Openai·Backend Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

OpenAI backend interview that threw a full interpreter design problem at me. Not your typical coding round, this one wanted tokenization, parsing strategy, expression evaluation, the works. Left feeling like I'd only scratched the surface of what they were actually looking for.

Questions Asked (1)

Build a toy language interpreter from scratch: support variable assignment, arithmetic, conditionals, and output statements. Walk through tokenization, your parsing approach, how you evaluate expressions, and how you handle edge cases like undefined variables and operator precedence.

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's scope and constraints, then walk through the pipeline: tokenizer, parser (e.g., recursive descent with precedence climbing), and evaluator. Emphasize design choices, trade-offs, and how you handle edge cases like undefined variables and operator precedence.

Pro tip: Demonstrate production-level thinking by discussing error handling with source locations and suggesting a visitor pattern for extensibility. This shows you consider maintainability and user experience, not just a quick hack.

1. Clarify Requirements and Scope

Ask clarifying questions about the language features, syntax, and constraints (e.g., single vs. multiple statements, variable scoping). This ensures you build the right thing and shows you think before coding.

2. Design the Tokenizer

Explain how to convert the input string into tokens (e.g., identifiers, numbers, operators, keywords). Mention handling whitespace, comments, and multi-character operators.

3. Design the Parser

Describe your parsing approach, such as recursive descent with precedence climbing for expressions. Explain how you build an AST and handle statements like assignment, conditionals, and output.

4. Design the Evaluator

Explain how to traverse the AST to execute statements and evaluate expressions. Discuss environment management for variables and control flow for conditionals.

5. Handle Edge Cases and Errors

Detail how you detect and report errors like undefined variables, type mismatches, and syntax errors. Include source location information for better diagnostics.

Key Points to Mention

Tokenization: using regex or manual scanning, handling multi-character operators and keywords.
Parsing: recursive descent with precedence climbing (or Pratt parsing) to correctly handle operator precedence and associativity.
AST design: node types for expressions and statements, and how they enable clean evaluation.
Evaluation: environment (symbol table) for variable storage, and control flow for conditionals.
Error handling: undefined variables, division by zero, type errors, with line/column info.
Extensibility: using visitor pattern or similar to add new features easily.

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