Meeting 11 - Operational Semantics

Bor-Yuh Evan Chang

Tuesday, October 1, 2024

\(\newcommand{\TirName}[1]{\text{#1}} \newcommand{\inferrule}[3][]{ \let\and\qquad \begin{array}{@{}l@{}} \TirName{#1} \\ \displaystyle \frac{#2}{#3} \end{array} } \newcommand{\infer}[3][]{\inferrule[#1]{#2}{#3}} \)

Meeting 11 - Operational Semantics

What questions does your neighbor have?

In-Class Slides
In-Class Jupyter
Book Chapter

Announcements

• HW1 scores to be released later today
• HW3 due Friday (or Monday?) 6pm
• Peer Reviews on Unit 2 Thu-Fri Oct 3-4 to prepare for the exam
• Exam on Units 1-2 next Tue Oct 8
  • 50 minutes in class
  • Accommodation letters confirmed (some weeks ago)?
    • Look out for note on Piazza about the extended-time location
    • During class time
  • Practice are all of the learning activities: HWs, Labs, Peer Review

Today

• Feedback Rubric
• Peer Reviews
• Preview HW3
• Operational Semantics
• Triage Your Questions
  • Lab 2?

Feedback Rubric

4 - Exceeding (E)

Student has met and exceeded the learning objective.

3 - Proficient (P)

Student has proficiently met the learning objective.

2 - Approaching (A)

Student is approaching the learning objective.

1 - Novice (N)

Student is far from meeting the learning objective.

0 - Zero (Z)

Student has made no attempt towards the learning objective.

• Emphasizing learning activities over summative assessments.
• Focus our attention on providing feedback on where you are at in meeting the learning objectives.
• Deemphasizes “getting each point possible” and instead emphasizes giving you insight into where you are at in your learning journey.
• “Getting the answers” is deemphasized in favor of going deeper and synthesizing what you learn.

Feedback Rubric

4 - Exceeding (E)

Student demonstrates mastery and shows the ability to apply and transfer learning with depth and complexity.

Student demonstrates synthesis of the underlying concepts. Student can go beyond merely describing the solution to explaining the underlying reasoning and discussing generalizations.

3 - Proficient (P)

Student demonstrates competence.

Student is able to explain the overall solution and can answer specific questions. While the student is capable of explaining their solution, they may not be able to confidently extend their explanation beyond the immediate context.

2 - Approaching (A)

Student demonstrates partial understanding.

Student may able to describe the solution but has difficulty answering specific questions about it. Student has difficulty explaining the reasoning behind their solution.

• Uses the full range of scores between 0-4 (not 50%-100%).
• A score of 2 is not itself a failing grade, but it an indicator that you need to make some additional effort to reach a proficient score.
• A score of 3 is a proficient grade, indicating you have met the basic learning objective.
• Philosophy: Challenge you and make learning opportunities available. Give clear feedback on where you are at in your learning journey (in a competitive world). But do not penalize grade-wise.

Grading Scale

Level	Percentage Range	Letter Grade
Exceeding (E)	88% – 100%	A
	82% – 88%	A-
Proficient (P)	75% – 82%	B+
	68% – 75%	B
	61% – 68%	B-
	56% – 61%	C+
	51% – 56%	C
Approaching (A)	46% – 51%	C-
	42% – 46%	D+
	38% – 42%	D
	34% – 38%	D-

• Proficient (P) everywhere (an average score of 3) is a B+ and solidly in the B range. Achieve Exceeding (E) in a few places gets you to an A-.
• Intentionally, the B range (B- to B+) is the largest.
• Exams will enable to you to show your learning level.

Peer Reviews

Goal: Staff-organized 30-minute review session to prepare for the exam.

• A day or two before the the sessions, the instructors will release a set of review questions on the unit (think: practice exam questions).
• You sign up for a 30-minute slot to take turns asking each other these questions and giving feedback on where your partner is at on their learning journey (Novice, Approaching, Proficient, Exceeding).
• Show up at your slot with the review questions and your laptop ready. No upfront prep other than completing the homework and lab.
• A member of the Feedback Staff will join most sessions to offer guidance but will not be an interviewer. They are a third-party participant.
• You do not “grade” your partner on their learning level but only assess to what extent your partner was an active participant. The Feedback Staff will do the same.

Questions?

• Review:
  • What is relation between judgment forms, inference rules, and judgments?

Questions?

Operational Semantics

Big Picture

What is the purpose of a language specification?

What is the difference between syntax and semantics?

• We talked about grammars to unambiguously describe the syntax of a language.

• We will now talk about a way to precisely describe the semantics of a language, called operational semantics.

• Comment: There are several ways that PL folks describe semantics of PLs that have different properties. We will focus on one class called “operational semantics” that focuses on

  • “How program expressions/statements execute?”
  • “Describing an interpreter for the language.”

• Interested in learning more about semantics: take CSCI 5535.

Lab 2: JavaScript is Bananas

JavaScript

true + 2

JavaScript

"Hello, " + "World!"

JavaScript

"b" + "a" + "n" + - "a" + "a" + "s"

How can we describe how to implement a interpreter for all JavaScripty programs?

• ECMA-262 standard is a rather precise specification based on natural language prose, but the descriptions are quite verbose.

• Introduce some mathematical notation that enables us to specify semantics with less ambiguity in a very compact form.

• Like any mathematical notation, its precise and compact nature makes it easier, for example, to spot errors or inconsistencies in specification. However, there will necessarily be a learning curve to reading the notation.

Evaluation Judgment Form

\[ e \Downarrow v \]

“Expression \(e\) evaluates to value \(v\).”

A set of inference rules defining such an evaluation judgment form is called a big-step operational semantics for expressions \(e\).

Closed Expressions

\[ \begin{array}{rrrl} \text{expressions} & e& \mathrel{::=}& v\mid e_1 \mathbin{\mathit{bop}} e_2 \\ \text{values} & v& \mathrel{::=}& n \\ \text{binary operators} & \mathit{bop}& \mathrel{::=}& \texttt{+} \\ \text{numbers} & n \end{array} \]

defined trait Expr
defined trait Bop
defined class Binary
defined class N
defined object Plus
defined function isValue
e_oneplustwo: Binary = Binary(bop = Plus, e1 = N(n = 1.0), e2 = N(n = 2.0))

EvalNum and EvalPlus Rules

\[ \inferrule[EvalNum]{ }{ ??? } \]

\[ \inferrule[EvalPlus]{ }{ ??? } \]

\inferrule[EvalNum]{
}{
  n \Downarrow n
}

\inferrule[EvalPlus]{
  e_1 \Downarrow n_1
  \and
  e_2 \Downarrow n_2
}{
  e_1 \mathtt{+} e_2 \Downarrow \num_1 + \num_2
}

EvalNum and EvalPlus Code

defined function eval

defined function eval
res2_1: Binary = Binary(bop = Plus, e1 = N(n = 1.0), e2 = N(n = 2.0))
v_oneplustwo: Expr = N(n = 3.0)

Dynamic Typing

Booleans

\[ \begin{array}{rrrl} \text{values} & v& \mathrel{::=}& b \\ \text{booleans} & b \end{array} \]

defined class B
defined function isValue
e_true: B = B(b = true)
e_trueplustwo: Binary = Binary(bop = Plus, e1 = B(b = true), e2 = N(n = 2.0))

EvalVal

\[ \inferrule[EvalVal]{ }{ v \Downarrow v } \]

defined function eval

defined function eval
res5_1: Binary = Binary(bop = Plus, e1 = N(n = 1.0), e2 = N(n = 2.0))
v_oneplustwo: Expr = N(n = 3.0)
res5_4: B = B(b = true)
v_true: Expr = B(b = true)

MatchError

scala.MatchError: B(true) (of class ammonite.$sess.cmd3$Helper$B)
  ammonite.$sess.cmd5$Helper.eval(cmd5.sc:6)
  ammonite.$sess.cmd6$Helper.<init>(cmd6.sc:2)
  ammonite.$sess.cmd6$.<clinit>(cmd6.sc:7)

• The particular issue is that \(+\) can only apply to numbers in the \(\TirName{EvalPlus}\) rule: specifically, \(n_1 + n_2\).
• When the operator does not apply to input values, this is called a type error.
• If we detect type error at run time, then this is called dynamic typing.

DynamicTypeError

defined class DynamicTypeError

defined function eval

defined function eval

TypeError: in expression Binary(Plus,B(true),N(2.0))
  ammonite.$sess.cmd9$Helper.eval(cmd9.sc:9)
  ammonite.$sess.cmd10$Helper.<init>(cmd10.sc:2)
  ammonite.$sess.cmd10$.<clinit>(cmd10.sc:7)

Coercions

toNumber

\(\fbox{$v \rightsquigarrow n$}\)

\(\inferrule[ToNumberNum]{ }{ n \rightsquigarrow n }\)

\(\inferrule[ToNumberTrue]{ }{ \mathbf{true} \rightsquigarrow 1 }\)

\(\inferrule[ToNumberFalse]{ }{ \mathbf{true} \rightsquigarrow 0 }\)

defined function toNumber

defined function toNumber

EvalVal and EvalPlus

\[ \inferrule[EvalVal]{ }{ ??? } \]

\[ \inferrule[EvalPlus]{ }{ ??? } \]

\(\inferrule[EvalVal]{ \phantom{\Downarrow} }{ v \Downarrow v }\)

\(\inferrule[EvalPlus]{ e_1 \Downarrow v_1 \and e_2 \Downarrow v_2 \and v_1 \rightsquigarrow n_1 \and v_2 \rightsquigarrow n_2 }{ e_1 \mathbin{\texttt{+}} e_2 \Downarrow n_1 + n_2 }\)

Variables

Syntax

\[ \begin{array}{rrrl} \text{expressions} & e& \mathrel{::=}& x\mid\mathbf{const}\;x\;\texttt{=}\;e_1\texttt{;}\;e_2 \\ \text{variables} & x, y \end{array} \]

defined class Var
defined class ConstDecl
e_const_i_two_trueplusi: ConstDecl = ConstDecl(
  x = "i",
  e1 = N(n = 2.0),
  e2 = Binary(bop = Plus, e1 = B(b = true), e2 = Var(x = "i"))
)
defined type Env
empty: Env = Map()
defined function lookup
defined function extend

Value Environments

\[ \begin{array}{rrrl} \text{value environments} & E, \mathit{env}& \mathrel{::=}& \cdot \mid E[\mathop{}x \mapsto v] \end{array} \]

Semantics

\[ \inferrule[EvalVar]{ }{ ??? } \]

\[ \inferrule[EvalConstDecl]{ }{ ??? } \]

\(\inferrule[EvalVar]{ \phantom{\Downarrow} }{ E \vdash x \Downarrow E(x) }\)

\(\inferrule[EvalConstDecl]{ E \vdash e_1 \Downarrow v_1 \and E[\mathop{}x \mapsto v_1] \vdash e_2 \Downarrow v_2 }{ E \vdash \mathbf{const}\;x\;\texttt{=}\;e_1\texttt{;}\;e_2 \Downarrow v_2 }\)

Variables, Numbers, and Booleans

\(\inferrule[EvalVar]{ \phantom{\Downarrow} }{ E \vdash x \Downarrow E(x) }\)

\(\inferrule[EvalConstDecl]{ E \vdash e_1 \Downarrow v_1 \and E[\mathop{}x \mapsto v_1] \vdash e_2 \Downarrow v_2 }{ E \vdash \mathbf{const}\;x\;\texttt{=}\;e_1\texttt{;}\;e_2 \Downarrow v_2 }\)

\(\inferrule[EvalVal]{ \phantom{\Downarrow} }{ E \vdash v \Downarrow v }\)

\(\inferrule[EvalNeg]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow n_1 }{ E \vdash \mathop{\texttt{-}} e_1 \Downarrow - n_1 }\)

\(\inferrule[EvalNot]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow b_1 }{ E \vdash \mathop{\texttt{!}} e_1 \Downarrow \lnot b_1 }\)

\(\inferrule[EvalArith]{ E \vdash e_1 \Downarrow v_1 \and E \vdash e_2 \Downarrow v_2 \and v_1 \rightsquigarrow n_1 \and v_2 \rightsquigarrow n_2 \and \mathit{bop}\in \left\{ \texttt{+}, \texttt{-}, \texttt{*}, \texttt{/} \right\} }{ E \vdash e_1 \mathbin{\mathit{bop}} e_2 \Downarrow n_1 \mathbin{\mathit{bop}} n_2 }\)

\(\inferrule[EvalAndTrue]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{true} \and E \vdash e_2 \Downarrow v_2 }{ E \vdash e_1 \mathbin{\texttt{\&\&}} e_2 \Downarrow v_2 }\)

\(\inferrule[EvalAndFalse]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{false} }{ E \vdash e_1 \mathbin{\texttt{\&\&}} e_2 \Downarrow v_1 }\)

\(\inferrule[EvalOrTrue]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{true} }{ E \vdash e_1 \mathbin{\texttt{||}} e_2 \Downarrow v_1 }\)

\(\inferrule[EvalOrFalse]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{false} \and E \vdash e_2 \Downarrow v_2 }{ E \vdash e_1 \mathbin{\texttt{||}} e_2 \Downarrow v_2 }\)

\(\inferrule[EvalIfTrue]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{true} \and E \vdash e_2 \Downarrow v_2 }{ E \vdash e_1\;\texttt{?}\;e_2\;\texttt{:}\;e_3 \Downarrow v_2 }\)

\(\inferrule[EvalIfFalse]{ E \vdash e_1 \Downarrow v_1 \and v_1 \rightsquigarrow \mathbf{false} \and E \vdash e_3 \Downarrow v_3 }{ E \vdash e_1\;\texttt{?}\;e_2\;\texttt{:}\;e_3 \Downarrow v_3 }\)

\(\inferrule[EvalEquality]{ E \vdash e_1 \Downarrow v_1 \and E \vdash e_2 \Downarrow v_2 \and \mathit{bop}\in \left\{ \texttt{===}, \texttt{!==} \right\} }{ E \vdash e_1 \mathbin{\mathit{bop}} e_2 \Downarrow v_1 \mathbin{\mathit{bop}} v_2 }\)

\(\inferrule[EvalInequality]{ E \vdash e_1 \Downarrow v_1 \and E \vdash e_2 \Downarrow v_2 \and v_1 \rightsquigarrow n_1 \and v_2 \rightsquigarrow n_2 \and \mathit{bop}\in \left\{ \texttt{<}, \texttt{<=}, \texttt{>}, \texttt{>=} \right\} }{ E \vdash e_1 \mathbin{\mathit{bop}} e_2 \Downarrow n_1 \mathbin{\mathit{bop}} n_2 }\)