Algebraically compact module: Difference between revisions

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An '''attribute grammar''' is a formal way to define [[Attribute (computing)|attributes]] for the productions of a [[formal grammar]], associating these attributes to values. The evaluation occurs in the nodes of the [[abstract syntax tree]], when the language is processed by some [[parser]] or [[compiler]].
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The attributes are divided into two groups: ''synthesized'' attributes and ''inherited'' attributes. The synthesized attributes are the result of the attribute evaluation rules, and may also use the values of the inherited attributes. The inherited attributes are passed down from parent nodes.
 
In some approaches, synthesized attributes are used to pass semantic information up the parse tree, while inherited attributes help pass semantic information down and across it. For instance, when constructing a language translation tool, such as a compiler, it may be used to assign semantic values to syntax constructions. Also, it is possible to validate semantic checks associated with a grammar, representing the rules of a language not explicitly imparted by the syntax definition.
 
Attribute grammars can also be used to translate the syntax tree directly into code for some specific machine, or into some [[intermediate language]].
 
One strength of attribute grammars is that they can transport information from anywhere in the abstract syntax tree to anywhere else, in a controlled and formal way.{{Fact|date=February 2007}}
 
==Example==
 
The following is a simple [[Context-free grammar]] which can describe a language made up of multiplication and addition of integers.  
<code>
  '''Expr''' &rarr; '''Expr''' + '''Term'''
  '''Expr''' &rarr; '''Term'''
  '''Term''' &rarr; '''Term''' * '''Factor'''
  '''Term''' &rarr; '''Factor'''
  '''Factor''' &rarr; "(" '''Expr''' ")"
  '''Factor''' &rarr; ''integer''
</code>
 
The following attribute grammar can be used to calculate the result of an expression written in the grammar. Note that this grammar only uses synthesized values, and is therefore an [[S-attributed grammar]].
<code>
  '''Expr<sub>1</sub>''' &rarr; '''Expr<sub>2</sub>''' + '''Term''' [ '''Expr<sub>1</sub>'''.value = '''Expr<sub>2</sub>'''.value + '''Term'''.value ]
  '''Expr''' &rarr; '''Term''' [ '''Expr'''.value = '''Term'''.value ]
  '''Term<sub>1</sub>''' &rarr; '''Term<sub>2</sub>''' * '''Factor''' [ '''Term<sub>1</sub>'''.value = '''Term<sub>2</sub>'''.value * '''Factor'''.value ]
  '''Term''' &rarr; '''Factor''' [ '''Term'''.value = '''Factor'''.value ]
  '''Factor''' &rarr; "(" '''Expr''' ")" [ '''Factor'''.value =  '''Expr'''.value ]
  '''Factor''' &rarr; ''integer'' [ '''Factor'''.value = strToInt(''integer''.str) ]
</code>
 
==Synthesized attributes==
 
Let <math>G=<V_n,V_t,P,S></math> be an Attribute grammar, where
* <math>V_n</math> is the set of non terminal symbols
* <math>V_t</math> is the set of terminal symbols
* <math>P</math> is the set of [[Formal_grammar#The_syntax_of_grammars|productions]]
* <math>S</math> is the distinguished symbol, that is the start symbol
 
<math>A.a</math> is a synthesized attribute if,
* <math>A \rightarrow \alpha \in P</math>
* <math>\alpha = \alpha_1 \ldots \alpha_n, \forall_{i, 1 \leq i \leq n}: \alpha_i \in (V_n \cup V_t)</math>
* <math>\{\alpha_{j1}, \ldots ,\alpha_{jm}\} \subseteq \{\alpha_1, \ldots ,\alpha_n\}</math>
* <math>A.a = f(\alpha_{j1}.a_1, \ldots ,\alpha_{jm}.a_m)</math>
 
==Inherited attributes==
An ''inherited attribute'' at a node in parse tree is defined using the attribute values at the parent or siblings. Inherited attributes are convenient for expressing the dependence of a programming language construct on the context in which it appears. For example, we can use an inherited attribute to keep track of whether an identifier appears on the left or the right side of an assignment in order to decide whether the address or the value of the identifier is needed.  
{| class="wikitable"
|-
! Production rule !! Semantic rule
|-
| S → T L || L.in:=T.type
|-
| T → int || T.type:=integer
|-
| T → float || T.type:=float
|-
| T → char || T.type:=char
|-
| T → double || T.type:=double
|-
| L → L<sub>1</sub>, id || L<sub>1</sub>.in=L.in
enter_type(id.entry, L.in)
|-
| L → id || enter_type(id.entry, L.in)
|}
 
==Special types of attribute grammars==
* [[L-attributed grammar]] : ''inherited attributes'' can be evaluated in one left-to-right traversal of the abstract syntax tree
* [[LR-attributed grammar]] : an L-attributed grammar whose ''inherited attributes'' can also be evaluated in [[bottom-up parsing]].
* [[ECLR-attributed grammar]] : a subset of LR-attributed grammars where equivalence classes can be used to optimize the evaluation of inherited attributes.
* [[S-attributed grammar]] : a simple type of attribute grammar, using only ''synthesized attributes'', but no ''inherited attributes''
 
==See also==
*[[Affix grammar]]
*[[Van Wijngaarden grammar]]
*[[Syntax-directed translation]]
 
==External links==
*[http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue4/Why_Attribute_Grammars_Matter Why Attribute Grammars Matter], The Monad Reader, Issue 4, July 5, 2005. (This article narrates on how the formalism of attribute grammars brings [[aspect-oriented programming]] to [[functional programming]] by helping writing [[catamorphism]]s [[compositionality|compositional]]ly. It refers to the [http://www.cs.uu.nl/wiki/bin/view/HUT/AttributeGrammarSystem Utrecht University Attribute Grammar] system as the implementation used in the examples.)
*[http://www.haskell.org/haskellwiki/Attribute_grammar Attribute grammar] in relation to [[Haskell (programming language)|Haskell]] and [[functional programming]].
*''Semantics of context-free languages'', by [[Donald Knuth|Don Knuth]], is the original paper introducing attributed grammars
*D. E. Knuth: [http://www-cs-faculty.stanford.edu/~knuth/papers/gag.tex.gz The genesis of attribute grammars]. ''Proceedings of the international conference on Attribute grammars and their applications'' (1990), LNCS, [http://www.springerlink.com/content/w35h4669q16j/ vol. 461], 1–12. Some informal, historical information.
*Jukka Paakki: Attribute grammar paradigms—a high-level methodology in language implementation. ''ACM Computing Surveys'' '''27''':2 (June 1995), 196–255.
 
[[Category:Formal languages]]
[[Category:Compiler construction]]
[[Category:Parsing]]

Latest revision as of 10:42, 8 November 2014

A lagging computer is actually annoying and may be quite a headache. Almost every person whom uses a computer faces this issue some time or the other. If the computer equally suffers from the same problem, there are it hard to continue functioning as usual. In such a situation, the thought, "what must I do to create my PC run faster?" is repeated plus infuriating. There's a answer, still!

But registry is easily corrupted and damaged whenever you may be utilizing a computer. Overtime, without right repair, it can be loaded with errors and incorrect or missing information which might create a system unable to work correctly or apply a certain task. And whenever the system cannot discover the correct info, it may not learn what to do. Then it freezes up! That is the real cause of your trouble.

The Windows registry is a system database of info. Windows plus additional software shop a lot of settings and other information inside it, and retrieve such information within the registry all the time. The registry is furthermore a bottleneck in which because it really is the heart of the operating system, any problems with it could cause errors and bring the running program down.

The problem with nearly all of the people is that they do not wish To spend income. In the cracked adaptation one refuses to have to pay anything and will download it from internet easily. It is simple to install because well. But, the issue comes whenever it happens to be not able to identify all possible viruses, spyware and malware in the system. This is considering it really is obsolete in nature and refuses to receive any standard changes within the webpage downloaded. Thus, a system is accessible to difficulties like hacking.

Google Chrome crashes on Windows 7 when the registry entries are improperly modified. Missing registry keys or registry keys with wrong values will lead to runtime errors plus thereby the issue happens. We are recommended to scan the entire system registry plus review the result. Attempt the registry repair procedure utilizing third-party tuneup utilities 2014 software.

2)Fix your Windows registry to speed up PC- The registry is a complex section of the computer that holds different kinds of data from the aspects we do on your laptop each day. Coincidentally, over time the registry usually become cluttered with information and/or will get certain type of virus. This is really critical and you MUST receive this problem fixed right away, otherwise we run the risk of the computer being permanently damage and/or the sensitive information (passwords, etc.) is stolen.

To accelerate the computer, you just require to be able to do away with all these junk files, allowing the computer to find what it wants, whenever it wants. Luckily, there's a tool which enables you to do this conveniently plus instantly. It's a tool called a 'registry cleaner'.

Fortunately, there's a simple method to fix virtually all the computer errors. You merely have to be capable to fix corrupt registry files on a computer. And to do that, we may merely use a tool recognised as a registry cleaner. These easy pieces of software actually scan through the PC and fix each corrupt file which might cause a issue to Windows. This allows the computer to utilize all of the files it wants, that not just speeds it up - and stops all of the mistakes on your program also.

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