← Openai Interview Insights

Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jun 2026Remote

Summary

OpenAI interview that basically asked me to build a mini interpreter from scratch. One question, but it had a lot of moving parts and I wasn't totally prepared for how deep it went.

Questions Asked (1)

Q1

Design and implement a small toy language that supports node and function constructs. This includes defining the data structures for nodes and functions, specifying how functions are declared and called, and writing an evaluator that can interpret programs written in this language.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

I underestimated this at first.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the language's scope and requirements, then define a minimal but extensible AST for nodes and functions. Walk through the design of the evaluator, emphasizing environment handling and recursion, and finally discuss trade-offs and potential extensions.

Pro tip: Demonstrate incremental development: begin with a simple version that supports basic node and function constructs, then iteratively add features like closures or error handling. This shows you can deliver working solutions while managing complexity.

1. Clarify Requirements and Scope

Ask questions to understand the expected complexity, syntax preferences, and evaluation semantics. Define what 'node' and 'function' mean in this context (e.g., AST nodes, function declarations with parameters and body).

2. Design the Abstract Syntax Tree (AST)

Define data structures for nodes (e.g., literals, variables, function calls) and functions (e.g., name, parameters, body). Use a class hierarchy or tagged unions for clarity and extensibility.

3. Implement the Evaluator

Write an evaluator that traverses the AST, using an environment to store variable bindings and function definitions. Handle function calls by creating a new scope with parameter bindings and evaluating the body.

4. Handle Recursion and Scoping

Ensure functions can call themselves and other functions, and that variable scoping follows lexical rules. Discuss how to manage the call stack and prevent infinite recursion.

5. Discuss Trade-offs and Extensions

Talk about design choices such as eager vs. lazy evaluation, error handling, and performance. Suggest possible extensions like closures, higher-order functions, or a parser.

Key Points to Mention

  • AST design: use classes or discriminated unions for nodes and functions, with clear separation of concerns.
  • Environment: implement a chain of scopes (e.g., a dictionary with parent pointer) to support lexical scoping and closures.
  • Function representation: store parameters and body, and capture the defining environment for closures.
  • Evaluation strategy: recursive tree-walking interpreter, with pattern matching or visitor pattern for node handling.
  • Error handling: detect and report undefined variables, arity mismatches, and stack overflows.
  • Trade-offs: simplicity vs. performance, extensibility vs. complexity, and potential for compilation vs. interpretation.

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