Signatures and grammars Signatures and grammars Why manual - - PowerPoint PPT Presentation

Signatures and grammars

• Why manual disambiguation in SDF?

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• Fully declarative definition of syntax
• Implicit disambiguation can be wrong
• More programming languages can be parsed
• More programming languages can be parsed
• Modularity
• Fewer non-terminals/sorts
• Separation of concerns: form of rules is independent of

disambiguation

• Costs?
• Intellectual effort
• False safety, grammar may still be ambiguous

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Signatures and grammars

• Associativity of binary operators

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• Priorities between binary operators
• Transitivity of priorities
• Prefix expressions and binary expressions
• Postfix expression with binary operator (slide 3+4)
• Overloaded comma’s in expression languages (slide

Overloaded comma s in expression languages (slide 5+6)

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Signatures and grammars

module Expressions exports sorts E lexical syntax [\ \t\n] -> LAYOUT context-free start-symbols E context-free syntax "e" -> E E "[" E "]" -> E E "+" E -> E {left} Input sentence: e + e [ e + e ]

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Signatures and grammars

module Expressions t

• The "< >" notation is used to

exports sorts E lexical syntax [\ \t\n] -> LAYOUT

restrict the filtering behavior

• f priorities to certain

arguments.

[\ \t\n] > LAYOUT context-free start-symbols E context-free syntax "e" -> E

• In this case, if a "+" is a direct

child of the first "E" in the "[ ]" production, it is filtered.

context-free priorities E "[" E "]" -> E <0> > E "+" E > E {l ft}

• No other direct children are

filtered but the ones listed between the angular brackets.

E "+" E -> E {left}

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module Expressions exports sorts E lexical syntax [\ \t\n] -> LAYOUT context-free start-symbols E context-free syntax "e" -> E "f" "(" {E ","}+ ")" -> E E "," E -> E {left} Input sentence: f(e,e)

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module Expressions exports

• The priority definition reveals

exports sorts E lexical syntax [\ \t\n] -> LAYOUT t t f t t b l E

some of the implementation details of SDF, by showing

• ne of the productions that

are automatically generated

context-free start-symbols E context-free syntax "e" -> E "f" "(" {E ","}+ ")" -> E

are automatically generated for you.

• There is no way around this,

unless you would like to

context-free priorities { non-assoc: E -> {E “,”}+ E "," E -> E {left} }

unless you would like to remove the comma separated argument list and use the binary comma operator binary comma operator instead for parsing your commas.

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Signatures and grammars

• Longest match

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• Runaway whitespace and comments
• Syntactic overloading between identifiers and

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• Useless cloning of identifier classes
• Dynamically reserved types
• Dynamically reserved types
• Dangling else and related ambiguities

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Signatures and grammars

• Runaway whitespace and comments

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module Layout exports exports sorts P S context-free start-symbols P lexical syntax lexical syntax [\ \t\n] -> LAYOUT context-free syntax context-free syntax "[" S* "]" -> P "s" -> S

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Signatures and grammars

• These trees have typical symptoms:

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• a nullable symbol (for example a star list), which

recognizes the empty language

• two layout nodes surrounding the nullable non terminal
• two layout nodes surrounding the nullable non-terminal

take turns in accepting the layout.

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Signatures and grammars

module Layout exports sorts P S context-free start-symbols P lexical syntax [\ \t\n] -> LAYOUT "%" ~[\%]* "%" -> LAYOUT context-free restrictions LAYOUT? -/- [\ \t\n] LAYOUT? -/- [\%] context-free syntax "[" S* "]" -> P "s" -> S

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Signatures and grammars

• Syntactic overloading between identifiers and keywords

module Keywords exports sorts S E context-free start-symbols S lexical syntax [A-Za-z][A-Za-z0-9]* -> E [\ \t\n] -> LAYOUT context-free syntax E "(" {E ","}* ")" ";" -> S "return" E ";" -> S "(" E ")" -> E {bracket}

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Signatures and grammars

• The symptoms of this ambiguity are:

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• The top two nodes of the ambiguity differ
• There is more than one instance of syntactic
• verloading:
• verloading:

− "return" keyword, and the bracket syntax "( )" infer with other parts of the syntax − ";" is allocated to a different production.

• Characters that are grouped under a keyword node

("return") in one alternative, end up under an identifier ( return ) in one alternative, end up under an identifier node in another ("E").

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Signatures and grammars

module Keywords exports sorts S E context-free start-symbols S lexical syntax [A-Za-z][A-Za-z0-9]* -> E [\ \t\n] -> LAYOUT "return" -> E {reject} context-free syntax E "(" {E ","}* ")" ";" -> S "return" E ";" -> S "(" E ")" -> E {bracket}

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Signatures and grammars

• Useless cloning of identifier classes

module Clone exports sorts Identifier ClassName TypeName Declaration context-free start-symbols Declaration lexical syntax [\ \t\n] -> LAYOUT [a-z]+ -> Identifier [a-z]+ -> ClassName context-free syntax Identifier -> TypeName ClassName -> TypeName TypeName Identifier ";" -> Declaration

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Signatures and grammars

• Symptoms are very clear:

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• The two top productions of the ambiguity are different
• The two top productions of the ambiguity are injections

(chain rules) (chain rules)

• Each chain rule introduces a lexical (identifier) that has

the same definition, but a different non-terminal name

• Each alternative recognizes exactly the same

characters in exactly the same way, modulo non- terminal names

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Signatures and grammars

• Useless cloning of identifier classes

module Clone exports sorts Identifier Declaration context-free start-symbols Declaration lexical syntax [\ \t\n] -> LAYOUT [a-z]+ -> Identifier context-free syntax Identifier Identifier ";" -> Declaration

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Signatures and grammars

• Dangling else and related ambiguities

module DanglingElse exports sorts S E lexical syntax [\ \t\n] -> LAYOUT context-free restrictions LAYOUT? -/- [\t\n\ ] context-free start-symbols S context-free syntax "expr" -> E "if" E "then" S+ -> S "if" E "then" S+ "else" S+ -> S "other" -> S

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Signatures and grammars

• Symptoms in these two trees:

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• Top nodes of the derivations have different productions

(this is not always the case in this kind of ambiguity)

• Both trees exercise the same production rules but in
• Both trees exercise the same production rules, but in

different vertical order

• One of the productions is a prefix of the other,
• and, these two productions are the ones that have

swapped vertical order between the two derivations

• In both derivations, the deeply nested statement is the

In both derivations, the deeply nested statement is the

• nly, or the last, statement of a list of statements

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Signatures and grammars

• Dangling else and related ambiguities

module DanglingElse exports sorts S E lexical syntax [\ \t\n] -> LAYOUT context-free restrictions LAYOUT? -/- [\t\n\ ] context-free start-symbols S context-free syntax "expr" -> E "if" E "then" S+ -> S {avoid} "if" E "then" S+ "else" S+ -> S "other" -> S

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