Object models for computer aided design

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61 :􁪽􋴚􁪽􋴵 5;E:e􁪽􋴚 􁪽􋴚􁪽􋵬ll be de􁪽􋴚􁪽􋴚􁪽􋵥ed in this chapter as an exte􁪽􋴵5ion to the Frcqra􁪽􋴚􁪽􋵩ing language Pascal [7) • Pascal 􁪽􋵡as c􁪽􋵯osen be;ause it is fairly 􁪽􋵥ell known and because it alrE􁪽􋴚􁪽􋵙Y 􁪽􋴡!5 rr2􁪽􋵹y cf the data 5tructurin􁪽􋵦 facilities we will Fascal also h.akes an i􁪽􋵰portant distinction betweer. 􁪽􋵥e􁪽􋴺:􁪽􋵩i􁪽􋴚􁪽􋵴 the type of a structure and declaring instances of tho2e tY?ES as variables. 􁪽􋵨his distinction is useful for 􁪽􋵯o􁪽􋵅El:ng 5i􁪽􋵣ce the model defi􁪽􋵩ition can si􁪽􋵰ply be viewed as an exte:􁪽􋴚􁪽􋵬l ty?e definition 􁪽􋵨hich is glocal to all of the application pro􁪽􋵲rams which make 􁪽􋵰p the system. The choice of a host proqrarr􁪽􋵩i􁪽􋵧g 13n?uage is 􁪽􋵨hen other considerations dictate the choice of a less 􁪽􋵡ata structure sup?ort as ?art of the implementation. This :5 the case, for Exarrple, in the Fortran based systerr described in Appendix I. Although lacking the elegance and c:a=􁪽􋵴ty 􁪽􋵨hat could be ac􁪽􋴚􁪽􋵥eved using i mere suitable host lan?􁪽􋵡age, this Fortran based implerrentaticn serves both as a ve􁪽􋵩icle for testing the consistency of these ideas and as a ?ractical, working 􁪽􋵣cdeli􁪽􋴿? systerr.