Parsley
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?
Parsley is distributed on Hackage, and can be added by depending on the package parsley
.
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:
data WQ a = WQ a (Code a)
This gives us combinators like:
pure :: WQ a -> Parser a
satisfy :: WQ a -> Parser a
char :: Char -> Parser a
char c = satisfy (WQ (== 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
package for more information.
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 (see parsley-core
).
References
Talks
For talks on how writing parsers changes when using Parsley see either of these:
For the technical overview of how Parsley works:
For information about how to write parsers cleanly: