Questions tagged [parsers]

Questions about algorithms that decide whether a given string belongs to a fixed formal language.

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184 views

Handling epsilon productions in recursive descent parsing

I am working on a recursive descent parser for lambda calculus. In my grammar, after removing left-recursion, I am left with the following two productions: ...
65 views

Is this a correct grammar for untyped lambda calculus?

I am trying to write a recursive-descent parser for untyped lambda calculus. While researching the way of formulating the grammar, I managed to put together something like this: ...
149 views

How to design a LL(1) grammar for basic regular expression?

I try to design a LL(1) grammar to parse the basic regular expression. Here's the origin grammar.（\| is the escape character, since | is a special character in grammar's pattern). ...
141 views

Proof that the grammar is LL(2)

I am given the following grammar: $S \rightarrow AabAba \\ A \rightarrow a | \epsilon$ and I have to prove it is LL(2). I know what LL(k) means - one can choose a production based on k characters ...
45 views

Why can't exhaustive search parsing stop after |w| + 1 derivations?

If my grammar does not have productions of the form $A\rightarrow\lambda$ and $A\rightarrow B$ for some variables $A$ and $B$ then I know that each step in the derivation must involve an increase in ...
14 views

41 views

Expanding an Earley parse forest

I have a small context-free grammar, which I would like to use to turn input strings into parse trees. I need to get all possibilities in case of ambiguity (so shift-reduce is out), and there's quite ...
62 views

How do I build a left derivation tree from an expression including right-to-left associativity?

Suppose I want to make a left derivation tree of an expression, but the expression includes operators that are right-to-left associative. Would I still expand the left-most variable first in my tree? ...
47 views

Can we use Operator precedence parsing with this grammar?

This Grammar: A-->Bbab|aa B-->b I think we can't because basically there is no relation between terminals? like no relation between a and b! or am i missing something? and the language of this ...
24 views

Parsing table confusion [closed]

I've been researching the different answers I've been given to this, would somebody just be able to give me a clear answer of the difference between Shift, reduce and go-to. - In a parsing table. Of ...
132 views

Is there a name for this tree traversal?

This came when I was looking at block scoping in compiler design. { A { B } { C {D} {E} } } For the above pseudo code, the order in which the scope ...
287 views

Can a YACC parse an ambiguous grammar

I found a question while studying compilers :- Given a grammar E->E+E/E×E/number The above grammar is fed to a YACC with semantic rules for parsing. What will be the action of the YACC. 1) it ...
27 views

Name for a property of grammars

Parsing s-expressions is almost trivial. This is true for both automated processes (and thus implies ease of implementation of such processes) and for a human readers of printed s-expressions. It is ...
104 views

Why doesn't the C/C++ compilers create different tokens for different types of numbers?

Based on GeeksforGeeks and many other sites, the C/C++ compilers will create the same token for float/int etc. However if we ...
173 views

Should we eliminate left recursion before using NPDA to simulate CFG?

I read the book "Introduction to the Theory of. Computation, Third Edition by Michael Sipser". It says: If a language is context free, then some pushdown automaton recognizes it. Let A be a ...
58 views

Generate an AST from an indefinite grammar and logic

As said in the title, I have to generate an AST from a grammar and logic (or a"syntax" if you prefer). This grammar and syntax are both indefinite in the program, meaning that it is the user (a ...
35 views

Formalism for unambiguous context-sensitive languages?

I know that there are some languages that are unambiguous but not context-free (this is a good fact). Is there any kind of formalism for describing unambiguous context-sensitive languages? Even better ...
33 views

extend ECMAScript by adding new keywords and syntax

I would like to extend ECMAScript by adding new keywords and syntax to it, for example: |variableName| stipulates a special kind of variable which I would like to be able to include in a normal ...
141 views

Conversion of ambiguous left recursive grammar to LL(1)

I have been strugling with this LL(1) transformation for far longer than expected and am getting quite desperate. We are given a grammar $G = \left\{V_N, V_T, S, \Phi \right\}$, with the non-terminal ...
48 views

How to define whitespace for certain rules but else ignore it

I am trying to make a programming language for the first time and wonder how it handles whitespace. I can see CFG by default seem to just ignore whitespaces, but what if I want it to use it sometimes? ...
99 views

CFG - Left factoring in recursive nested productions

I'm attempting to convert a CFG into an LL(1) grammar for predictive parsing in a compiler. I've been able to left factor and eliminate left recursion and ambiguity for every case in the grammar, with ...
49 views

Best Approach To Parse Human Readable Text To Machine Readable (CSV)

I am attempting to parse a large amount of text that cannot be readily used by other software due to the human readable design. However, each "section" of the text has the same format. By "section", I ...
1k views

Dangling else problem

I was reading a book about compiler design, then in the CFG section, they introduced the dangling-else problem. They offered to match the the else with the last <...
78 views

Examples of context-free grammars with worst-case complexity

What are some examples of context-free grammars that necessarily trigger cubic worst-case complexity for GLR parsers? I have seen a mention of the example S $\rightarrow$ SSS | SS | "a" but I would ...
249 views

How do we evaluate semantic actions in L attributed SDT?

I came across following problem: $S→ER$ $R→∗E\{print(′∗′);\}R∣ε$ $E→F+E\{print(′+′);\}∣F$ $F→(S)∣id\{print(id.value);\}$ Here $id$ is a token that represents an integer and $id.value$ ...
185 views

Not getting the clear picture of Goto actions , reduce and shift actions in SLR Parsing

I am going through the following lecture notes on LR parsing : https://www3.nd.edu/~dthain/courses/cse40243/fall2016/chapter4b.pdf For the following grammar : The following LR automation has been ...
65 views

Is there a way to efficiently parse this grammar?

Consider the following context-free grammar: $T \to (*) \mid (T*) \mid (*T) \mid (T*T)$ This effectively forms a notation for binary trees: A binary tree is a $*$, with optional children on either ...
531 views

Number of nodes of a parse tree when the Grammar is in CNF?

There happened to be a following question - In any Chomsky Normal Form grammar $G$, how many nodes will all parse trees for terminal strings of length $n>0$ generated by G have? I tried for ...
40 views

The book I am learning about compilers from states that $M \to \alpha N \beta$ will generate a constraint that $\mathrm{FOLLOW}(M)$ is contained in $\mathrm{FOLLOW}(N)$ when $\beta$ is nullable. This ...
338 views

Tokenization Problem

Yes, this is a quiz question. It's from a self-paced course, but the answer just isn't correct to me no matter how I look at it. There isn't really an active community to consult. My Regular ...
346 views

LL1 parsing algorithm for strings generated by a given grammar

How to describe a $\operatorname{LL(1)}$ parsing algorithm for strings generated by a given grammar? I have to design a parser for a specific grammar. Let $G$ be the grammar described as: S \...
223 views

How to build a parser for syntactic analysis of regular expressions?

Is it possible to build a parsing algorithm that build a parse tree using a top-down method that simulate a derivation process for regular expressions, like we do for type 2 languages? My guess ...
2k views

What is a “sentential form”?

I do know what a "left-most-" and a "right-most derivation" is, but I stumble across the term "sentential form" and its specific differences "right-sentential form" as well as "left-sentential form" ...
62 views

Compiler Question : Does the parser detect Errors in these examples or the Semantic routine?

For example, which part will detect these errors, Parser or Semantic Routine? 1) int Array[10.5] ( wrong size ) 2) int x="some string" 3) A[10.5] + 3 (not in declaration, but in the middle of ...
42 views

What does the $^*$ operator do in the theory of parsing?

I'm currently reading through Aho's "Theory of parsing, translation and compiling" on my own and I got a bit confused about the closure operator on language $L$ with alphabet $\Sigma$ ($L^*$). The ...
567 views

Proof of every regular language has a LL(1) grammar

I tried some examples and found that LL(1) grammar always exist. I tried searching for formal proof but didn't find any. Can someone give a formal proof for the above statement?