Formalisation in Constructive Type Theory of Stoughton's Substitution for the Lambda Calculus

In [Stoughton, A., Substitution revisited, Theor. Comput. Sci. 59 (1988), pp. 317–325], Alley Stoughton proposed a notion of (simultaneous) substitution for the Lambda calculus as formulated in its original syntax – i.e. with only one sort of symbols (names) for variables – and without identifying α-convertible terms. According to such formulation, the action of substitution on terms is defined by simple structural recursion and an interesting theory arises concerning the connection to α-conversion. In this paper we present a formalisation of Stoughton's work in Constructive Type Theory using the language Agda, which reaches up to the Substitution Lemma for α-conversion. The development has been quite inexpensive e.g. in labour cost, and we are able to formulate some improvements over the original presentation. For instance, our definition of α-conversion is just syntax directed and we prove it to be an equivalence relation in an easy way, whereas in [Stoughton, A., Substitution revisited, Theor. Comput. Sci. 59 (1988), pp. 317–325] the latter was included as part of the definition and then proven to be equivalent to an only nearly structural definition as corollary of a lengthier development. As a result of this work we are inclined to assert that Stoughton's is the right way to formulate the Lambda calculus in its original, conventional syntax and that it is a formulation amenable to fully formal treatment.