Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Apr 2026Remote

Summary

Got a coding round for a Software Engineer role at OpenAI that was pretty deep into type system territory. The core problem was implementing type inference for a toy language, which sounds manageable until you're actually in it and realize how many edge cases there are.

Questions Asked (1)

You have a Node class representing types in a small toy language. Nodes can be primitives (char, int, float), generic placeholders (T1, T2, etc.), or tuples of other Nodes. Implement an infer_return(input_type, actual_type) function that walks both type trees simultaneously, records what concrete types each generic maps to, raises an error on conflicting bindings or structural mismatches, and returns the final generic-to-type mapping. Then discuss how you'd extend this to nested generics and use the result to resolve a return type.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

I started okay, got the basic tree walk going and handled the primitive case fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the Node structure and the inference goal, then outline a recursive simultaneous traversal that builds a mapping while checking for conflicts and structural mismatches. After implementing the core algorithm, discuss extensions for nested generics and how the mapping resolves return types, emphasizing error handling and edge cases.

Pro tip: Mention that you'd use a union-find or substitution map to handle transitive bindings and avoid infinite loops, and that you'd write unit tests for conflicting bindings and nested tuples to ensure robustness.

1. Clarify assumptions and define data structures

Confirm the Node representation (e.g., primitives, generics, tuples) and define a mapping structure (e.g., dictionary) to store generic-to-type bindings. Discuss error types for conflicts and mismatches.

2. Design recursive inference algorithm

Outline a recursive function that compares input_type and actual_type: if both are primitives, check equality; if input is generic, bind or verify; if both are tuples, recurse element-wise; otherwise raise structural mismatch.

3. Handle conflicts and structural mismatches

Explain how to detect conflicting bindings (e.g., T1 bound to int and float) and raise an error. Also handle cases where tuple lengths differ or node kinds don't match.

4. Extend to nested generics and return type resolution

Discuss how the same recursive approach naturally handles nested generics (e.g., T1 inside a tuple). Then show how to use the final mapping to substitute generics in a return type template to get a concrete type.

5. Discuss edge cases and optimizations

Mention handling of repeated generics, recursive types, and performance considerations. Suggest using memoization or union-find for efficiency in large type trees.

Key Points to Mention

Recursive simultaneous traversal of two type trees
Maintaining a mapping from generic placeholders to concrete types
Conflict detection when a generic is bound to different types
Structural mismatch handling (e.g., tuple vs primitive, different tuple lengths)
Substitution of generics in return type using the final mapping
Handling nested generics and potential recursive types

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