Haskell, 10.12. Exercice. N'oubliez pas la solution de nos problmes de planification (par ex. les cinq maris jaloux) l'aide des files prioritaires ! Haskell et parseurs monadiques (1) Ceci est un tutoriel de l'approche monadique au parsing. Il peut vous tre utile dans votre cours de compilation. L'approche fonctionnelle, monadique l'analyse syntaxique est une petite merveille algorithmique, permettant de travailler directement avec les lments des parseurs (analyseurs de syntaxe) de manire presque si simple, comme si ces modules taient des fragments de grammaire sous-jacente, et o le plug-in des procdures smantiques (constructeurs des rsultats du parsing) est facile et lisible. On profitera de nos connaissances de la monade non-dterministe (listes). D'abord, il faut savoir quelque chose sur les grammaires formelles, et ici nous ne pouvons consacrer trop de temps...
Imaginons - afin de nous rapprocher au mode de penser monadique - que la donne qui nous intresse, c'est ce rsultat, l'arbre en question. Packrat Parsing: a Practical Linear-Time Algorithm with Backtracking. Abstract Packrat parsing is a novel and practical method for implementing linear-time parsers for grammars defined in Top-Down Parsing Language (TDPL). While TDPL was originally created as a formal model for top-down parsers with backtracking capability, this thesis extends TDPL into a powerful general-purpose notation for describing language syntax, providing a compelling alternative to traditional context-free grammars (CFGs). Common syntactic idioms that cannot be represented concisely in a CFG are easily expressed in TDPL, such as longest-match disambiguation and "syntactic predicates," making it possible to describe the complete lexical and grammatical syntax of a practical programming language in a single TDPL grammar.
Packrat parsing is an adaptation of a 30-year-old tabular parsing algorithm that was never put into practice until now. Full Thesis In PDF or PostScript Pappy: a Parser Generator for Haskell Example Arithmetic Expression Parsers Example Java Language Parsers Enjoy! Haskell - Writing a Regular Expression parser in Haskell: Part 2 - Matthew Manela - Farblondzshet in Code. The first module in my simple regular expression parse is called RegexToNFA. This module exposes the types that make up a finite state machine and also the functions to convert a regular expression string into a finite state machine. My structure for a FSM follows closely from the mathematical definition: I have the value which you transition on as a Maybe Char (which I alias as TransitionValue).
This allowed me to define epsilon as Nothing data constructor. With this structure defined my goal now is to convert a regular expression pattern such as: (a|b)* into a FiniteMachine. In order to do this there is a lot of state that I need to keep track of which naturally leads to the use of the State monad. To do this I set up a structure for what data I want to be kept track of and then create a state monad using that structure: This structure is passed between functions to allow them to see the current state of the parsing and create a new state.
Interp.pdf (Objet application/pdf) Let's build a compiler (in Haskell): Part 7 - Parser Combinators at AlephNullPlex. Tags: < Part 6 Our parser has served us well so far. We can successfully parseAndEmit simple mathmatical expressions as well as assignments. However the technique used so far, one big parse function, has some serious drawbacks. Not least is the fact that it is starting to get too long and complicated. We also have a major issue with implementing things such as parentheses, multi-character factors and variables and alternatives such as what looks like an identifier may actually end up being a function call. In this article we will rewrite out compiler using a technique that has been used in parsers, especially those written in functional languages, for several decades now. Parser Combinators First of all, we need to define what a parser is: parse - 3. And naturally a parser is something that performs a parse operation. A combinator is the "...use of a [Higher Order Function] as an infix operator in a function-definition...
" We can then write combinator functions that take two parsers. A simple regex engine in Haskell. UPDATE: sorear from #haskell pasted a cool version of this here. His version is a Parsec Parser that returns another Parsec Parser! How cool is that? His version doesn't actually return the matches themselves, just a Bool, but still, it's a clever hack. I recently wrote a simple regex engine (and parser) in Haskell. The implementation is far from optimal, but I'm still pretty excited about how easy it was, especially the parser. I'm going to walk over the implementation and give examples. Import Text.ParserCombinators.Parsec data Match = MkMatch String String deriving (Show, Eq) type Matcher = Match -> [Match] We'll need Text.ParserCombinators.Parsec later. MatchOne :: (Char -> Bool) -> Matcher matchOne _ (MkMatch _ "") = [] matchOne f (MkMatch xs (y:ys)) | f y = [MkMatch (xs ++ [y]) ys] | otherwise = [] matchOne takes a predicate function and returns a Matcher.
*Main> matchOne (=='x') (MkMatch "" "xyz") [MkMatch "x" "yz"] Saying =='x' all the time is tedious: Example: Now for the parser: Rediscover the Joy of Coding :: Reading Haskell – Part 4. In the last three articles I covered the overall structure, lexer, and parser of a simple expression evaluator.
This articles concludes by presenting the evaluator and main loop. At this stage we are able to take a string, tokenize it, and then build a tree representing the expression. We now need to be able to reduce the tree down to a single value (or error). Expression Reduction reduce :: (MonadError ExprError m) => ExpressionTree -> m Int We’re returning an Int, but as before, the context for this return value is an error state.The reducer is a really simple recursive function. Reduce (Node (NNumber _ v) _) = return v Operator nodes recursively reduce their left and right child nodes, then apply the operator: reduce (Node (NOperator p _ op) (lhs:rhs:[])) = do x <- reduce lhs y <- reduce rhs case op x y of Right v -> return v Left m -> throwError $ ExprErrorAt m p At this point we’re “collapsing” the error context.
Main Loop main = interact $ unlines . map (unlines . formatResult) . lines. 117337.pdf (Objet application/pdf) Haskell Textile to HTML/Latex parser. Yayfun. I like writing in textile (RedCloth), it's less intrusive than doing "pure" latex when I'm authoring documents but I still get that nice warm fuzzy of being able to author docs in plain text. Yeah, I'm a geek. Anyway since I couldn't find any textile to latex parsers I whipped one up in Haskell for a reasonable subset of textile (it's not very big).
I'm sure it's not 100% compatible with the other implementations out there but it's for my purposes and I'm going to be open sourcing it when the opens.snepo.com is finally (whenever that may be).. You'll notice that I hardcoded in the header and footer of the latex output. I will eventually be expanding this so that I can specify a style and it will read the header and footer from a template. I haven't done any performance testing but it feels pretty speedy Anyway, here's the source code of the parser: module Rc2Latex (rc2latex, rc2html) where import Text.ParserCombinators.Parsec import Prelude hiding (break) import Debug.Trace -- Scanner. Neil Mitchell - TagSoup. TagSoup is a library for parsing HTML/XML.
It supports the HTML 5 specification, and can be used to parse either well-formed XML, or unstructured and malformed HTML from the web. The library also provides useful functions to extract information from an HTML document, making it ideal for screen-scraping. The library provides a basic data type for a list of unstructured tags, a parser to convert HTML into this tag type, and useful functions and combinators for finding and extracting information.
Related Projects TagSoup for Java - an independently written malformed HTML parser for Java. Including links to other HTML parsers. Downloads Tags: haskell library popular released. Parsec, a fast combinator parser. Daan Leijen University of Utrecht Dept. of Computer Science PO.Box 80.089, 3508 TB Utrecht The Netherlandsdaan@cs.uu.nl, 4 Oct 2001 Introduction Parsec is an industrial strength, monadic parser combinator library for Haskell. It can parse context-sensitive, infinite look-ahead grammars but it performs best on predictive (LL[1]) grammars. Combinator parsing is well known in the literature and offers several advantages to YACC or event-based parsing. About this document This document ships in the following formats: Postscript (PS). Compatibility The core library is written in Haskell98. Compiling with GHC Parsec is distributed as a package with GHC.
Suppose that the library is compiled in the directory c:\parsec. Ghc -c myfile -ic:\parsec When your linking the files together, you need to tell GHC where it can find libraries (-L) and to link with the Parsec library too (-l): ghc -o myprogram myfile1.o myfile2.o -Lc:\parsec -lparsec Compiling with Hugs Reporting bugs History. Parser un format simple en Haskell avec Parsec.