This chapter develop a small language of numbers and booleans, serving as a straightforward vehicle for the introduction of serveral fundamental concepts like, abstract syntax tree, evaluation and runtime errors.

Syntax

BNF-like Definition

Terms of this language are defined as below:

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t ::=
true
false
if t then t else t
0
succ t
pred t
iszero t

where t is called meta-variable. At every point where the symbol t appears, we may substitute any terms.

Inductive Definition

The set of terms is the smallest set $T$ s.t.

  1. $\{0, true, false\} \subseteq T$
  2. if $t_1 \in T$, then ${succ\ t_1, pred\ t_1, iszero\ t_1} \subseteq T$
  3. if $t_1, t_2, t_3 \in T$, then $if\ t_1\ then\ t_2\ else\ t_3 \in T$

Concrete Definition

For each natural number $i$, define a set $S_i$ as follows:

  • $S_0 = \phi$
  • $S_{i+1} = \{0, true, false\} \cup \{succ\ t_1, pred\ t_1, iszero\ t_1 | t_1 \in S_i\} \cup \{if\ t_1\ then\ t_2\ else\ t_3 | t_j \in S_i\}$
  • $S = \cup_i S_i$

Three definition above are equivalent.

Semantics

The semantics of an language is how its terms are evaluated. Three approaches had been proposed to formalize semantics:

  • Operational Semantics (used here) specifies the behavior of a programming language by defining a simple abstract machine for it. This machine is “abstract” in the sense that it uses the terms of the language as its machine code, rather than some low-level microprocessor instruction set. For simple languages, a state of the machine is just a term, and the machine’s behavior is defined by a straition function that for each state, either gives the next state, or declares that the machine has halted. (字节码语义,Compare with languages like Python or Java)
  • Denotational Semantics. The meaning of a term is taken to be some mathematical object, instead of a sequence of machine states. Giving denotational semantics for a language consists of finding a collection of semantic domains and then defining an interpretation function mapping terms into elements of these domains.
  • Axiomatic semantics (omitted)

Semantics with only Booleans

Firstly we consider a simpler situation where only booleans get involved.

The evaluation process can be summarized as three rules:

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if true then t2 else t3 -> t2 [E-IFTRUE]
if false then t2 else t3 -> t3 [E-IFFALSE]
t1 -> t1' || if t1 then t2 else t3 -> if t1' else t2 then t3 [E-IF]

A rule consists of one conclusion and zero or more premises. For example, in rule E-IF, t1 -> t1' is the premise and if t1 then t2 else t3 -> if t1' else t2 then t3 the conclusion.

Note that in textbook premises are written above conclusion with a horizontal line in the middle, but here I use a notation that is more convenient to type in.

A subset of terms should be defined as possible final results of evaluation, which are called values. Here they are just the constants true, false, 0.

Note that $\rightarrow$ can be viewed as a binary relation over $T$, i.e., a subset of $T \times T$.

The third rule E-IF specifies the evaluation order of an expression, i.e., clauses are always evaluated after their guards.

DEFINITION instance of an inference rule An instance of an inference rule is obtained by consistently replacing each metavariable by the same term in the rule’s conclusion and all its premises (if any). e.g., if true then true else (if false then false else false) -> true is an instance if E-IFTRUE.

DEFINITION satisfy A rule is satisfied by a relation if, for each instance of the rule, either the conclusion is in the relation or one of the premises is not (to ensure evaluation can proceed).

DEFINITION one-step evaluation relation denoted as $\rightarrow$, is the smallest binary relation on terms satisfying the three rules. When the pair (t, t') is in the relation, we say that t -> t' is derivable. (“smallest” implies that t -> t' is derivable iff it is justified by the rules).

DEFINITION normal form A term $t$ is in normal form if no evaluation rule applies to it.

Facts:

  • Every value is in normal form.
  • When only booleans involved, every term in normal form is a value.
  • Every term can be evaluated in values.

Semantics with Booleans and Numbers

Now we let numbers in. Several new rules should be added:

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t1 -> t1' || succ t1 -> succ t1' [E-SUCC]
pred 0 -> 0 [E-PREDZERO]
pred (succ nv1) -> nv1 [E_PREDSUCC]
t1 -> t1' || pred t1 -> pred t1' [E-PRED]
iszero 0 -> true [E-ISZERO]
iszero (succ nv1) -> false [E-ISZEROSUCC]
t1 -> t1' || iszero t1 -> iszero t1'[E-ISZERO]

and new values:

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v ::=
...
nv
nv ::=
0
succ nv # positive integer

DEFINITION stuck term A closed term is stuck if it is in normal form but not a value. e.g. if 0 then true else true or iszero false.

Stuckness gives us a simple notion of run-time error for this simple machine. Intuitively, it characterizes the situation where the operational semantics does not know what to do because the program has reached a “meaningless state”.

Stuckness can be prevented by introducing a new term called $wrong$ and augment the opereational semantics with rules that explicitly generate $wrong$ ain all the situations where the present semantics gets stuck.

Evaluation Theorems

If $t \rightarrow t1$ and $t \rightarrow t2$, then $t1 = t2$. (Can be proved by performing induction on the structure of t)

Other Forms of Evaluation Relation

DEFINITION mutli-step evaluation $\rightarrow^*$ is the reflexive, transitive closure of one-step evaluation.

If $t \rightarrow^\star t1$ and $t \rightarrow^\star t2$, where $t1$ and $t2$ are in normal form, then $t1 = t2$.

For every term $t$ there is some normal form $t’$ s.t. $t \rightarrow^\star t’$.

DEFINITION big-step evaluation (omitted) formulates the notion of “this term evaluates to that final value”.