happy

I have two snippets of Happy code here, one that uses normal precedence rules, and one that uses context-dependent precedence rules (both of which are described here ). Normal: %left '+' %left '*' %% Exp :: { Exp } : Exp '+' Exp { Plus $1 $3 } | Exp '*' Exp { Times $1 $3 } | var { Var $1 } Context-dependent: %left PLUS %left TIMES %% Exp :: { Exp } : Exp '+' Exp %prec PLUS { Plus $1 $3 } | Exp '*' Exp %prec TIMES { Times $1 $3 } | var { Var $1 } Given the input: a * b + c * d The normal version gives: Plus (Times (Var "a") (Var "b")) (Times (Var "c") (Var "d")) whereas the context-dependent

I'm writing a compiler for a class I'm taking. The class isn't specifically Haskell but I'm using Haskell to write my compiler and interpreter. I have a cabal package setup to hopefully make it easy for my prof to run/compile. I have happy and alex in the build-tools field for both executables but Cabal ignores that and then complains that it cannot find the modules that Happy and Alex should be generating. If I manually run: alex LimpScanner.x happy LimpParser.y then cabal runs perfectly. I thought I had cabal automatically running them earlier but perhaps I remember imperfectly. limp.cabal:

I'm working on a school project where I have to build an interpreter for a simple language using Alex + Happy in Haskell. After looking through the documentation I understand most of it, but would like to see a full blown example on using the tools. Not on building interpreters, but on building lexers and parsers, yes. See the example for a lexical analyzer in Alex, here , combined with an intro to Happy here . I found the haskell.x and haskell.y files distributed in the darcs repos for Alex and Happy useful. You can find those here and here. I wrote a series of posts at bjbell.wordpress.com

To learn how to write and parse a context-free grammar I want to choose a tool. For Haskell, there are two big options: Happy, which generates a parser from a grammar description and *Parsec, which allows you to directly code a parser in Haskell. What are the (dis)advantages of either approach? External vs internal DSL The parser specification format for Happy is an external DSL, whereas with Parsec you have the full power of Haskell available when defining your parsers. This means that you can for example write functions to generate parsers, use Template Haskell and so on. Precedence rules

I am currently playing with the happy parser generator. Other parser generators can give nice messages like "unexpected endline, expected 'then'". With happy I just get the current Tokens and the position of the error. Can you give me an example of how to get error messages like above? There is a Happy feature that I have authored for this purpose. See my blog post: Toward better GHC syntax errors It was merged in this pull request RFC: On parse error - show the next possible tokens . Generally, from what I've heard, if you want nice parser errors, use Parsec instead of Happy. 来源： https:/