Algol 68 - ACM Digital Library

92 van Wijngaarden, et al.ALGOL 68 Revised Report93The relevant production tree (with elision of "NEST',"with TAG', and with invisible subtrees omitted) is:Iwidening coerciondereferencing coercion6.1.1. SyntaxA)B)C)D)E)F)G)a)b)c)d)e)f)g)h)'strong real identifier coercee'I 6.1.1.a* 'widened to real identifier'I 6.5.1.a'dereferenced to integral identifier'I 6.2.1.a'applied' and[ 'unchanged from reference to integral identifier'[ 6.1.l.f'reference to integral identifier' (coercend)[ 4.S.l.b, 9.1.f I'letter i symbol' .}STRONG{a,66a} :: FIRM{B} ; widened to{65a,b,c,d} ; rowed to{66a} ;voided to{67a,b}.FIRM{A,b} :: MEEK{C} ; united to{64a}.MEEK{B,c,d,62a,63a,64a,65a,b,c,d} :: unchanged from{f} ;dereferenced to{62a} ; deprocedured to{63a}.SOFT{e,63b} :: unchanged from{f} ; softly deprocedured to{63b}.FORM :: MORF ; COMORF.MORF :: NEST selection ; NEST slice ; NEST routine text ;NEST ADIC formula ; NEST call ;NEST applied identifier with TAG.COMORF :: NEST assignation ; NEST identity relation ;NEST LEAP generator ; NEST cast ; NEST denoter ;NEST format text.strong MOlD FORM coercee{5A,B,C,D,A341i} :where (FORM) is (MORF), STRONG{A} MOlD MORF ;where (FORM) is (COMORF), STRONG{A} MOlD COMORF,unless (STRONG MOLD) is (deprocedured to void).firm MODE FORM coercee{5A,B,C,D,542c} : FIRM{B} MODE FORM.meek MOlD FORM coercee{5A,B,C,D} : MEEK[C} MOlD FORM.weak REFETY STOWED FORM coercee{5A,B,C,D} :MEEK{C} REFETY STOWED FORM,unless (MEEK) is (dereferenced to)and (REFETY) is (EMPTY).soft MODE FORM coercee{5A,B,C,D} : SOFT{D} MODE FORM.unchanged from MOlD FORM{C,D,67a,b} : MOlD FORM.* SORT MOID coercee : SORT MOID FORM coercee{a,b,c,d,e}.* MOID coercend : MOID FORM.{Examples:a) 3.14 (in x := 3.14)b) 3.14 (in x + 3.14)c) sin (in sin (x))d) xl (in xl [2] := 3.14)e) x (in x := 3.14) }{For 'MOLD FORM" (rule f), see the cross-references inserted insections 5.I.A,B,C,D before "coercee". Note, however, that a 'MOLD FORM'may be a blind alley. Blind alleys within this chapter are not indicated.}{There are five sorts of syntactic position. They are:• "strong" positions, i.e., actual-parameters, e.g., x in sin(x), sources;e.g., x in y:=x, the ENCLOSED-clause of a cast, e.g., (nil) inref book (nil), and statements, e.g., y := x in (y : = x; x : = 0);• "firm" positions, i.e., operands, e.g., x in x+y;• "meek" positions, i.e., enquiry-clauses, e.g., x>O in (x~Olx[O),boundscripts, e.g., i in xl [i], and the PRIMARY of a call, e.g., sin insin (x);• "weak" positions, i.e., the SECONDARY of a selection and thePRIMARY of a slice, e.g., xl in xl [i];• "soft" positions, i.e., destinations, e.g., x in x := y and one of theTERTIARYs of an identity-relation, e.g., x in xx :=: x.Strong positions also arise in balancing (3.2.1.e).In strong positions, all six coercions may occur; in firm positions,rowing, widening and voiding are forbidden; in meek and weak positions,uniting is forbidden also, and in soft positions only deproceduring isallowed. However, a dereferenccd-to-STOWED-FORM may not be directlydescended from a weak-STOWED-FORM-coercee (rule d) for, otherwise,x :=xl [i] would be syntactically ambiguous (although, in this case, notsemantically). Also, a deprocedured-to-void-COMORF may not be directlydescended from a strong-void-COMORF-coercee (rule a) for, otherwise,(proc void engelfriet; proc void rijpens = skip; engelfriet : = rijpens; skip)would be ambiguous.}6.2. Dereferencing{Dereferencing serves to obtain the value referred to by a name, as inx:=y, where y yields a name referring to a real number and it is thisnumber which is assigned to the name yielded by x. The a priori mode ofy, regarded as a coercend, is 'reference to real' and its a posteriori mode,when y is regarded as a coercee, is 'real'.}6.2.1. Syntaxa) dereferenced to{61C} MODEl FORM :MEEK{61C} REF to MODE2 FORM,where MODE2 deflexes to MODEl{47a,b,c,-}.