LF has a single type constructor, that allows us to construct new types from existing ones, namely the dependent product type $\inline \small \prod$. Its behaviour is governed by the following formation rule:

If A:type and B(x) is inhabitable for any x:A, then $\inline \small \prod_{x:A}B(x)$ has type type

$\inline \small \prod_{x:A}B(x)$ is the type of functions, that take an element a:A and return an element b:B(a). Note, that the return type of the function may depend on the argument. If B does not depend on the argument, we get the simple function type we already used:

If B does not depend on x, then $\inline \small A\to B := \prod_{x:A}B$.

Next, we need a way to construct elements of these function types. This is done using the λ-operator, which is governed by the following introduction rule:

If A:type and t(x):B(x) for any x:A, then $\inline \small \lambda\; x:A.\; t(x)$ has type $\inline \small \prod_{x:A}B(x)$.

So λ allows us to construct functions out of terms. Of course, functions are useless if we can’t apply them. The behaviour of function applications is governed by this elimination rule:

If f has type $\inline \small \prod_{x:A}B(x)$ and a:A, then f a has type B(a) (where B(a) is the result of substituting x by a in B(x))

Additional computation rules make sure, that e.g. λ x. (f x) is the same as λ y. (f y), which is the same as f, etc., but those rules are intuitive enough to not require much explanation.

### Judgments-as-Types

LF is a logical framework, i.e., it is intended to allow for conveniently specifying the syntax and proof theories of various logics. This might be surprising, given that LF only provides dependent function types. This is where the judgments-as-types-paradigm comes into play. It is based on the observation, that there is a natural correspondence between the judgments of a logic (e.g. “φ holds”, or in symbols : ⊦ φ) and some associated type A being inhabited (i.e. there is some a with a:A):

• We introduce a type o for the expressions of our logic,
• we associate every judgment J with a type ⊦J,
• we interpret the type TJ as the type of proofs that the judgment J holds,
• correspondingly we can “prove” the judgment J by constructing an element of the type ⊦J.
• The syntax of our logic thus reduces to functions, that construct new propositions from atomic propositions - i.e. new elements of the type o from declared elements of o,
• The proof theory of our logic corresponds to constructing elements of the judgment types ⊦J (for some judgment J).

The last point is worth looking at more closely: Usually, proof rules are denoted in the form of:

$\dfrac{\vdash \phi_1, ..., \vdash\phi_n}{\vdash\phi}$

, which translates to:

If (judgment φ_1 is provable andand judgment φ_n is provable), then judgment φ is provable.

, where a context is a list of assumptions. Since the judgments correspond to types, such a rule translates to a function taking proofs of the premises as arguments and returning a proof of the conclusion, i.e.

$\inline \prod_{p_1:\vdash\phi_1}...\prod_{p_n:\vdash\phi_n}\vdash\phi$

In the case of propositional logic, we have a single judgment for every proposition φ:prop, namely the judgment ⊦ φ that φ holds. Correspondingly, we declared the function proof, that maps a proposition φ to the associated type of proofs ⊦ φ. The syntax we have already taken care of, by declaring the connectives as functions on the type prop.

In the next section we will use dependend function types to implement the rules of the natural deduction calculus as functions, that yield elements of the proof types ⊦ φ.

> 4 - Natural Deduction for Intuitionistic Propositional Logic