BACKUS NAUR FORM TUTORIAL PDF

Keeping you updated with latest technology trends, Join DataFlair on Telegram 1. An important part of QlikView functioning is the QlikView scripting as nearly all the operations being performed on the data or on the application are done through script commands. You must have seen and dealt with commands and scripting before while working with any computer language. Now a thing to wonder is how these computer languages are formulated. Backus Naur Form is a metalanguage. Metalanguages are the languages which are used to explain other natural or computer languages.

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Our tree will have a root: one non-terminal representing our entire document. The root will contain other non-terminals that will contain other non-terminals and so on. The picture below show how we can go from a stream of tokens or terminals to an AST, which groups terminals into a hierarchy of non-terminals. We have seen that non-terminals represent structures at different levels.

Some of them can contain other statements. We are going to see it in the next section. Each production rule tells us how a non-terminal can be composed. We are now going to see the different elements that we can use in such rules. Terminal We have seen that a terminal can be defined in-line, specifying a string or a regular expression, or can be defined elsewhere and simply referred to in a rule.

The latter method is not technically part of EBNF, but it is commonly used. However this can lead to left-recursive rules that are typically forbidden. For more details see the paragraph on recursion in grammars. Alternative There are some constructs, or portion of constructs, that can be defined in different ways. To represent this case we use alternatives. In other words it can be optional. However it is more common to find them represented by a question mark following the optional element.

Zero or More Times An element can appear zero or more times no upper limit. However it is more common to find them represented by an asterisk following the element to repeat. One or More Time An element can appear one or more times no upper limit. This is not present in standard EBNF, however this is equivalent to a sequence in which the same elements is present twice and the second time it is followed by an asterisk, so that the same element is effectively present always at least once. Grouping We can group multiple elements together by using round parenthesis.

This is typically used because a modifier optional, zero-or-more, one-or-more must be applied to a set of elements. It can also be used to control the precedence of operators. A Few Things to Consider We have seen what constructs we can use to define production rules. Recursion in Grammars: Left or Right? EBNF lets us define recurring grammars.

Recurring grammars are grammars that have recurring production rules, i. Left recurring grammars are more common. Consider this example: expression: INT expression PLUS expression ; This production rule is left recurring because the expression could start with… an expression. The fact is that many tools that process EBNF grammars cannot deal with that because they risk to enter infinite loops.

There are ways to refactor left recurring rules, however they lead to less clear grammars. So, if you can, just use modern tooling that deal with left and right recurring grammars. In the languages we are going to define we can have many different rules and we need to define a specific order to consider them.

The traditional way to manage precedence is to define a list of different rules that refers to each other. At the top level we will have the more abstract rules, like expression, at the bottom level we will have rules representing the smallest components, like a single term. Later this can be possibly combined using the plus or the minus sign.

Then the less than operator can be used and this can be part of an expr, the top level rule considered. This is a relatively simple example but in richer languages we could have intermediate rules like test and sum, to represent different groups of operators with the same precedence.

Thing about the multiplication, division, power, comparison operators, logical operators, array access, etc. There are many possible ways to build expressions and you need to define an order of precedence. Now, some modern tools use just the order in which alternatives are defined to derive the precedence rules.

We have list of parameters, list of variables, all sorts of lists. We could have lists that must have at least one element and lists which can have zero elements.

What does it mean? However they are all semantically incorrect, because they do not respect additional constraints on how the language should be used. These semantics constraints are used in addition to the grammar, after the structures of the language have been recognized. By combining syntactic and semantic rules we can express what is valid in our language.

How do you define semantic rules? By writing code that works on the AST. You cannot do that in the EBNF grammar. Where to Go From Here? An EBNF grammar is useful to support discussion and to communicate with other langue designers. Typically you may want to do more with it: you want to build actual parsers and tools to process languages from your EBNF grammars. You could be interested in learning more about parsing. We have prepared different articles explaining how to proceed depending on the language you prefer to work with:.

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QlikView Backus Naur Form – Example of BNF in QlikView

Our tree will have a root: one non-terminal representing our entire document. The root will contain other non-terminals that will contain other non-terminals and so on. The picture below show how we can go from a stream of tokens or terminals to an AST, which groups terminals into a hierarchy of non-terminals. We have seen that non-terminals represent structures at different levels.

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Backus-Naur Form Syntax

Top-down parsing LL The easiest way of parsing something according to a grammar in use today is called LL parsing or top-down parsing. It works like this: for each production find out which terminals the production can start with. This is called the start set. Then, when parsing, you just start with the start symbol and compare the start sets of the different productions against the first piece of input to see which of the productions have been used. Of course, this can only be done if no two start sets for one symbol both contain the same terminal. If they do there is no way to determine which production to choose by looking at the first terminal on the input. The number in the parenthesis tells you the maximum number of terminals you may have to look at at a time to choose the right production at any point in the grammar.

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