ramsey@NCoast.ORG (Cedric Ramsey) (08/07/90)
I am curious about the K&R2 grammar for a declaration. The grammar for a declaration is as follows (section A8 pg. 210): declaration: declaration-specifiers init-declarator-list_opt declaration-specifiers: storage-class-specifier declaration-specifers_opt type-specifier declaration-specifier_opt type-qualifier declaration-specifier_opt I will not write the grammer for the other productions because they can be obtained from the book. Anyway, my concern is this. The production doesn't specify any precedence or order for what should be seen first. For instance, according to this grammar, the following declarations: const char static i; or static const char i; or static char const i; All would be valid. I don't recall reading anywhere in the manual a mention of how the precedence is to be taken. I guess a programmer would have to use his or her common sense and write: static const char i = 0xHH;. You guys with ansi compilers can you get away with those ,strang ?, kind of declarations. However, the ansi standard is clear on other precedence rules. Take expressions, for example just by looking at the grammar you can see that && has higher precedence than || and * higher than +. The semantic rules talk about how type specifiers may specified with a type qualifier and the semantics for the const/volatile qualifiers but does not say which should come first. I came to the conclusion that their grammer is not exactly Backus-Naur form. What do you guys think. Please no flames, just thrill me!
diamond@tkou02.enet.dec.com (diamond@tkovoa) (08/07/90)
In article <1990Aug7.012657.8398@NCoast.ORG> ramsey@NCoast.ORG (Cedric Ramsey) writes: >The grammar for a declaration is as follows (section A8 pg. 210): >declaration: declaration-specifiers init-declarator-list_opt >declaration-specifiers: storage-class-specifier declaration-specifers_opt > type-specifier declaration-specifier_opt > type-qualifier declaration-specifier_opt >The production doesn't specify any precedence Precedence is not really a concern, because ALL of the input storage-class-specifiers, type-specifiers, and type-qualifiers apply to the same init-declarator-list. However, if you mean which one is closest to the init-declarator-list, bound by a node in the syntax tree most closely located to the leaves, it is the storage-class-specifier, type-specifier, or type-qualifier which appears closest to the init-declarator-list in the input, i.e. the rightmost one. >or order for what should be seen first. The grammar says that any of them may be seen first. >For instance, according to this grammar, the following declarations: >const char static i; or >static const char i; or >static char const i; >All would be valid. Yes, they are all valid. >I don't recall reading anywhere in the manual a mention >of how the precedence is to be taken. I guess a programmer would have to use >his or her common sense and write: static const char i = 0xHH;. const static char i = 0xHH; has exactly the same effect. Common sense is needed for another human to read your program, not for a compiler to process it. >You guys with ansi compilers can you get away with those ,strang ?, kind of >declarations. Pre-ANSI compilers didn't all have const, but the static and the char could be in either order. ANSI suggests that a later ANSI standard might require the static to go first. >However, the ansi standard is clear on other precedence rules. Take >expressions, for example just by looking at the grammar you can see >that && has higher precedence than || and * higher than +. The grammar specifies which operators go closest to the leaves of the syntax tree. Some wording in the standard must suggest that the operations appear to be executed in some precedence that corresponds to the syntax tree. >The semantic rules talk about how type specifiers may specified with a >type qualifier and the semantics for the const/volatile qualifiers but >does not say which should come first. It says that any of them should come first. It also says that a = b + c; and a = c + b; are both legal statements. It doesn't require the b and c to be in any particular order. Does this still bother you? >I came to the conclusion that their grammer is not exactly Backus-Naur form. Backus-Naur form uses <non-terminals>, terminals, and ::=. For example: <declaration> ::= declaration-specifiers init-declarator-list <declaration> ::= declaration-specifiers <declaration-specifiers> ::= <storage-class-specifier> <declaration-specifers> <declaration-specifiers> ::= <storage-class-specifier> <declaration-specifiers> ::= <type-specifier> <declaration-specifers> <declaration-specifiers> ::= <type-specifier> <declaration-specifiers> ::= <type-qualifier> <declaration-specifers> <declaration-specifiers> ::= <type-qualifier> Other constructs can carry exactly the same meaning, using more readable notations. Backus-Naur form does not require a grammar to be context-free, to be unambiguous, or to have your strange property. I don't think anyone has ever named a kind of grammar that, for each valid input string, would prevent all other permutations of that string from being valid. And I'm sure that no one has named a kind of grammar that does that only for some parts of some input strings. -- Norman Diamond, Nihon DEC diamond@tkou02.enet.dec.com This is me speaking. If you want to hear the company speak, you need DECtalk.
steve@taumet.com (Stephen Clamage) (08/07/90)
ramsey@NCoast.ORG (Cedric Ramsey) writes: | I am curious about the K&R2 grammar for a declaration... |...according to this grammar, the following declarations: |const char static i; or |static const char i; or |static char const i; |All would be valid. That's right. They are. -- Steve Clamage, TauMetric Corp, steve@taumet.com
henry@zoo.toronto.edu (Henry Spencer) (08/08/90)
In article <1990Aug7.012657.8398@NCoast.ORG> ramsey@NCoast.ORG (Cedric Ramsey) writes: >... The production doesn't >specify any precedence or order for what should be seen first. For instance, >according to this grammar, the following declarations: > >const char static i; or >static const char i; or >static char const i; > >All would be valid... They are indeed all valid. Furthermore, they all mean exactly the same thing, so there is no question of "precedence" involved. Many people would consider only the middle one to be good style, mind you, and ANSI has hinted that the first one might become illegal eventually (i.e. the storage class would be required to be first). -- The 486 is to a modern CPU as a Jules | Henry Spencer at U of Toronto Zoology Verne reprint is to a modern SF novel. | henry@zoo.toronto.edu utzoo!henry