<aside> 💡 The core idea of the HtDD recipe is this:

When identified, the structure of the information gives us the structure of the data representing this information, which gives us the structure of a function processing this data, and guides the creation of tests (check-expect) for the function.

∴ remember that identifying the structure of the information in the problem domain is a crucial first step in systematic program design.

</aside>

cond expressions

• The cond keyword is a multi-armed conditional — it’s a highly readable way to run multiple conditionals in parallel (like an if-elif-else structure in Python).

• Its structure is as follows:

  (cond [(condition) result] ; if
  			[(condition) result] ; elif
  			[(condition) result] ; elif
  			...
  			[else (answer)]) ; else clause is optional
  

• If the topmost condition evaluates to false, it gets “deleted” and we move on to the next statement. This process repeats until 1 condition evaluates to true. When that happens, the result corresponding to the true condition is returned.

  (cond [(> 1 2) "bigger"]
  			[(= 1 2) "equal"]
  			[(< 1 2) "smaller"])
  ---------------------------------------------------------------------------
  (cond [false "bigger"] ; false -> condition will be deleted
  			[(= 1 2) "equal"]
  			[(< 1 2) "smaller"])
  ---------------------------------------------------------------------------
  (cond [(= 1 2) "equal"]
  			[(< 1 2) "smaller"])
  ---------------------------------------------------------------------------
  (cond [false "equal"] ; false -> condition will be deleted
  			[(< 1 2) "smaller"])
  ---------------------------------------------------------------------------
  (cond [(< 1 2) "smaller"])
  ---------------------------------------------------------------------------
  (cond [true "smaller"]) ; true -> result will be returned
  ---------------------------------------------------------------------------
  "smaller"
  

Data definitions

• Data definitions are parts of programs which describe

  1. how data of a new data type is formed
  2. how information in the problem domain (the scope of the actual problem outside the program) can be represented as data in the program
  3. (inversely,) how data in the program can be interpreted into information in the problem domain
  4. a template which can be used for operating on the newly-defined data.

• Data definitions make programs simpler to understand and simplifies the design of functions involving the newly-defined data.