## The Recipes

### Design Recipe for Abstraction

- Are the function definitions similar? The answer is certainly
yes, if they are based on the same template. If not, reconsider
abstracting over them. -- Mark the differences in pairs and connect
the pairs via lines; we call those difference lines.
- Create a function definition that looks just like one of the
two. Give it a new distinct name. Add one parameter to the function
header per "difference lines". Also add those parameters to the
recursive calls. Use the parameters where the different expressions
used to be.
- Can you re-define the original functions using the abstraction you
created? Do so.
- Do the re-definitions pass the test suites for the original
functions? If not, you're in trouble.
- Develop a general contract for the abstraction.
State the purpose statement in terms of "..." terms. See page 313 in
How to Design Programs.

### Design Recipe for Structural Data

- What kinds of data are involved? Create data definitions. If
they look complex, construct examples according to the data
definitions, just to make sure they work.
- What kinds of data does the function consume? Which kind does it
produce? And what is its purpose? (in one line)
- Can you make up examples of inputs? What should the function
produce for these inputs?
- Let's construct the template:
- Does the data definition (of the main argument) mention clauses?
If so, use a cond with as many cases as there are clauses in the
data definition.
- How can you distinguish these kinds of data with conditions
involving the main parameter?
- Are structs involved? If so, write down all the selector
expressions. (Do so on a per-clause basis.)
- Does the data definition involve any self-references (or
cross-references)? If so, use recursion in the template to express
these "arrows".

- Let's code:
- Can you deal with the simple cond cases? Your examples should give
you some hints.
- What do the expressions in the recursive cases compute? Use the
purpose statement of the function to figure out what the recursive
function application computes.
- How can you combine the results of these expressions so that the
function returns the desired value?

- Did you turn the examples into tests? You may want to do this as
you develop functional examples.

Last modified: Wed Feb 7 22:10:37 EST 2007