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Openai·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

Got a coding question for an ML Engineer role at OpenAI that was more software design than anything ML-related. Basically had to implement a small type system with nodes and functions and get the toString rendering right for nested types.

Questions Asked (1)

Q1

Design Node and Function classes to represent a toy type system, and implement toString so that primitive types print as their name, tuple types print as a parenthesized comma-separated list, and function types print as `(params) -> return_type`, all recursively expanded for nested types.

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

My first instinct was to just make everything a string field and call it done, which would've totally broken the recursive nesting.

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AI HintsAI Generated

Suggested Approach

Start by defining a base Node class with an abstract toString method, then create subclasses for primitive, tuple, and function types. Implement toString recursively in each subclass, ensuring proper formatting and handling of nested types. Discuss trade-offs such as using inheritance vs. composition and how to handle edge cases like empty tuples or functions with no parameters.

Pro tip: Mention that you would use the Composite pattern to treat individual types and compositions uniformly, and that you'd consider adding an equals method for type checking, which is crucial in ML type systems.

1. Define the base Node class

Create an abstract Node class with an abstract toString method to serve as the common interface for all types.

2. Implement primitive types

Create a Primitive class that extends Node, storing the type name and overriding toString to return the name.

3. Implement tuple types

Create a Tuple class that extends Node, storing a list of element types. Override toString to recursively call toString on each element and join with commas inside parentheses.

4. Implement function types

Create a Function class that extends Node, storing parameter types and a return type. Override toString to format as (param1, param2) -> returnType, recursively calling toString on all types.

5. Test and discuss trade-offs

Test with nested types (e.g., function returning a tuple) and discuss design choices like using inheritance vs. composition, and handling edge cases.

Key Points to Mention

  • Use of inheritance and polymorphism to define a common interface for all types.
  • Recursive toString implementation to handle nested types naturally.
  • Proper formatting: parentheses for tuples and functions, commas for tuple elements, and arrow for function types.
  • Edge cases: empty tuple, function with no parameters, and deeply nested types.
  • Trade-offs: inheritance vs. composition, and potential need for equals/hashCode for type equality.
  • Extensibility: how to add new type constructors (e.g., list, map) without modifying existing code.

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