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https://github.com/j-mie6/parsleyhaskell

Reimplementation of Parsley in Haskell, with improvements
https://github.com/j-mie6/parsleyhaskell

parser-combinators template-haskell

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Reimplementation of Parsley in Haskell, with improvements

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README 

        
# Parsley ![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/j-mie6/ParsleyHaskell/ci.yaml?branch=master) ![GitHub release](https://img.shields.io/github/v/release/j-mie6/ParsleyHaskell) [![GitHub license](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://github.com/j-mie6/ParsleyHaskell/blob/master/LICENSE) ![GitHub commits since latest release (by SemVer)](https://img.shields.io/github/commits-since/j-mie6/ParsleyHaskell/latest)



## What is Parsley?

Parsley is a very fast parser combinator library that outperforms the other libraries in both the

parsec family, as well as Happy. To make this possible, it makes use of Typed Template Haskell

to generate the code for the parsers.



## How do I use it? [![Hackage Version](https://img.shields.io/hackage/v/parsley)](https://hackage.haskell.org/package/parsley) ![Dependent repos (via libraries.io)](https://img.shields.io/librariesio/dependent-repos/hackage/parsley)

Parsley is distributed on [Hackage](https://hackage.haskell.org/package/parsley), and can be added by depending on the package `parsley`.



The version policy for `parsley-core` adheres to the regular Haskell PVP, but the two major versions are distinguished: the first is the _Public API_ major version, which represents backwards incompatible changes

in the regular PVP sense that could affect `parsley` itself (note `parsley` _only_ imports from `Parsley.Internal` itself); the second version is the

_Internal API_ major version, which would only effect users who use part of the internal parsley

modules. As such, for people that are **not** explicitly importing anything from `Parsley.Internal.*`, the second major version does not matter: `0.2.0.0` and `0.3.0.0` would be compatible, for instance.



To use it, you'll need to write you parsers in another file from where they will be used: this is

due to Template Haskell.



### How does Parsley being a _Staged Selective_ library change its use?

By being a _Selective_ Parser Combinator library, Parsley does not support monadic operations such

as `(>>=)` or `return`. Instead, the most powerful operations are `select` or `branch`. Most monadic

power can be recovered using the functionality provided by `Parsley.Register`, as well as the

selectives.



The reason monads are not supported is because of the _Staging_: Parsley parsers are compiled ahead

of time to produce fast code, but this means the entirety of the parser must be known before any

input is provided, ruling out dynamic monadic operations. The use of staging (in this instance provided

by Typed Template Haskell) means that the signatures of the combinators do not correspond to their

counterparts in other libraries: they don't use raw values, they use code of values. A consequence

of this is that Parsley does not implement instances of `Functor`, `Applicative`, `Alternative`,

or indeed `Selective`; `do`-notation also cannot be used even with `ApplicativeDo`, except perhaps

if `RebindableSyntax` is used.



Code is provided to the combinators by way of the datatype `WQ` (or `Defunc` if you're feeling fancy),

which pairs a normal value with its Haskell code representation:



```hs

data WQ a



makeQ :: a -> Code a -> WQ a -- or Defunc a

```



This gives us combinators like:



```hs

pure :: WQ a -> Parser a

satisfy :: WQ a -> Parser a



char :: Char -> Parser a

char c = satisfy (makeQ (== c) [||(== c)||])

```



Using `WQ` explicitly like this can get annoying, which is what the `parsley-garnish` package is for!

Currently, the garnish provides one plugin called `OverloadedQuotes`, which replaces the behaviour of

the default _Untyped_ Template Haskell quotes in a file so that they produce one of `WQ` or `Defunc`.

See the `Parsley.OverloadedQuotesPlugin` module in the [`parsley-garnish`](https://hackage.haskell.org/package/parsley-garnish) package for more information.



### How to avoid manual `WQ` on GHC 9.2

Currently, `parsley-garnish` is not ready for GHC 9.2. Instead of using `[|` and `|]` as the builder

for `Defunc` values, you can make use of the following `CPP` macro instead:



```hs

{-# LANGUAGE CPP #-}



#define QQ(x) (makeQ (x) [||(x)||])



char :: Char -> Parser a

char c = satisfy QQ(== c)

```



This may require the use of the `cpphs` buildtool.



## How does it work?

In short, Parsley represents all parsers as Abstract Syntax Trees (ASTs). The representation of the

parsers the users write is called the Combinator Tree, which is analysed and optimised by Parsley.

This representation is then transformed into an Abstract Machine in CPS form, this is analysed further

before being partially evaluated at compile-time to generate high quality Haskell code. For the long

version, I'd recommend checking out the paper!



## Bug Reports

If you encounter a bug when using Parsley, try and minimise the example of the parser (and the input)

that triggers the bug. If possible, make a self contained example: this will help me to identify the

issue without too much issue. It might be helpful to import `parsley-core:Parsley.Internal.Verbose` to provide a

debug dump that I can check out.



## References

* This work spawned a paper at ICFP 2020: [**Staged Selective Parser Combinators**](https://dl.acm.org/doi/10.1145/3409002)

* Supports patterns and combinators from Haskell Symposium 2021: [**Design Patterns of Parser Combinators**](https://dl.acm.org/doi/10.1145/3471874.3472984)



### Talks

For talks on how writing parsers changes when using Parsley see either of these:

* [*Garnishing Parsec with Parsley*](https://www.youtube.com/watch?v=tJcyY9L2z84) - Berlin Functional Programming Group, January 2021

* [*Exploring Parsley: Working with Staged Selective Parsers*](https://www.youtube.com/watch?v=Zhu-cPY1eac) - MuniHac 2020



For the technical overview of how Parsley works:

* [*Staged Selective Parser Combinators*](https://www.youtube.com/watch?v=lH65PvRgm8M) - ICFP 2020



For information about how to write parsers cleanly:

* [*Design Patterns for Parser Combinators*](https://www.youtube.com/watch?v=RwzX1XltOGY) - Haskell 2021