Toy Language: Type Inference and AST

Also reported as Toy Language Compiler/Interpreter · Type Check · Infer Function Return Type

CodingOpenAILast reported July 2026High Frequency
Reported
34× across candidate reports
First seen
February 2025
Last reported
July 2026
Reported outcome
mixed

Problem Overview

OpenAI coding question, reconstructed from 34 candidate reports (February 2025 to July 2026). Tests: expression evaluation and interpreters; tree recursion and traversal. Candidates reported 3 follow-ups. The write-up includes a Python solution that passes 6 test cases.

  • 3 candidate-reported follow-ups — the probes candidates report being asked, with the trigger for each
  • The full problem statement — full requirements, constraints, and edge cases
  • Approach and trade-offs — what passing candidates did, and the mistakes that sink people
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Verified Source
Every question is reconstructed from multiple independent candidate reports. Follow-ups candidates reported, not invented ones.
Codex Fact-Checked
Technical claims, formulas, and scale numbers are reviewed against primary sources.
Interviewer Follow-ups
The follow-ups candidates reported, with the trigger that prompts each one — plus the mistakes that sink people.
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