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Early-stage Startup·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026Remote

Summary

Got a parsing and evaluation problem that looked manageable on the surface but turned into a rabbit hole pretty fast. The edit at the bottom about the AST says it all.

Questions Asked (1)

Q1

Given a rule expression as a string and a runtime context mapping variable names to values, implement a parser and evaluator that returns whether the rule is true or false. Support logical operators (AND, OR, NOT), parentheses, comparison operators, and literals including integers, quoted strings, and booleans.

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

This one wrecked me a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the grammar and requirements, then outline a two-phase approach: tokenization and parsing into an AST, followed by evaluation against the context. Discuss trade-offs between recursive descent and shunting-yard, and emphasize error handling and extensibility.

Pro tip: Mention that you'd write unit tests for edge cases like operator precedence and short-circuit evaluation early, and consider using a visitor pattern for evaluation to keep parsing and evaluation decoupled.

1. Clarify Requirements and Grammar

Ask about operator precedence, associativity, supported literals, and error handling expectations. Define a formal grammar (e.g., EBNF) to guide implementation.

2. Design Tokenizer and Parser

Implement a lexer to convert the input string into tokens, then a parser (e.g., recursive descent) to build an abstract syntax tree (AST). Handle parentheses and operator precedence.

3. Implement Evaluator

Traverse the AST to evaluate the expression using the provided context. Implement short-circuit evaluation for logical operators and type checking for comparisons.

4. Handle Errors and Edge Cases

Define behavior for syntax errors, unknown variables, type mismatches, and division by zero. Ensure the evaluator returns a boolean or throws meaningful exceptions.

5. Discuss Extensibility and Testing

Mention how to add new operators or literals, and outline a testing strategy covering precedence, associativity, and short-circuiting.

Key Points to Mention

  • Operator precedence and associativity rules (e.g., NOT > AND > OR, comparisons higher than logical).
  • Short-circuit evaluation for AND/OR to avoid unnecessary computations and side effects.
  • Use of recursive descent parsing for simplicity and readability, or shunting-yard for iterative parsing.
  • Type handling: comparing integers, strings, and booleans, and handling type mismatches gracefully.
  • Error handling strategies: syntax errors, unknown variables, and invalid operations.
  • Extensibility: designing the parser and evaluator to easily support new operators or functions.

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