SICP Exercise 4.3 eval using data-directed style

Exercise 4.3.  Rewrite eval so that the dispatch is done in data-directed style. Compare this with the data-directed differentiation procedure of exercise 2.73. (You may use the car of a compound expression as the type of the expression, as is appropriate for the syntax implemented in this section.).

 SOLUTION

 The code and tests are here. 

 Note: The tests demonstrate that the data-directed dispatch happens correctly. Ignore the subsequent (environment related) errors for now. (These will be corrected in the upcoming exercises.)

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