← Openai Interview Insights

Openai·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

OpenAI ML Engineer interview that was basically a mini language implementation challenge. You build a tiny interpreter from scratch, handle variable assignment, arithmetic, and some control flow, then show what state the program ends up in. More CS fundamentals than ML, which threw me a bit.

Questions Asked (1)

Q1

Build a small interpreter for a toy language: support variable assignment, arithmetic expressions, and basic control flow, then return the resulting program state or output.

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

Spent the first few minutes overthinking the parser design when I should've just started with a simple recursive descent approach.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the toy language's syntax and semantics, then outline a classic interpreter pipeline: lexing, parsing to an AST, and tree-walking evaluation. Emphasize modular design and discuss trade-offs like tree-walking vs. bytecode compilation, especially in the context of ML systems where interpreters are used for expression graphs.

Pro tip: Connect the interpreter design to ML frameworks (e.g., how PyTorch/TensorFlow evaluate computation graphs) and mention that a tree-walking interpreter is often sufficient for small DSLs, but bytecode or JIT compilation can offer performance benefits for larger workloads.

1. Clarify Requirements and Scope

Ask about the language's features: what arithmetic operators, control flow constructs (if/while), and variable scoping rules are needed. Confirm the expected output (program state or printed values).

2. Design the Interpreter Architecture

Propose a modular pipeline: lexer -> parser -> AST -> evaluator. Discuss whether to use a tree-walking interpreter or compile to bytecode, and justify based on simplicity vs. performance.

3. Define the Grammar and AST

Sketch a simple grammar (e.g., using EBNF) and define AST node types for assignments, binary operations, if/while statements, and blocks. This ensures a clear separation between syntax and semantics.

4. Implement Evaluation with Environment

Describe how to evaluate expressions and statements using an environment (symbol table) for variable storage. Handle control flow by recursively evaluating blocks and conditions.

5. Discuss Trade-offs and Extensions

Compare tree-walking vs. bytecode compilation in terms of speed, memory, and complexity. Mention potential extensions like functions, closures, or integration with ML tensors.

Key Points to Mention

  • Lexical analysis and parsing techniques (e.g., recursive descent)
  • Abstract Syntax Tree (AST) design and traversal
  • Environment/symbol table for variable storage and scoping
  • Control flow implementation via recursive evaluation
  • Trade-offs between tree-walking and bytecode interpreters
  • Relevance to ML: interpreting computation graphs or DSLs for model definition

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