The lexer hack

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原文链接：http://www.cnblogs.com/ainima/p/6331936.html

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本文探讨了C语言中由于语法上下文敏感性导致的lexer hack问题，即编译器难以区分变量名与类型名。文章详细介绍了这一问题的具体表现，并讨论了包括回溯解析在内的多种解决方案。

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The lexer hack

From Wikipedia, the free encyclopedia

In computer programming, the lexer hack (as opposed to "a lexer hack") describes a common solution to the problems in parsing ANSI C, due to the reference lexical grammar being context-sensitive. In C, classifying a sequence of characters as a variable name or a type name requires contextual information of the phrase structure, which prevents one from having a context-free lexer based only on the regular grammar of token classes.

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Problem[edit]

The problem is that in the following code, the lexical class of A cannot be determined without further contextual information:

 
     (A)*B

This code could be multiplication of two variables, in which case A is a variable; written unambiguously:

 
     A * B

Alternatively, it could be casting the dereferenced value of B to the type A, in which case A is a typedef-name; written in usual human-readable form, but still ambiguously from the point of view of the grammar:

 
     (A) *B

In more detail, in a compiler, the lexer performs one of the earliest stages of converting the source code to a program. It scans the text to extract meaningful tokens, such as words, numbers, and strings. The parser analyzes sequences of tokens attempting to match them to syntax rules representing language structures, such as loops and variable declarations. A problem occurs here if a single sequence of tokens can ambiguously match more than one syntax rule.

This ambiguity can happen in C if the lexer does not distinguish between variable and typedef identifiers.^[1] For example, in the C expression:

 
     (A) * B

the lexer may find these tokens:

left parenthesis
identifier 'A'
right parenthesis
operator '*'
identifier 'B'

The problem is precisely that the lexical class of A cannot be determined without further context: the parser can interpret this as variable A multiplied by B or as type A casting the dereferenced value of B. This is known as the "typedef-name: identifier" problem, due to the name of the problematic production rule.^[2]

The hack solution[edit]

The solution generally consists of feeding information from the parser's symbol table back into the lexer. That is, rather than functioning as a pure one-way pipeline from the lexer to the parser, there is a backchannel from the parser back to the lexer. This mixing of the lexer and parser is generally regarded as inelegant, which is why it is called a "hack".

Without added context, the lexer cannot distinguish type identifiers from other identifiers without extra context because all identifiers have the same format. With the hack in the above example, when the lexer finds the identifier A it should be able to classify the token as a type identifier. The rules of the language would be clarified by specifying that typecasts require a type identifier and the ambiguity disappears.

The problem also exists in C++ and parsers can use the same hack.^[1]

Alternative solutions[edit]

This problem does not arise (and hence needs no "hack" in order to solve) when using lexerless parsing techniques, as these are intrinsically contextual. These are generally seen as less elegant designs, however, because they lack the modularity of having a concurrent lexer and parser in a pipeline.

Some parser generators, such as the yacc-derived BtYacc ("Backtracking Yacc"), give the generated parser the ability to try multiple attempts to parse the tokens. In the problem described here, if an attempt fails because of semantic information about the identifier, it can backtrack and attempt other rules.^[3]

The relatively new LLVM-based parser Clang handles the situation a completely different way, namely by using a non-reference lexical grammar. Clang's lexer does not attempt to differentiate between type names and variable names: it simply reports the current token as an identifier. The parser then uses Clang's semantic analysis library to determine the nature of the identifier. This allows a much cleaner separation of concerns and encapsulation of the lexer and parser, and is therefore considered a much more elegant solution than The Lexer Hack by most modern software design metrics.^[4] This is also the approach used in most other modern languages, which do not distinguish different classes of identifiers in the lexical grammar, but instead defer them to the parsing or semantic analysis phase, when sufficient information is available.