# Mechanized metatheory for a \(\lambda \)-calculus with trust types

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## Abstract

As computer programs become increasingly complex, techniques for ensuring trustworthiness of information manipulated by them become critical. In this work, we use the Coq proof assistant to formalise a \(\lambda \)-calculus with trust types, originally formulated by Ørbæk and Palsberg. We give formal proofs of type soundness, erasure and simulation theorems and also prove decidability of the typing problem. As a result of our formalisation a certified type checker is derived.

## Keywords

Trust Type systems Proof assistants Soundness proofs## 1 Introduction

Ensuring security of information manipulated by computer systems is a long-standing and increasingly important problem. There is little assurance that current computer systems keep data integrity and traditional (theoretical and practical) approaches to express and enforce security properties are, in general, unsatisfactory [46, 53].

One of such traditional approaches to protect data confidentiality is access control: privileges are required to access files or objects containing confidential data and information release is restricted according to some policy. Access control checks can restrict release but not propagation of information. Once information is released, a program can transmit it in some form and, since it is not feasible to suppose that all programs in a system are trustworthy, one cannot ensure that confidentiality is maintained. In order to guarantee that information is used only in accordance with relevant policies, it is necessary to analyse how information flows within the program. As modern computing systems are complex artefacts, automating such analysis is required [46].

An approach to ensure security property of computer software consists of the use of type systems in order to control information flow in software [46]. In programming languages with *security types*, variables and expressions types have annotations that indicate policies to be ensured by the compiler on uses of such data. This approach has the following benefits: (1) since these policies are checked at compile-time, there is no run-time overhead; (2) once security policies are expressed by a type system, standard techniques for guaranteeing type system soundness can be used to certify that security policies are enforced in an end-to-end way in the whole program.

However, proofs of programming language formalisms (e.g. type systems and semantics) are usually long and error prone. In order to give more reliability to these proofs, programming language researchers have been developing, in recent years, a large number of works devoted to machine assisted proofs [1, 6, 10, 28].

- 1.
type soundness, using standard small-step call-by-value semantics. Intuitively, type soundness property ensures that if a program is well-typed it does not cause any run-time errors,

- 2.
erasure and simulation theorems [38, Sections 3.3 and 3.4]. Erasure and simulation theorems ensure that the \(\lambda \)-calculus with trust types is a restriction of the simply typed \(\lambda \)-calculus. Together these theorems ensure that after type-checking a term, we can simply erase all trust constructs and annotations and evaluate it using the rules of the simply typed \(\lambda \)-calculus,

- 3.
decidability of type checking. From this proof we extract a certified type checker for the language.

The rest of this paper is organised as follows. Section 2 presents a brief introduction to the Coq proof assistant and its features used in our formalisation. Section 3 briefly reviews the syntax and defines a small-step semantics for the \(\lambda \)-calculus with trust types. Section 4 presents the non-syntax directed type system for the \(\lambda \)-calculus with trust, as proposed in [38], and proves its type soundness property. We also define a syntax directed version of this original type system and prove soundness and completeness between these two versions. Finally, we prove that the typing problem for this calculus is decidable. Section 6 presents related work and Sect. 7 concludes.

## 2 A taste of Coq proof assistant

Coq is a proof assistant based on the calculus of inductive constructions (CIC) [5], a higher order typed \(\lambda \)-calculus extended with inductive definitions. Theorem proving in Coq follows the ideas of the so-called BHK-correspondence,^{1} where types represent logical formulas and \(\lambda \)-terms represent proofs [49]. Thus, the task of checking if a piece of text is a proof of a given formula corresponds to checking if the term that represents the proof has the type corresponding to the given formula.

However, writing a proof term whose type is that of a logical formula can be a hard task, even for very simple propositions. In order to make the writing of complex proofs easier, Coq provides *tactics*, which are commands that can be used to construct proof terms in a more user friendly way.

The source code in Fig. 1 shows some basic features of the Coq proof assistant: types, functions and proof definitions. In this example, a new inductive type is defined to represent natural numbers in Peano notation. This type is formed by two data constructors: O, that represents the number \(0\); and S, the successor function. For instance, in this notation the number \(2\) is represented by the term S (S O) of type nat.

The command Fixpoint allows the definition of structural recursive functions. Function plus defines the sum of two unary natural numbers, in a straightforward way. It is noteworthy that, in order to maintain logical consistency, all functions in Coq must be total.

Besides the declaration of inductive types and functions, we can define and prove theorems in Coq. Figure 1 shows an example of a simple theorem about function plus, namely that, for an arbitrary value n of type nat, we have that plus n 0 = n. The command Theorem allows us to state some formula that we want to prove and it starts the *interactive proof mode*, in which tactics can be used to produce the wanted proof term. In an interactive section of Coq (after enunciation of theorem plus_O_r), we must prove the following goal:

After command Proof., one can use tactics to build, step by step, a term of the given type. The first tactic, intros, is used to move premisses and universally quantified variables from the goal to the hypothesis. Now, we need to prove:

The quantified variable n has been moved from the goal to the hypothesis. Now, we can proceed by induction over the structure of n. This can be achieved by using tactic induction, that generates one goal for each constructor of type nat. This will leave us with the following two goals to be proved:

The goal plus 0 0 = 0 holds trivaly by the definition of plus. Tactic reflexivity proves trivial equalities, after reducing both sides of the equality to their normal forms. The next goal to be proved is:

The hypothesis IHn’ is the automatically generated induction hypothesis for this theorem. In order to finish this proof, we need to transform the goal to use the inductive hypothesis. To do this, we use the tactic simpl, which performs reductions based on the definition of function plus. This changes the goal to:

Since the goal now has as a subterm the exact left hand side of the hypothesis IHn’, we can use the rewrite tactic, which replaces some term by another using some equality in the hypothesis. Now, we have the following goal:

This can be proved immediately using the reflexivity tactic. This tactic script builds the following term:

Instead of using tactics, one could instead write CIC terms directly to prove theorems. This is, however, a complex task, even for simple theorems like plus_O_r, since the manual writing of proof terms requires knowledge of the CIC type system. Thus, tactics frees us from the details of constructing type correct CIC terms.

An interesting feature of Coq is the possibility of defining inductive types that mix computational and logic parts. This allows us to define functions that compute values together with a proof that this value has some desired property. The type sig, also called “subset type”, is defined in the Coq’s standard library as:

The exist constructor takes two arguments: the value x of type A—that represents the computational part—and an argument of type P x—the “certificate” that the value x has the property specified by the predicate P. As an example of a sig type, consider:

This type represents a function that returns the predecessor of a natural number n, together with a proof that the returned value p really is the predecessor of n. Defining functions using the sig type requires writing the corresponding logical certificate. As with theorems, we can use tactics to define such functions.

Using the command Extraction pred_certified we can discard the logical part of this function definition and get a certified implementation of this function in OCaml [50], Haskell [23] or Scheme [16]. The OCaml code of this function, obtained through extraction, is the following:

## 3 \(\lambda \)-Calculus with trust types

This section reviews some motivations for the use of trust types and gives definitions of the syntax and semantics of the trust \(\lambda \)-calculus, which differ from the original definitions in [38] as follows: (1) we use a small-step call-by-value semantics, and (2) without loss of generality, we consider only one base type: bool. Extensions to include other type constructors are straightforward.

### 3.1 Motivations

Data manipulated by computer programs can be classified as *trusted* or *untrusted*. Trusted data come from trusted sources, like company databases, program constants, cryptographically verified network data. All other data are considered untrusted [38].

Trust analysis is especially important in web applications, where user input data can be used to exploit security vulnerabilities, using attacks such as cross-site scripting (XSS). XSS attacks can occur when a user is able to “dump” HTML text in a dynamically generated page [44]. Through this vulnerability, it is possible to inject JavaScript code to steal cookies, in order to acquire session privileges. Such a threat occurs due to a lack of verification on input data since, ideally, HTML code cannot be considered as valid input.

In order to avoid such invalid inputs, one can insert checks that ensure data trustworthiness. But, how can we guarantee that all paths, in which probably untrusted information flows, pass all required checks? The solution proposed by Ørbæk and Palsberg [38] is to use a type system to track the flow of untrusted data in a program.

The language considered is a \(\lambda \)-calculus with additional constructs to check if some piece of data can be trusted and mark data as trusted or untrusted. If \(e\) is some program expression, then *trust e* indicates that the result of \(e\) can be trusted. Dually, *distrust e* indicates that the result of \(e\) cannot be trusted and *check e* indicates that \(e\) must be trustworthy. Well-typed programs do not have any sub-expression *check e* where \(e\) has an untrusted type.

### 3.2 Syntax of types and terms

The syntax of types and terms used in our formalisation is identical to [38], except that we require type annotations in every \(\lambda \)-abstraction. We restrict ourselves to type annotated \(\lambda \)-terms, since our main interest is the development of a correct type checker for this language. Allowing non-annotated \(\lambda \)-abstractions characterises a type inference problem that would require a formalisation of a unification algorithm. The formalisation of a unification algorithm has been studied elsewhere [26, 32]. We let a formalisation of the type inference problem for this trust-calculus for future work.

The id type, used in the definition of term, represents a generic identifier with a decidable function for testing equality and its simple definition is omitted to avoid unnecessary distraction.

### 3.3 Small-step operational semantics

In order to prove type soundness, we follow the standard approach of using a small-step operational semantics for proving progress and preservation theorems [39]. This differs from the approach adopted in [38], where the semantics of the trust \(\lambda \)-calculus is formalised using a reduction semantics, with no predefined order of evaluation, and the Church–Rosser property and a subject reduction theorem are proved. The Church–Rosser property ensures that computation in the trust \(\lambda \)-calculus is deterministic and subject reduction guarantees that reduction preserves typing [2, 20].

Let us first define the notion of *value*, i.e. a term that cannot be further reduced according to the intended semantics. We distinguish two kinds of values: primitive values and untrusted values. Primitive values (represented by metavariable \(v\)) are boolean constants and \(\lambda \)-abstractions. An untrusted value (represented by metavariable \(u\)) is a term of the form (*distrust v*), where \(v\) is a primitive value. Untrusted values arise as normal forms of terms that do not have any *check* construct.

The small-step semantics of the trust \(\lambda \)-calculus is an extension of the standard call-by-value semantics for the simply typed \(\lambda \)-calculus. The required extensions deal with trust specific constructs (terms trust, distrust and check). As usual, semantics for \(\lambda \)-calculi rely on substitution. For any \(e_{1},\,e_{2}\) and \(x\), we define \([x\,\mapsto \,e_{1}]\,e_{2}\) to be the result of substituting every *free* occurrence of variable \(x\) in \(e_{2}\), that follows the standard definition of capture free substitution [2, 20].

^{2}

*values*and \(u\) denotes an

*untrusted value*, as defined in Fig. 4. Most of the rules are standard, but some deserve attention. Rules \(\mathtt{Trust_c}\), \(\mathtt{Distrust_c}\), \(\mathtt{Distrust_{ca1}}\), \(\mathtt{Distrust_{ca2}}\), \(\mathtt{Trust_v}\) and \(\mathtt{Check_v}\) are rules for eliminating redundant uses of trust related constructs. For example, rule \(\mathtt{Distrust_c}\) specifies that distrusting a value twice is the same as distrusting it once. The other contraction rules have similar meanings.

We denote by \(\rightarrow ^{*}\) the reflexive, transitive closure of the small-step semantics. If a term \(e\) is not a value (primitive or untrusted), and \(e\) cannot be further reduced according to the rules of the small-step semantics, let’s say that \(e\) is *stuck*. An example of a stuck term is *check*(*distrust true*); since *check* only reduces trusted values, this term does not reduce to any other term and it is not a primitive or untrusted value.

The main purpose of the type system is to rule out all programs that contain stuck expressions such as *check*(*distrust t*), for some term \(t\).

The following lemma states the property that the proposed semantics is deterministic.

### **Lemma 1**

(Determinism of small-step semantics) For any \(e_{1}, e_{2}\) and \(e_{3}\), if \(e_{1}\rightarrow e_{2}\) and \(e_{1}\rightarrow e_{3}\) then \(e_{2} = e_{3}\).

### *Proof*

Induction over the derivation of \(e_{1}\rightarrow e_{2}\) and case analysis on the last rule used to conclude \(e_{1}\rightarrow e_{3}\).

## 4 Type system

The type system proposed in [38] is based on the Curry version of simply typed \(\lambda \)-calculus. Since our main interest is the development of a certified type-checker and proofs about the type system, we use a variation of a Church like type system for the simply typed \(\lambda \)-calculus. The type system is defined in Fig. 8, as a set of rules for deriving judgements \(\Gamma \,\vdash \,e : \tau \), meaning that term \(e\) has type \(\tau \), in typing context \(\Gamma \) (which contains type assumptions for the free variables in \(e\)). When \(e\) is a well-typed closed term, we omit \(\Gamma \) and simply write \(\vdash \,e:\tau \).

\(\Gamma ,\,x:\tau \) is the standard notation for extending typing context \(\Gamma \) with a new assumption, after deleting from \(\Gamma \) any type assumption for \(x\). We let \(\Gamma (x)=\tau \) if \(x\,:\,\tau \in \Gamma \). Typing contexts are represented in Coq by lists of pairs of identifiers and types. Definitions of typing contexts, functions and properties over them (and their corresponding lemmas) are straightforward.

Trust annotations in types are subjected to a subtyping relation \(s\preceq s'\), meaning that trust type \(s\) is a subtype of \(s'\), which is defined as the smallest reflexive relation (encoded as an inductive type) such that (the only non-reflexive element of relation \(\preceq \) is ) tr\(\preceq \)dis.

The meaning of the typing rules for boolean constants, variables, subtyping and abstractions is standard. Constants and functions written by the programmer are considered as trusted, following [38]. The rules T-Trust and T-Distrust “cast” the trust type of an expression to trust and untrust, respectively, and rule T-Check checks whether an expression has a trusted type. In rule T-App, the annotated type of the actual argument is required to match the annotated type of the formal argument. This includes trustworthiness. The trust of the result of an application is the least upper bound of the trust of that function result type and the trust of the function type itself. We let \(s \vee s'\) denote the maximum between the trust types \(s\) and \(s'\).

We can now proceed to prove that the type system enjoys the type soundness property. In order to do this, we need to prove some lemmas about the typing relation, namely, inversion lemmas for the typing relation and canonical forms lemmas [39]. We will not state each one of these “infrastructure” lemmas here, but only sketch the key ones. Type soundness of the \(\lambda \)-calculus with trust types essentially guarantees that well typed terms do not stuck by checking trustworthiness of an untrusted term. This comes as a consequence of two properties: (1) progress, which guarantees that any well typed closed term reduces to a value; and (2) preservation, that ensures that term reduction preserves types. These results are formalised below.

### **Theorem 1**

(Progress) If \(\;\vdash e:\tau \), then either \(e\) is a value, or it is an untrusted value, or there exists some term \(e'\) such that \(e\rightarrow e'\).

### *Proof*

Induction over the derivation of \(\;\vdash e:\tau \) using canonical form lemmas.

### **Lemma 2**

(Substitution lemma) If \(\Gamma ,x:\tau ' \vdash e:\tau \) and \(e'\) is such that \(\Gamma \vdash e':\tau '\), then \(\Gamma \vdash [x\mapsto e']\,e:\tau \).

### *Proof*

Induction over the structure of \(e\) using the corresponding inversion lemma for the typing relation in each case.

### **Theorem 2**

(Preservation) If \(\;\vdash e:\tau \) and \(e \rightarrow e'\), then \(\;\vdash e':\tau '\), for some \(\tau '\) s.t. \(\tau ' \le \tau \).

### *Proof*

Induction over the derivation of \(\;\vdash e:\tau \) and case analysis over the last rule used to conclude \(e\rightarrow e'\), using Lemma 2.

### **Corollary 1**

(Type soundness) If \(\;\vdash e\,:\,\tau \) and \(e\rightarrow ^{*}e'\), then \(e'\) is not stuck \((\)i.e., \(e'\) is not of the form \(\mathtt{check e''}\), where \(e''\) has an untrusted type\()\).

### *Proof*

Induction over \(e\rightarrow ^{*}e'\) using Theorems 1 and 2.

### 4.1 Syntax directed type system

The syntax directed type system is presented in Fig. 10 as a set of rules for deriving judgements of the form \(\Gamma \,\vdash ^{D}\,e\,:\,\tau \). The rules are almost the same as the ones in Fig. 8 except for the application rule that now includes, as a premise, a test of the subtyping relation \(\tau ' \le _{D} \tau \), which represents a function that is true if and only if \(\tau ' \le \tau \) holds. Termination, soundness and completeness of the subtyping test function follows the approach in [39] and their proofs are straightforward.

The next theorems state soundness and completeness of the syntax directed type system, and their proofs are in the companion Coq scripts.

### **Theorem 3**

(Soundness) If \(\Gamma \vdash ^{D} e : \tau \), then \(\Gamma \vdash e:\tau \)

### *Proof*

Induction on the derivation of \(\Gamma \vdash ^{D} e :\tau \).

### **Theorem 4**

(Completeness) If \(\Gamma \vdash e:\tau \), then \(\Gamma \vdash ^{D} e:\tau '\) for some \(\tau '\) such that \(\tau ' \le \tau \).

### *Proof*

Induction on the derivation of \(\Gamma \vdash e:\tau \).

Finally, we prove that the typing problem for the trust \(\lambda \)-calculus is decidable, that is, we prove that, given a typing context \(\Gamma \) and term \(e\), it is decidable whether there exists a type \(\tau \) such that \(\Gamma \vdash ^{D} e\,:\,\tau \). Because of the constructive nature of this proof, a certified algorithm for type checking an expression can be extracted from it. This theorem is stated as the following piece of Coq source code.

Predicate has_type_alg represents the syntax directed type system of Fig. 10. Intuitively, this theorem means that either there exists a type \(t\) such that has_type_alg ctx e t is provable or there is no such type t.

## 5 Erasure and simulation

As pointed out in [38], the type system for the \(\lambda \)-calculus with trust types is just a restriction of the classic (in our formalisation) Church type system for \(\lambda \)-calculus. This notion is formalised by an erasure function that converts terms, types and contexts from the trust calculus to simply typed \(\lambda \)-calculus.

Following [38], we write these erasure functions using notation \(|\phi |\), where \(\phi \) is used as a term, type or context.

### **Lemma 3**

(Lemma 12 of [38]) For any trust types \(\tau \) and \(\tau '\) such that \(\tau \le \tau '\) we have that \(|\tau |\,=\,|\tau '|\).

### *Proof*

Induction over the derivation of \(\tau \le \tau '\).

### **Theorem 5**

(Erasure) If \(\Gamma \,\vdash \,e\,:\,\tau \), then we have that \(|\Gamma |\,\vdash ^{C}\,|e|\,:\,|\tau |\).

### *Proof*

Induction over \(\Gamma \,\vdash \,e\,:\,\tau \).

For any well typed term, we can erase all trust, distrust and check constructs and evaluate the resulting term using a standard semantics of \(\lambda \)-calculus. In practice, this means that after type-checking a term, we can erase all trust related constructs and evaluate the term without any performance penalties [38]. This fact is expressed by the following theorem.

### **Theorem 6**

(Simulation) If \(\vdash \,e\,:\,\tau \) and \(|e|\rightarrow ^{*}e'\) then there exists \(e_{1}\) such that \(e\rightarrow ^{*}e_1\) and \(|e_1|=e'\).

### *Proof*

Induction over \(e\).

## 6 Related work

*Language support*. The use of language based techniques for protecting information has as its most prominent example the security mechanism implemented by the Java run-time environment, which defines a set of security policies for applets [30, 54].

Recently, an extension of Haskell was designed to deal with some language features that can be used to bypass the type system, referential transparency and module encapsulation [51]. The approach used by Safe Haskell is to classify modules and packages as safe, trust and unsafe based on its source code or in compiler pragmas that can be used to declare a possibly unsafe module as trustworthy. The Safe Haskell extension is available in the GHC compiler version 7.2 [45]. The authors used it to implement a web-based version of a Haskell interpreter, but no formal description of the safety inference process was given.

*Type systems for security*. The work of Volpano et al. was the first to use type systems to enforce security policies by a compiler [52]. They defined the lattice based analysis proposed by Denning in [15] as a type system for a prototypical imperative language with first order procedures. Their type system relies on polymorphism, thus allowing that commands and expression types depend on the context in which they occur. The calculus proposed by Ørbæk and Palsberg in [38] does not support polymorphism as well as our formalisation. In order to provide polymorphism in the \(\lambda \)-calculus with trust types, we need to deal with the combination of structural subtyping and parametric polymorphism that was developed in [22, 47]. We leave this extension for future work.

Another proposal for a type system for ensuring security was described in [19], where a type system for a purely functional language was given and extensions like concurrency support were also discussed. The core calculus presented in [19] supports products and sums types and their related term constructs (projections and case expressions, respectively) and they prove type soundness results for closed expressions like the present work. Extending the formalisation of the \(\lambda \)-calculus with trust types with sum and product types is straightforward (e.g. [39, Chapter 15]).

The JFlow type system [35] is used in a language that extends Java with security types. Unlike other works in security type systems [19, 52], JFlow applies the idea of a type system to ensure security policies over information flow on a full programming language, but no formal soundness proof is presented. A production compiler for this language is available [34] and was used in the development of a secure voting system [13]. Later, Simonet et al. uncovered a couple of flaws in JFlow type system [41], during his development of an extension of ML with types for information flow control, called Flow-Caml [42]. Following ML tradition of providing full type inference, Simonet developed a type inference algorithm for polymorphism and subtyping that was used as basis for Flow-Caml type system [47].

Barthe et al. [4] describe a security type system for a low level language with jumps and calls and prove that information flow types are preserved by the compilation from a high level imperative language. A mechanised proof that the type system proposed in Barthe’s work is sound was given in [24], using the Coq proof assistant. Compared to the present work, Kammüler’s formalisation deals with the problems of the lack of structure present in low level languages which make control flow more intricate than in a pure functional language.

As pointed out by Ørbæk and Palsberg in [38], security analysis focus on avoiding that classified information leaks out of a system to unprivileged users. The formalised type system ensures that untrustworthy information does not flow *into* the system. So, a trust type system can be seen as the “dual” of security type systems.

*Use of Proof Assistants*. Proof assistants have been used with success in several verification tasks. The Compcert project aims to develop a certified C compiler for embedded systems programming, using Coq proof assistant [27]. Several intermediate results were reported such as a mechanisation of a subset of C semantics [6], verification of the Compcert back-end [28] and a formalisation of C memory model [29]. The works of Chlipala describe compilers for small functional languages also using Coq proof assistant [10, 11] and proofs about low level programs using separation logic [12]. The main differential of Chlipala works was the use of dependent types and proof automation to enable maintainability of proof scripts. Coq was also used with success in the formalisation of well known mathematical theorems such as the four colour theorem and the Feit–Thompson theorem by Georges Gonthier team at Microsoft-INRIA joint research center [17, 18].

Agda is a dependently typed language [37], based on Martin Löf’s type theory [31], that can be used as a proof assistant. Licata et al. [33] developed a library, called Aglet, for embedding secure-typed programming in Agda. Security policies are, in Aglet, proof terms that ensure some security policy. In order to ease the task of writing proof terms, this library provides an implementation of proof search procedure.

Isabelle/HOL [36] is a proof assistant that has been used in several projects like the formalisation of a general purpose operating system kernel, in which C code can be extracted from the produced Isabelle theories [25]. The Archive of Formal Proofs [21] is a online repository for Isabelle developments that contains several formalisations of programming languages and mathematical theorems, such as the formal proofs of Volpano et al. type system for security [48, 52].

A common issue in the formalisation of programming languages metatheory using proof assistants is how to deal with languages whose syntax supports variable binding. The main issue with variable binding is the possibility of variable capture in substitutions. Several techniques were developed to avoid variable capture such as de Bruijn indexes [7], high-order abstract syntax [14], locally nameless [8] and nominal logic [40]. For a detailed survey on binding techniques we refer the interested reader to [1]. In our work, we decided to avoid the extra complications of dealing with binding techniques (such as de Bruijn indexes and high-order abstract syntax) considering the formalisation of typing properties for closed terms only. There is a library for using the locally nameless approach for Coq proof assistant [9], but it relies on nonconstructive features that would not allow us to extract a verified type-checker from the developed formalisation.

## 7 Conclusion

We presented an axiom-free, fully constructive Coq formalisation of \(\lambda \)-calculus with trust types. The major differences between the original formulation of the trust \(\lambda \)-calculus and its presentation in this work is the use of a small-step semantics, instead of a reduction semantics, and a Church style, instead of a Curry style, type system. This allowed us to give concise proofs of type soundness, erasure and simulation theorems.

We also presented a syntax directed formulation of the original type system, that is sound and complete with respect to the former. Decidability of type checking is proved using this syntax directed version and a correct type checker can be extracted from this proof.

Future directions for extending this work include: (1) a reviewed formalisation which deals with free variables capture on substitutions, that could be used as a basis for the formalisation of other properties of the calculus; (2) modifying the type system in order to account for polymorphism; (3) formalising the type inference problem for the calculus and extracting a certified type inferencer; and (4) extending the calculus to a high level functional language.

## Footnotes

- 1.
Abbreviation of Brower, Heyting, Kolmogorov, de Bruijn and Martin-Löf Correspondence. This is also known as the Curry–Howard “isomorphism”.

- 2.
Several techniques can be used in order to deal with the problem of free variable capture on substitution, such as de Bruijn indexes [7], locally nameless representation [8] and high-order abstract syntax [14]. More about the formalisation of programming languages syntax with variable binding can be found at [1].

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