Revised report on the algorithmic language Algol-68

e) CHOICE STYLE nishf34ag :

where (CHOICE) is (choice using boolean), STYLE f94f,-g token ; where (CHOICE) is (CASE), STYLE esacf94f,-g token.

f) NOTION token :

pragmentf92ag sequence option, NOTION symbolf94a,b,c,d,e,f,g,hg.

g)*token : NOTION tokenffg.

h)*symbol : NOTION symbolf94a,b,c,d,e,f,g,hg.

9.2. Comments and pragmats

f A source of innocent merriment. Mikado, W.S.Gilbert.g

fA pragment is a comment or a pragmat. No semantics of pragments is given and therefore the meaning (2.1.4.1.a) of any program is quite una ected by their presence. It is indeed the intention that comments should be entirely ignored by the implementation, their sole purpose being the enlightenment of the human interpreter of the program.

Pragmats may, on the other hand, convey to the implementation some piece of information a ecting some aspect of the meaning of the program which is not de ned by this Report, for example:

the action to be taken upon over ow (2.1.4.3.h) or if the scope rule is violated (as in 5.2.1.2.b), e.g., pr over ow check on pr, pr over ow check o pr, pr scope check on pr or pr scope check o pr;

the action to be taken upon completion of the compilation process, e.g., pr compile only pr, pr dump pr or pr run pr;

that the language to be implemented is some sublanguage or superlanguage of ALGOL 68, e.g., pr nonrec pr (for a routine-text which may be presumed to be non-recursive);

that the compilation may check for the truth, or attempt to prove the correctness, of some assertion, e.g.:

int a, b; read((a, b)) pr assert a 0 ^ b > 0 pr; int q := 0, r := a;

while r b pr assert a = b q + r ^ 0 < r pr do (q +:= 1, r -:= b) od

pr assert a = b q + r ^ 0 r ^ r < b pr .

They may also be used to convey to the implementation that the source text is to be augmented with some other text, or edited in some way, for example:

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some previously compiled portion of the particular-program is to be invoked, e.g., pr with segment from album pr;

the source text is continued on some other document, e.g., pr read from another le pr;

the end of the source text has been reached, e.g., pr nish pr.

The interpretation of pragmats is not de ned in this Report, but is left to the discretion of the implementer, who ought, at least, to provide some means whereby all further pragmats may be ignored, for example:

pr pragmats o pr .g

f pr algol 68 pr begin

proc pr nonrec pr pr = void: pr; pr

end

pr run pr pr ? pr

Revised Report on the Algorithmic Language ALGOL 68.g

9.2.1. Syntax

A) PRAGMENT :: pragmat ; comment.

a)pragmentf80a,91f,A341b,h,A348a,b,c,A349a,A34Abg : PRAGMENTfbg.

b)PRAGMENTfag : STYLE PRAGMENT symbolf94b,-g, STYLE PRAGMENT itemfcg sequence option, STYLE PRAGMENT symbolf94b,-g.

fSTYLE :: brief ; bold ; style TALLY.g

c) STYLE PRAGMENT itemfbg :

character glyphf814cg ; STYLE other PRAGMENT itemfdg.

d) A production rule may be added for each notion designated by STYLE other PRAGMENT item fc, for which no hyper-rule is given in this Reportg each of whose alternatives is a symbol f1.1.3.1.fg, di erent from any terminal production of character glyph f8.1.4.1.cg, and such that no terminal production of any STYLE other PRAGMENT item is the corresponding STYLE PRAGMENT symbol . fThus comment /c comment might be a comment, but /c /c /c could not.g

fExamples:

a) pr list pr /c source program to be listed /c c) l ? g

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9.3. Representations

a)A construct in the strict language must be represented in some \representation language" such as the \reference language", which is used in this Report. Other representation languages specially suited to the supposed preference of some human or mechanical interpreter of the language may be termed \publication" or \hardware" languages. fThe reference language is intended to be used for the representation of particular-programs and of their descendents. It is, however, also used in Chapter 10 for the de nition of the standard environment.g

b)A \construct in a representation language" is obtained from the terminal production T f1.1.3.2.fg of the corresponding construct in the strict language f1.1.3.2.eg by replacing the symbols in T by their representations, as speci ed in 9.4 below in the case of the reference language.

fThus, the strict-language particular-program whose terminal production is

bold begin symbol skip symbol bold end symbol gives rise to the reference language particular-program begin skip end.g

c) An implementation f2.2.2.cg of ALGOL 68 which uses representations which are su ciently close to those of the reference language to be recognized without further elucidation, and which does not augment or restrict the available representations other than as provided for below f9.4.a, b, cg, is an \implementation of the reference language". fE.g., begin, begin, BEGIN,

.beginy and 'begin' could all be representations of the bold-begin-symbol in an implementation of the reference language; some combination of holes in a punched card might be a representation of it in some hardware language.g

9.4. The reference language

a) The reference language provides representations for various symbols, including an arbitrarily large number of TAX-symbols fwhere TAX :: TAG ; TAB ; TAD ; TAM.g. The representations of some of them are speci ed below

y This is an intentional deviation from the text of the original Report, allowed because it is \pragmatic remark". All known implementions of ALGOL 68 use \POINT stropping", \UPPER stropping" or even \RES stropping" f, see [20]g, the latter in clear contradiction to the Report. These stropping regimes should be mentioned, at least. The examples drawn from real programming are therefore typeset in teletype font. The original proposes \single apostroph stropping" at this position. \Matched apostroph stropping", common in in ALGOL 60, seems not to have been used much.

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f9.4.1g, and to these may be added suitable representations for style-TALLY- letter-ABC-symbols and style-TALLY-monad-symbols and any terminal productions of STYLE other PRAGMENT item f9.2.1.dg and of other string item f8.1.4.1.dg. Representations are not provided for any of these fbut they enable individual implementations to make available their full character sets for use as characters, to provide additional or extended alphabets for the construction of TAGand TAB-symbols, and to provide additional symbols for use as operatorsg. There is not, however, fand there must not be,g except in representations of the standard-, and other, preludes f10.1.3.Step 6g, any representation of the letter-aleph-symbol or the primal-symbol. fFor the remaining TAX-symbols, see 9.4.2. There are, however, some symbols produced by the syntax, e.g., the brief-pragmat-symbol, for which no representation is provided at all. This does not preclude the representation of such symbols in other representation languages.g

b) Where more than one representation of a symbol is given, any of them may be chosen. Moreover, it is su cient for an implementation of the reference language to provide only one. Also, it is not necessary to provide a representation of any particular MONAD-symbol or NOMAD-symbol so long as those that are provided are su cient to represent at least one version f10.1.3.Step 3g of each operator declared in the standard-prelude.

fFor certain di erent symbols, one same or nearly the same representation is given; e.g., the representation \:" is given for the routine-symbol, the colon-symbol and the up-to-symbol and \:" for the label-symbol. It follows uniquely from the syntax which of these four symbols is represented by an occurrence, outside comments, pragmats and string-denotations, of any mark similar to either of those representations. It is also the case that \.." could be used, without ambiguity, for any of them, and such might indeed be necessary in implementations with limited character sets. It may be noted that, for such implementations, no ambiguity would be introduced were \(/" and \/)" to be used as representations of the style-ii-sub-symbol and the style-ii-bus- symbol, respectively.

Also, some of the given representations appear to be composite; e.g., the representation \:=" of the becomes-symbol appears to consist of \:", the representation of the routine-symbol, etc., and \=", the representation of the equals-symbol and of the is-de ned-as-symbol. It follows from the syntax that \:=" can occur, outside comments, pragmats and string-denotations, as a representation of the becomes-symbol only (since \=" cannot occur as the representation of a monadic-operator). Similarly, the other given composite representations do not cause ambiguity.g

c) The fact that the representations of the letter-ABC-symbols given

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f9.4.1.ag are usually spoken of as small letters is not meant to imply that the corresponding capital letters could not serve equally well. fOn the other hand, if both a small letter and the corresponding capital letter occur, then one of them is presumably the representation of some style-TALLY-letter-ABC- symbol or of a bold-letter-ABC-symbol. See also 1.1.5.b for the possibility of additional ABC s in a variant of the language.g

d) A \typographical display feature" is a blank, or a change to a new line or a new page. Such features, when they appear between the symbols of a construct in the reference language, are of no signi cance and do not a ect the meaning of that construct. However, a blank contained within a stringor character-denotation is one of the representations of the spacesymbol f9.4.1.bg rather than a typographical display feature. Where the representation of a symbol in the reference language is composed of several marks fe.g., to, :=g, those marks form one findivisibleg symbol and, unless the contrary is explicitly stated f9.4.2.2.a, cg, typographical display features may not separate them.

9.4.1. Representations of symbols

a)Letter symbols

B)LETTERfAg :: letter ABCf94ag ; letter alephf-g ; style TALLY letter ABCf-g.

C)DIGITfAg :: digit zerof94bg ; digit onef94bg ; digit twof94bg ;

digit threef94bg ; digit fourf94bg ; digit vef94bg ;

digit sixf94bg ; digit sevenf94bg ; digit eightf94bg ; digit ninef94bg.

D)TABf48a,bg :: bold TAGfA,-g ; SIZETY STANDARDfEg.

E)STANDARDfDg :: integralf94eg ; realf94eg ; booleanf94eg ;

characterf94eg ; formatf94eg ; voidf94eg ; complexf94eg ; bitsf94eg ; bytesf94eg ; stringf94eg ; semaf94eg ; lef94eg ; channelf94eg.

F)TADf48a,bg :: bold TAGfA,-g ; DYADfGg BECOMESETYfJg ; DYADfGg cum NOMADfIg BECOMESETYfJg.

G)DYADfFg :: MONADfHg ; NOMADfIg.

H)MONADfG,Kg :: orf94cg ; andf94cg ; ampersandf94cg ;

di ers fromf94cg ; is at mostf94cg ; is at leastf94cg ; overf94cg ; percentf94cg ; windowf94cg ; oorf94cg ; ceilingf94cg ;

plus i timesf94cg ; notf94cg ; tildef94cg ; downf94cg ; upf94cg ; plusf94cg ; minusf94cg ; style TALLY monadf-g.

I)NOMADfF,G,Kg :: is less thanf94cg ; is greater thanf94cg ; divided byf94cg ; equalsf94cg ; timesf94cg ; asteriskf94cg.

J)BECOMESETYfF,Kg :: cum becomesf94cg ; cum assigns tof94cg ; EMPTY.

K)TAMf48a,bg :: bold TAGfA,-g ; MONADfHg BECOMESETYfJg ; MONADfHg cum NOMADfIg BECOMESETYfJg.

L) ABCfBg :: a ; b ; c ; d ; e ; f ; g ; h ; i ; j ; k ; l ; m ; n ; o ; p ; q ; r ; s ; t ; u ; v ; w ; x ; y ; z.

M) *DOP :: DYADfGg ; DYADfGgcum NOMADfIg.

fThe metanotion \ABC" is provided, in addition to the metanotion \ALPHA", in order to facilitate the de nition of variants of ALGOL 68 (1.1.5.b).g

9.4.2.2. Representation

a) The representation of each TAG-symbol not given above f9.4.1g is composed of marks corresponding, in order, to the LETTER s or DIGIT s contained in that TAG . These marks may be separated by typographical display features f9.4.dg. The mark corresponding to each LETTER ( DIGIT ) is the representation of that LETTER-symbol (DIGIT-symbol). fFor example, the

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