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Questions tagged [left-recursion]

A special kind of recursion, defined through a particular grammar property: grammar is left-recursive if we can find some non-terminal A which will eventually derive a sentential form with itself as the left-symbol.

Left-recursive grammar can be immediate or indirect:

Immediate left recursion

Immediate left recursion occurs in rules of the form

A → Aa | b

where a and b are sequences of nonterminals and terminals, and b doesn't start with A. For example, the rule

Expr → Expr + Term

is immediately left-recursive.

Indirect left recursion

Indirect left recursion in its simplest form could be defined as:

A → Ba | C
B → Ab | D

possibly giving the derivation A ⇒ Ba ⇒ Aba ⇒ ...

More generally, for the nonterminals A0, A1, ..., An, indirect left recursion can be defined as being of the form:

A0 → A1a_1 | ...
A1 → A2a_2 | ...
...
An → A0a_n+1 | ...

where a_1, a_2, ..., a_n are sequences of nonterminals and terminals.

Source: Wikipedia

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How to handle left recursion in FParsec

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Left-factoring grammar of coffeescript expressions

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Why does haskell stop when trying to add a string to the end of this string?

I am trying to add a string to the end of a string in Haskell. albumStr = "" main = do let albumStr = albumStr ++ "nothing" print albumStr Whenever i run this it just gets stuck on " in the console and i have to terminate it. Why? and how do…
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Parsing left-recursive grammar in sum-type inifinite recursion

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Prolog DCG: Match different symbols on a chain

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I'm trying to convert the following grammar production callExpr: primaryExpr | callExpr primaryExpr to a Parsec expression in Haskell. Obviously the problem is that it's left-recursive, so I'm trying to parse it recursive-ascent style. The…
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I'm trying to parse a simple language. The trouble comes with parsing function calls. I'm trying to tell it that a function call is an expression followed by left parenthesis, argument list, and right parenthesis. I have something like this: expr =…
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Left recursive ANTLR grammar

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Arunodoy Das Gupta
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Removing Left Recursion from CFG

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Grammar involving braces

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How to avoid left-recursion infinite loops in Fastparse?

I had a parser that worked well in Scala Packrat parser combinators. I would like to try something faster with the Fastparse library. However, it cannot handle left-recursion infinite loops. Is there any standard way to cope with that? sealed trait…
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I have been trying to build a SQL parser with the scala-parser-combinator library, which I've simplified greatly into the code below. class Expression case class FalseExpr() extends Expression case class TrueExpr() extends Expression case class…
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Removing Left Recursion with Terminals

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Removing Left Recursion in a Context Free Grammar

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