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Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026

Summary

OpenAI software engineer interview where the main challenge was building a working interpreter for a toy language from scratch, including tokenizing, parsing, and evaluating it live on the spot. The follow-up extensions made it way harder than the initial prompt suggested.

Questions Asked (1)

Q1

Implement a parser and interpreter for a small toy language that supports variable assignment, arithmetic expressions, basic control flow, and print statements. Tokenize the input, build some form of AST or evaluate inline, and execute the program correctly.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

This wrecked me a little.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the language features and then outline a classic pipeline: tokenizer, parser (to AST), and interpreter. Emphasize modular design, error handling, and testing, and be ready to discuss trade-offs between AST interpretation and direct evaluation.

Pro tip: Mention that you would use a recursive descent parser for simplicity and extendability, and that you'd write unit tests for each component to ensure correctness.

1. Clarify Requirements

Ask about the specific syntax and semantics: what arithmetic operators, control flow constructs (if/while), variable scoping, and print format are expected. Confirm whether the language is statically or dynamically typed.

2. Design the Architecture

Propose a modular pipeline: tokenizer -> parser -> AST -> interpreter. Discuss whether to evaluate directly from the AST or compile to bytecode, and justify your choice based on simplicity and performance.

3. Implement Tokenizer and Parser

Describe tokenizing with regular expressions or a hand-written lexer. For parsing, suggest recursive descent or Pratt parsing for expressions, and explain how to handle operator precedence and associativity.

4. Build the Interpreter

Explain how to traverse the AST, maintain an environment (symbol table) for variables, and execute control flow using conditional jumps or recursive evaluation. Mention handling runtime errors like undefined variables.

5. Test and Iterate

Outline a testing strategy: unit tests for each component, integration tests with sample programs, and edge cases (e.g., division by zero, nested loops). Discuss how to extend the language later.

Key Points to Mention

  • Tokenization techniques (regex, finite state machines) and handling of whitespace/comments
  • Parser design: recursive descent, operator precedence, and AST node types
  • Interpreter execution models: tree-walking vs. bytecode, and environment management
  • Error handling: syntax errors, runtime errors, and informative messages
  • Performance considerations: time/space complexity, and potential optimizations
  • Testing methodologies: unit tests, integration tests, and fuzzing

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