My previous take on higher-order logic was classical, channeling Church's simple theory of types and Andrews $Q_0$. In that last post, I was hinting at motivation. So I want to expand a bit now how this stuff can be useful for building software at large. I am posting some ramblings here.

# Equality is Fundamental

There are two different "levels" at which a programmer deals with some kind of reasoning that involves equality:

- while writing code: need to be able to tell when two objects are the same.
- when designing or arguing that code is correct: reasoning whether behavior like output values conform to a (however implicit) specification

In both cases, one needs to be able to identify whether objects (or behaviors or input-output pairs) are "the same" as expected ones. But the latter activity, reasoning about program correctness, happens in some form of logic. Much that has been written and is being written on formal logic is about mathematics and mathematical convention. This is true whether we talk about semantics of programming languages or program specification.

But for programmers, there are benefits to knowing formal reasoning even if one is not interested in the foundations of math, axiomatizations of set theory etc. It is no secret that in mathematics as in programming, you run into situations where there are multiple different ways to represent some data, and one needs to chose or translate between those representations. While programmers resign to writing "proto-to-proto" translation over and over (which often are not as trivial as it seems), in mathematics it is customary to leave out the awkward details, introduce "abuse of notiation" and quotienting to keep arguments informal, but concise.

The heart and soul of much mathematics consists of the fact that the “same” object can be presented to us in different ways. [1]

[1] Barry Mazur. When is one thing equal to some other thing?

# Equality in First Order Logic

We take a leap towards "formal systems", where objects are represented using (strings of) symbols, along with strict rules to form statements and derive new true statements from existing ones.

A typical definition of a first-order logic language contains constant and function symbols, predicate symbols, logical operators.

There is a "design choice" here whether equality is considered as
*built-in*, magically denoting exactly the equality in the underlying
sets, or whether equality is a predicate with certain special properties.
Wikipedia"
talks about "first-order logic with equality" (when built-in) and first-order
logic without equality (when it is not).

When equality does not come built-in, one needs to add axioms. The equality axioms are special, in the sense that they have to be preserved by all the function and relation symbols. This looks like:

- $\mathsf{eq}$ is reflexitve, symmetric, transitive
- if $\mathsf{eq}(x_1, y_1), \ldots, \mathsf{eq}(x_n, y_n)$, then $\mathsf{eq}(f(x_1,\ldots,x_n),f(y_1,\ldots,y_n)$ for all function symbols $f$ of arity $n$
- if $\mathsf{eq}(x_1, y_1), \ldots, \mathsf{eq}(x_n, y_n)$, then $R(x_1,\ldots,x_n) \Leftrightarrow R(y_1,\ldots,y_n)$ for all relation symbols $R$ of arity $n$

With a "defined" equality, is it is well possible to interpret logical statements involving $\mathsf{eq}$ in such a way that two distinct elements are regarded as equal according to $\mathsf{eq}$.

# Coding our own equality

When we are talking about practical software development, there is really no hope of defining a platonic ideals; yet when techniques of reasoning, informaticians are in various ways confined to results and methods that formalizing mathematics.

Take this example: We'd like our terms to represent free monoid on $X$, so we'd like these terms to satisfy the following equations (or identities): $$s \cdot 1 = s$$ $$t = 1 \cdot t$$ $$(s \cdot t) \cdot u = s \cdot (t \cdot u)$$

Consider definitions in some fictional language to express the "interface" of a monoid: $$ \begin{array}{ll} \mathsf{interface}~ M[X] \\ \quad \mathsf{unit}(): M[X]\\ \quad \mathsf{emb}(x: X): M[X]\\ \quad \mathsf{dot}(s: M[X], t: M[X]): M[X]\\ \quad \mathsf{eq}(s: M[X], t: M[X]): \mathsf{bool} \end{array}$$

Here, $\mathsf{unit}$ stands for $1$, $\mathsf{emb}(x)$ is for
embedding $x$ and $\mathsf{dot}$ is the combine operaion in $MX$.
If our fictional language has algebraic data types, we can use those to
build *terms* which represent the algebraic theory of monoids:
$$ \begin{array}{ll}
\mathsf{data}~ \mathsf{Mon}[X] \\
\quad \mathsf{Unit}(): \mathsf{Mon}[X]\\
\quad \mathsf{Emb}(x: X): \mathsf{Mon}[X]\\
\quad \mathsf{Dot}(s: \mathsf{Mon}[X], t: \mathsf{Mon}[X]): \mathsf{Mon}[X]\\
\quad \mathsf{eqMon}(s: \mathsf{Mon}[X], t: \mathsf{Mon}[X]): \mathsf{bool}
\end{array}$$

It is now really obvious what these constructors stand for, but we note that while the equality $\mathsf{eqMon}$ returns true whenever two terms are equal when considered as elements of the free monoid, there is at least one more equality relation on terms that is different ("structural equality" or "syntactic equality", which will consider $\mathsf{dot}(\mathsf{dot}(s, t), u)$ as distinct from $\mathsf{dot}(s, \mathsf{dot}(t, u))$).

Mathematically speaking, we want to consider the quotient set $\mathsf{Mon}[X] / \mathsf{eqMon}$, and work with equivalence classes.

# Formulas = Functions to Truth Values

After all this talk about equality in algebraic theories, we want to turn equality in the context of logic, specifically higher-order logic.

In classical logic, a proposition (logical statement) stands for a truth value, and there are exactly two of then (the two-value type "bool"). Boole was the first to associate propositional connectives with algebraic operations. In this perspective, the the truth values are part of the language, even though this was later abandoned as in Tarski-style semantics.

Programmers are quite used to having functions return $\mathsf{bool}$. In terms of mathematical logic, our custom-made equality "relation" $\mathsf{eqMon}$ above is in fact a function to the set of truth values. Could we not do away with relations and talk in terms of functions?

Yes, we can; using truth values and functions in a formal language of propositions is
the essential summary of classical higher order logic (or Church's simple theory of types.
There is a more compelling example: if you consider "set builder notation"
$$ \{ x \in A \ |\ \phi(x) \} $$
then this describes a subset of $A$. We can choose to describe $\phi$ as defining
a (unary) relation, but viewing it as a function to $\mathsf{bool}$ is no doubt
more natural in a context like `SELECT x FROM A WHERE `

$\phi$
where "relations" are given by their extensions (as tables) and computation in the form of expressions is available.

The point of (classical) higher-order logic is that using the $\lambda$ calculus and truth values, we have everything we need to define the meaning of all logical connectives, and the types prevent logical paradoxes of "naively-untyped set theory". Since the truth values are part of the language, it is possible to define the meaning purely through definitions that involve equality.

More on this and classical higher order logic in that last post on Higher Order Logic and Equality and the next one I'm going to write in a second, where I talk about the intuitionistic version.

I will just end with a note that there is no distinction between "predicate", "relation" and "set" is higher-order logic, and this is a good thing. An expression of type $\iota \rightarrow \omicron$ (where $\iota$ are the individuals and $\omicron$ the truth values) defines a (sub)set of individuals, a predicate on the set of individuals, a unary relation, all at the same time. $$\mathsf{even} := \lambda x:\iota. \Sigma \lambda y:\iota. \mathsf{eq}(x, \mathsf{times}(2, y))$$ $$\mathsf{greater} := \lambda x:\iota.\lambda y:\iota. \Sigma \lambda z:\iota. \mathsf{eq}(x, \mathsf{plus}(y,z))$$ $$\mathsf{lowerbound} := \lambda x:\iota.\lambda y:\iota \rightarrow \omicron. \Pi \lambda z:\iota. \mathsf{implies}(y(z), \mathsf{greater}(z, x))$$

The confusing bit for programmers may be that this thing is called simple type theory when in fact there is only one "sort" of individuals. This makes more sense when considering that it is no problem to come up with multiple "sorts" of individuals but that wouldn't give sets ($\iota \rightarrow \omicron$), sets of sets $(\iota \rightarrow \omicron) \rightarrow \omicron$, sets of sets of sets (and so on) of individuals. It's this distinction of hierarchies of sets/predicates/relations that earn Church's higher order logic the name "simple type theory": only terms with the right types are considered well-formed and meaningful.

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