Openai·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Interviewed for a software engineering role at OpenAI and got a type-system design question that felt more like a PL theory exercise than a typical coding screen. Interesting problem but I was not fully prepared for the data structure angle.

Questions Asked (1)

Design a Node class for a small toy language's type system that supports primitives (char, int, float), generic placeholders (T1, T2, ...), and tuples of Nodes. Implement constructors for each variant and a toString function that produces canonical string output like '(int, T1, (float, T2))'. The design should also make type inference easier down the line.

System DesignTechnical Trade-offsData Modeling

Author's notes

I spent the first few minutes just trying to figure out whether to use a tagged union or subclassing.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then propose a class hierarchy or tagged union design that cleanly separates the three variants. Emphasize how the design supports canonical string output and future type inference, and discuss trade-offs between simplicity and extensibility.

Pro tip: Mention that using a sealed class or tagged union with pattern matching makes adding new node types and implementing visitors (like type inference) easier, and that canonical toString should be deterministic and unambiguous.

1. Clarify Requirements and Constraints

Ask about the expected operations, performance needs, and whether the type system will be extended. Confirm that toString must produce canonical output and that type inference is a future goal.

2. Choose a Representation

Decide between a class hierarchy (inheritance) or a tagged union (e.g., sealed class in Kotlin/Scala, or a struct with a tag in C). Discuss trade-offs: inheritance is extensible but can lead to fragile base class; tagged union is simple but adding variants requires modifying all functions.

3. Define the Node Class and Constructors

Implement the Node class with variants for primitives (char, int, float), generic placeholders (T1, T2, ...), and tuples (list of Nodes). Provide constructors or factory methods for each variant, ensuring immutability.

4. Implement Canonical toString

Write a toString method that recursively produces the canonical string, e.g., '(int, T1, (float, T2))'. Ensure no extra spaces and consistent formatting.

5. Discuss Extensibility for Type Inference

Explain how the design supports type inference, e.g., by allowing pattern matching or visitor patterns to traverse the tree. Mention that generic placeholders can be unified and tuples can be decomposed.

Key Points to Mention

Use of sealed classes or tagged unions to represent variants safely and enable exhaustive pattern matching.
Immutability of Node instances to simplify reasoning and support persistent data structures.
Canonical toString implementation with recursion and proper handling of nested tuples.
Trade-offs between inheritance and tagged unions for extensibility and maintainability.
How the design facilitates type inference algorithms like unification or constraint solving.
Potential need for a visitor pattern or fold function to traverse and transform the type tree.

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