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This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Jane Street.

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This question is commonly asked for Machine Learning Engineer candidates at Jane Street during technical interviews.

Design and implement a tiny language runtime | Jane Street Interview Question

Quick Overview

This question evaluates proficiency in programming-language implementation—lexical analysis, recursive-descent parsing, AST construction, static semantic checks, and runtime execution (interpreter or VM)—as well as software design for extensibility, scoping, error reporting, and complexity reasoning.

Minimal Imperative Language — Design and Implementation Task

Context

You are asked to design and implement a minimal imperative language and its tooling. The language supports integers, variables, arithmetic, comparisons, conditionals, loops, and printing. You must decide and document a concrete grammar, implement a tokenizer and recursive-descent parser that produce an AST, perform basic static checks, and execute programs.

Language Specification (to implement)

Tokens: identifiers, integers, operators (+, -, *, /), comparisons (<, <=, >, >=, ==, !=), assignment (=), punctuation (;, (, ), {, }).
Statements:
1. Variable declaration: let x = expr;
2. Assignment: x = expr;
3. Conditional: if (expr) { ... } else { ... }
4. Loop: while (expr) { ... }
5. Print: print(expr);
Expressions: integer arithmetic and parentheses.

Deliverables

Implement a tokenizer and a recursive-descent parser that builds an AST for a grammar you define.
Implement static checks for undefined variables and integer-only type consistency.
Implement either:
- an AST interpreter, or
- a bytecode compiler plus VM. Choose one, implement it, and justify the choice.

Design Extensions and Discussion

How would you extend the design to support user-defined functions with lexical scoping and a call stack?
How would you organize classes and interfaces (OOD) to make adding new statements/operators low-friction while keeping the interpreter/compiler cohesive?
How will you implement syntax/semantic error reporting with line/column tracking and recovery to continue parsing?
What are the time and space complexities of tokenization, parsing, and execution with respect to program length?

Quick Overview

Context

Language Specification (to implement)

Tokens: identifiers, integers, operators (+, -, *, /), comparisons (<, <=, >, >=, ==, !=), assignment (=), punctuation (;, (, ), {, }).

Statements:

Variable declaration: let x = expr;
Assignment: x = expr;
Conditional: if (expr) { ... } else { ... }
Loop: while (expr) { ... }
Print: print(expr);

Expressions: integer arithmetic and parentheses.

Deliverables

Implement a tokenizer and a recursive-descent parser that builds an AST for a grammar you define.

Implement static checks for undefined variables and integer-only type consistency.

Implement either:

an AST interpreter, or
a bytecode compiler plus VM. Choose one, implement it, and justify the choice.

Design Extensions and Discussion

How would you extend the design to support user-defined functions with lexical scoping and a call stack?

How would you organize classes and interfaces (OOD) to make adding new statements/operators low-friction while keeping the interpreter/compiler cohesive?

How will you implement syntax/semantic error reporting with line/column tracking and recovery to continue parsing?

What are the time and space complexities of tokenization, parsing, and execution with respect to program length?

Design and implement a tiny language runtime

Quick Overview

Minimal Imperative Language — Design and Implementation Task

Context

Language Specification (to implement)

Deliverables

Design Extensions and Discussion

Solution

Comments (0)

Design and implement a tiny language runtime

Quick Overview

Minimal Imperative Language — Design and Implementation Task

Context

Language Specification (to implement)

Deliverables

Design Extensions and Discussion

Solution

Comments (0)