When are two expressions equal? Two possible answers are:

  • When they are syntatically identical.

  • When they are semantically equivalent: they produce the same value.

For example, are 42 and 41+1 equal? The syntactic answer would say they are not, because they involve different tokens. The semantic answer would say they are: they both produce the value 42.

What about functions: are fun x -> x and fun y -> y equal? Syntactically they are different. But semantically, they both produce a value that is the identity function: when they are applied to an input, they will both produce the same output. That is, (fun x -> x) z = z, and (fun y -> y) z = z. If it is the case that for all inputs two functions produce the same output, we will consider the functions to be equal:

if (forall x, f x = g x), then f = g.

That definition of equality for functions is known as the Axiom of Extensionality in some branches of mathematics; henceforth we'll refer to it simply as "extensionality".

Here we will adopt the semantic approach. If e1 and e2 evaluate to the same value v, then we write e1 = e2. We are using = here in a mathematical sense of equality, not as the OCaml polymorphic equality operator. For example, we allow (fun x -> x) = (fun y -> y), even though OCaml's operator would raise an exception and refuse to compare functions.

We're also going to restrict ourselves to expressions that are well typed, pure (meaning they have no side effects), and total (meaning they don't have exceptions or infinite loops).

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