P59. I still don't think it is necessary to use "square" instead of (* x x) everywhere we perform this operation, but I appreciate the idea of "abstraction". It is really a good idea (and practice) to abstract concepts in different levels and use them accordingly.
Most of the examples I read are in a top-down design manner: firstly use the abstract concepts, and then implement it in a lower level. The concerns for specific lower-level modules is similar to the way I did using unit test. But I am a little bit confuse how to perform the unit test in this manner. Especially for the closure using in LISP--implementing local helper functions right after building major function-- how can I make sure the helper functions are right before I run the program as a whole? If I separate a helper function as an independent one, it seems that quite a little redundant work has to perform when I move it back to the major function.
Anyway, SICP just makes me love Racket even more =D
cheers,
Days of our lives
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