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An OCaml modular and generalised parser combinator library.

Install the library and its dependencies viaOPAM:

opam install transept

or in yourproject-name.opam dependencies:

...depends: [  "transept" { >= "0.1.0" }  ...]  ...

This example is the traditional arithmetic expression language. This can be represented by the following abstract datatypes.In this first example we only care about significant items likefloat, parenthesis and finally operations.

typeoperation=  |Add  |Minus  |Mult  |Divtypeexpr=  |Numberoffloat  |BinOpofoperation*expr*expr

Direct style means we parse a stream of characters. In this case all characters are significant even spaces.

Transept provides modules in order to help parsers construction. In the next fragmentUtils contains basic functionslikeconstant. TheParser module is a is parser dedicated to char stream analysis andLiteralsis dedicated to string,float etc. parsing.

moduleUtils=Transept.UtilsmoduleCharParser=Transept.Extension.Parser.For_char_listmoduleLiterals=Transept.Extension.Literals.Make (CharParser)

Therefore we can propose a first parser dedicated to operations.

let operator=letopenUtilsinletopenCharParserin        (atom'+'<$> constantAdd)<|> (atom'-'<$> constantMinus)<|> (atom'*'<$> constantMult)<|> (atom'/'<$> constantDiv)

Then the simple expression and the expression can be defined by the following parsers.

let expr=(* sexpr ::= float | '(' expr ')'*)letrecsexpr()=letopenLiteralsinletopenCharParserinfloat<$> (funf ->Number f)<|> (atom'('&> do_lazy expr<& atom')')(* expr ::= sexpr (operator expr)?*)andexpr()=letopenCharParserin      do_lazy sexpr<&> opt (operator<&> do_lazy expr)<$>function|e1,None -> e1|e1,Some (op,e2) ->BinOp (op, e1, e2)in expr

Finally, a sentence can be easily parsed.

letparses=letopenUtilsinletopenCharParserin    parse (expr())@@Stream.build@@ chars_of_string s

With this solution we don't skip whitespaces. It means1+(2+3) is parsed when1 + (2 + 3) is not!

SinceTransept is a generalized version, it's possible to parse something other than characters. For this purpose ageneric lexer is proposed thanks to theGenlex module.

Transept provides modules in order to help parsers construction. In the next fragmentUtils contains basic functionslikeconstant. TheCharParser module is a parser dedicated to char stream analysis andStreamis dedicated toparsing using another parser.

moduleUtils=Transept.Utils.FunmoduleParser=Transept.Extension.Parser.For_char_listmoduleStream=Transept.Stream.Via_parser (Parser)moduleGenlex=Transept.Genlex.Lexer.Make (Parser)

moduleParser=Transept.Core.Parser.Make_via_stream    (Stream)    (structtypet=Transept.Genlex.Lexeme.tend)moduleToken=Transept.Genlex.Lexeme.Make (Parser)

Therefore, we can propose a first parser dedicated to operations.

let operator=letopenUtilsinletopenParserinletopenTokenin        (kwd"+"<$> constantAdd)<|> (kwd"-"<$> constantMinus)<|> (kwd"*"<$> constantMult)<|> (kwd"/"<$> constantDiv)

Then the simple expression and the expression can be defined by the following parsers.

let expr=(* sexpr ::= float | '(' expr ')'*)letrecsexpr()=letopenParserinletopenLexemeinfloat<$> (funf ->Number f)<|> (kwd"("&> do_lazy expr<& kwd")")(* expr ::= sexpr (operator expr)?*)andexpr()=letopenParserin      do_lazy sexpr<&> opt (operator<&> do_lazy expr)<$>function|e1,None -> e1|e1,Some (op,e2) ->BinOp (op, e1, e2)in expr()

Finally, a sentence can be parsed using parsers. First oneCharParser parses char stream and is used by theGenlex in order to create a streamof lexemes. The second oneParser is used to parse the previous lexeme stream.

letparses=letopenUtilsinletopenParserinlet parser=CharParser.Stream.build@@Utils.chars_of_string sinlet stream=Stream.buildGenlex.tokenizer parserin    parse (expr<& eos) stream

With this solution whitespaces are skipped by the generic lexer. It means1 + ( 2+ 3) is parsed correctly now.

AJSON Parser has been designed with this approch based on a low level parser producing tokens and a high level parser producing JSON terms from tokens.

MIT License

Copyright (c) 2020 Didier Plaindoux

Permission is hereby granted, free of charge, to any person obtaining a copyof this software and associated documentation files (the "Software"), to dealin the Software without restriction, including without limitation the rightsto use, copy, modify, merge, publish, distribute, sublicense, and/or sellcopies of the Software, and to permit persons to whom the Software isfurnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in allcopies or substantial portions of the Software.

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