6.001 Structure and Interpretation of Computer Programs. Copyright o 2004 by Massachusetts Institute of Technology Slide 15.2.15 Eval once again checks the type of this expression, deducesArithmetic calculator it is a number, and just returns that expression, literally a p1us*24(p1us*56}) pointer to that thing which is an internal representation for the What are the argument and return values of eval each number 24 time it is called in the evaluation of ine 17? (424)24 W业1让(P124@L让+56 A'(p1u+24(1u+56) 1. Arithmetic calculator Slide 152.16 4(p1us*56}》) Next, eval has to evaluate the second subexpression, so this is the eval of the expression shown at top right. As before he argument and return values of eval each time it is called in the evaluation of line 17? we are going to dispatch on type, i. e. check to see what kind of beast"this is, deduce that it is a"sum"and therefore pass this on to the right procedure to handle sums Wa124 Slide 15.217 Arithmetic calculator Once more, eval-sum will reduce to applying the addition p1us*24(p1ust56》 operation to whatever it gets by evaluating the subpieces. Thus, we need to extract the subexpressions and once again apply What are the argument and return values of eval each time it is called in the evaluation of line 17? eval to them. Notice the nice recursive unwinding that is going on here a1u'(plus*56》) sva124)24 1-到让鼻'(Pu*241u*56) 1. Arith metic calculator Slide 15.2.18 p1us*24(p1us*56》) ell this just unwraps one more time. Again, we will apply What are the argument and return values of eval each to whatever we get by evaluating the two pieces, and eval in he evaluation of line 17? both cases just dispatches on type, determines the expression is a number and returns the expression as the value (ea15)5a66 va1-积‘(Plu56)) (42424(c4(1us56)) A1让'(P1*24@1划+56 wA1'(P1uB+24(u*56)}6.001 Structure and Interpretation of Computer Programs. Copyright © 2004 by Massachusetts Institute of Technology. Slide 15.2.15 Eval once again checks the type of this expression, deduces it is a number, and just returns that expression, literally a pointer to that thing which is an internal representation for the number 24. Slide 15.2.16 Next, eval has to evaluate the second subexpression, so this is the eval of the expression shown at top right. As before, we are going to dispatch on type, i.e. check to see what kind of "beast" this is, deduce that it is a "sum" and therefore pass this on to the right procedure to handle sums. Slide 15.2.17 Once more, eval-sum will reduce to applying the addition operation to whatever it gets by evaluating the subpieces. Thus, we need to extract the subexpressions and once again apply eval to them. Notice the nice recursive unwinding that is going on here. Slide 15.2.18 Well this just unwraps one more time. Again, we will apply + to whatever we get by evaluating the two pieces, and eval in both cases just dispatches on type, determines the expression is a number and returns the expression as the value