Lemon is a
parser generator
In computer science, a compiler-compiler or compiler generator is a programming tool that creates a parser, interpreter, or compiler from some form of formal description of a programming language and machine.
The most common type of compiler- ...
, maintained as part of the
SQLite
SQLite ( "S-Q-L-ite", "sequel-ite") is a free and open-source relational database engine written in the C programming language. It is not a standalone app; rather, it is a library that software developers embed in their apps. As such, it ...
project, that generates a ''look-ahead LR parser'' (
LALR parser
In computer science, an LALR parser (look-ahead, left-to-right, rightmost derivation parser) is part of the compiling process where human readable text is converted into a structured representation to be read by computers. An LALR parser is a soft ...
) in the programming language
C from an input
context-free grammar
In formal language theory, a context-free grammar (CFG) is a formal grammar whose production rules
can be applied to a nonterminal symbol regardless of its context.
In particular, in a context-free grammar, each production rule is of the fo ...
. The generator is quite simple, implemented in one C source file with another file used as a template for output.
Lexical analysis
Lexical tokenization is conversion of a text into (semantically or syntactically) meaningful ''lexical tokens'' belonging to categories defined by a "lexer" program. In case of a natural language, those categories include nouns, verbs, adjectives ...
is performed externally.
Lemon is similar to the programs
Bison
A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised.
Of the two surviving species, the American ...
and
Yacc
Yacc (Yet Another Compiler-Compiler) is a computer program for the Unix operating system developed by Stephen C. Johnson. It is a lookahead left-to-right rightmost derivation (LALR) parser generator, generating a LALR parser (the part of a co ...
, but is incompatible with both. The grammar input format is different, to help prevent common coding errors. Other distinctive features include a
reentrant,
thread-safe output
parser
Parsing, syntax analysis, or syntactic analysis is a process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar by breaking it into parts. The term '' ...
, and the concept of ''non-terminal destructors'' that try to make it easier to avoid
memory leak
In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that memory which is no longer needed is not released. A memory leak may also happen when an objec ...
s.
SQLite uses Lemon with a hand-coded tokenizer to parse
SQL
Structured Query Language (SQL) (pronounced ''S-Q-L''; or alternatively as "sequel")
is a domain-specific language used to manage data, especially in a relational database management system (RDBMS). It is particularly useful in handling s ...
strings.
Lemon, together with
re2c and a re2c wrapper named Perplex, are used in
BRL-CAD as platform-agnostic and easily compilable alternatives to Flex and Bison. This combination is also used with STEPcode.
OpenFOAM expression evaluation uses a combination of
ragel and a version of lemon that has been minimally modified to ease C++ integration without affecting C integration.
[
] The parser grammars are augmented with
m4 macros.
Notes
References
*
*
External links
* {{Official website
Calculator with Lemon and Lex in C++ Example Lemon, bundled with Re2c and PerplexLemon Grove: Lemon along with sibling projects, forks and assets
Parser generators
Public-domain software with source code