Overview
This lecture covered the design recipe for creating functions in BSL (Beginning Student Language), focusing on structured steps for defining, documenting, and testing functions. The class also included clicker questions to reinforce evaluation rules and function design principles.
Clicker Questions and Evaluation Rules
- Early questions tested syllabus knowledge and function evaluation rules in BSL.
- BSL evaluates arguments left to right and substitutes values in the function body.
- Arithmetic and conditional expressions are stepped through based on evaluation rules.
- "if" expressions evaluate the question part first; only the relevant branch executes.
- "and" expressions stop evaluation when a false value is encountered.
Introduction to the Design Recipe
- The design recipe provides a step-by-step process to design reliable functions.
- Steps include intent statement, signature, purpose, stub, examples, template, and function body.
- Writing metadata (e.g., @htdf, @signature) documents intent and supports autograding.
- The recipe helps students learn habits crucial for solving complex problems later.
Steps in the Design Recipe
- Intent Statement: Declare the function being designed (e.g., @htdf topple).
- Signature: Specify input and output types (e.g., Image → Image).
- Purpose Statement: Describe function’s behavior in one line.
- Stub: Provide a placeholder function returning a value of the correct output type.
- Examples & Tests: Write check-expect statements showing expected behavior for different inputs.
- Template & Inventory: Use @template-origin and a skeletal version of the function to scaffold the body.
- Function Body: Complete the function using prior steps and tests as guidance.
Examples of Function Design
- Designed
topple, a function that rotates an image by 90° using image primitives.
- Wrote multiple
check-expect tests to clarify expected function output, especially edge cases.
- Discussed the importance of writing enough tests to distinguish correct from incorrect implementations.
Conventions and Best Practices
- Use question marks in predicate function names (e.g., tall?).
- Boolean output functions always end with a question mark.
- Add enough test cases, especially around boundaries, to specify behavior unambiguously.
- Maintain proper formatting and refer to the course style guide for code structure.
- Keep stubs and templates commented out as you progress.
Working with Metadata and Templates
- Metadata tags (e.g., @htdf, @signature, @template-origin) help describe and structure code.
- Templates provide a reusable starting point for the function body based on input type.
- As course progresses, templates will encapsulate more complex structures.
Key Terms & Definitions
- BSL (Beginning Student Language) — Teaching language used in Dr. Racket for this course.
- Design Recipe — Structured process for function design: intent, signature, purpose, stub, examples, template, implementation.
- Stub — Placeholder function returning a value of the correct output type.
- Template — Skeleton or partial implementation of a function, facilitating easier completion.
- check-expect — BSL construct for specifying and checking function output against expected results.
- Predicate — Function producing a boolean (true/false) result, named with a question mark.
Action Items / Next Steps
- Complete post-lecture exercises and submit using the provided starter files.
- Review BSL evaluation rules and function design recipe.
- Refer to the course style guide for code formatting.
- Ensure you are up to date with clicker setup and solve any technical issues via office hours or Piazza.
- Check grader feedback and fix errors in function signatures, tests, or templates before submission.