Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

OpenAI SWE coding round, one problem the whole session: build a type inference engine for a toy language. Sounds manageable until you're staring at nested generics and trying to remember how unification actually works under pressure.

Questions Asked (1)

Implement a type inference system for a toy language that supports primitives, generics, nested tuples, and function signatures. Start by implementing string representations for Node and Function types, then write a function that resolves a function's return type given concrete argument types by unifying generics and substituting them into the output type.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This one took me longer than I expected to get fully right.

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AI HintsAI Generated

Suggested Approach

Start by defining the type system with a clear recursive data structure for types, including primitives, generics, tuples, and functions. Implement string representations for Node and Function types to aid debugging. Then, design a unification algorithm that takes a function signature and concrete argument types, resolves generics by matching parameter types, and substitutes the resolved generics into the return type.

Pro tip: Emphasize the importance of handling nested generics and ensuring the unification algorithm is robust against edge cases like arity mismatches and recursive types. Mention that you would write unit tests for each component to validate correctness incrementally.

1. Define the Type System

Create a recursive type representation that includes primitives, generic type variables, tuples, and function types. This forms the foundation for all operations.

2. Implement String Representations

Write toString methods for Node and Function types to produce human-readable output, which is crucial for debugging and testing.

3. Design Unification Algorithm

Develop a unification function that matches a function's parameter types against concrete argument types, building a substitution map for generics.

4. Resolve Return Type

Apply the substitution map to the function's return type, recursively substituting generics to produce the concrete return type.

5. Test and Validate

Write test cases covering primitives, generics, nested tuples, and functions to ensure the inference works correctly and handles edge cases.

Key Points to Mention

Recursive data structures for types to handle nesting
Unification algorithm with substitution maps for generics
Handling of nested generics and tuples in substitution
Error handling for mismatched types or arity
Importance of string representations for debugging
Incremental testing and validation of each component

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