Jane Street·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Jane Street SWE interview, got a stack machine interpreter problem. Pretty self-contained but the type system angle made it trickier than it looked at first glance.

Questions Asked (1)

Implement an interpreter for a simple stack machine that supports PUSH, POP, POP_AND_PRINT, and ADD instructions, where values on the stack are either integers or strings and the ADD operation behaves differently depending on the types of the two operands.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

The ADD rule is where it gets you.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and edge cases, then outline a design using a stack and a dispatch mechanism for instructions. Implement the interpreter with clear type handling for ADD, and discuss potential extensions and trade-offs.

Pro tip: Demonstrate strong communication by walking through examples and edge cases before coding, and show awareness of production concerns like error handling and extensibility.

1. Clarify Requirements

Ask questions to confirm the instruction set, operand types, and behavior of ADD for different type combinations. Clarify error handling and input format.

2. Design the Interpreter

Choose a data structure for the stack (e.g., list) and a dispatch mechanism (e.g., switch or dictionary) to map instructions to operations. Consider how to represent values and handle type checking.

3. Implement Core Operations

Write functions for PUSH, POP, POP_AND_PRINT, and ADD. For ADD, implement type-specific behavior: integer addition, string concatenation, or mixed-type handling (e.g., convert to string or error).

4. Handle Edge Cases and Errors

Address scenarios like stack underflow, invalid instructions, and type mismatches. Decide on error reporting (exceptions, error codes) and ensure robustness.

5. Test and Extend

Walk through test cases covering all instructions and type combinations. Discuss potential extensions like additional instructions or support for other types, and trade-offs in design choices.

Key Points to Mention

Stack data structure and operations (push, pop, peek)
Type handling for ADD: integer addition, string concatenation, and mixed-type behavior
Error handling: stack underflow, invalid instructions, type errors
Instruction dispatch mechanism (e.g., switch statement, dictionary mapping)
Extensibility: adding new instructions or types
Testing strategy: unit tests for each instruction and edge cases

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.