Symparsec
Type level string parser combinators. A Parsec-like for
Symbol
s; thus, Symparsec! With many of the features you’d expect:
- define parsers compositionally, largely as you would on the term level
- define complex parsers, including mutually recursive ones (e.g. expression
parsers!)
- pretty, detailed parse errors
- good performance (probably? please help me benchmark!)
Requires GHC >= 9.6.
Examples
Define a type-level parser:
import Symparsec
import DeFun.Core
type PExample = Skip 1 *> Tuple (Isolate 2 NatHex) (Literal "_" *> TakeRest)
Use it to parse a type-level string (in a GHCi session):
ghci> :k! Run PExample "xFF_etc"
Run ...
= Right '( '(255, "etc"), "")
See the Symparsec.Example
namespace for further examples.
Why?
Via GHC.Generics
, we may inspect Haskell data types on the type level.
Constructor names are Symbols
. Ever reify these, then perform some sort of
checking or parsing on the term level? Symparsec does the parsing on the type
level instead. Catch bugs earlier, get faster runtime.
Also type-level Haskell authors deserve fun libraries too!!
Limitations
Symparsec defines a lot of low-level parsers you would expect on the term level,
even monadic (bind) and applicative (ap) parser combinators. The key limitation
Symparsec grapples with is Haskell having no type-level binders. This means:
- no
let
s, no where
s
- no
do
notation
The workaround is writing extra functions, and writing uglier functions.
Otherwise, I believe an average term-level Parsec-like parser should look
comparable to a type-level Symparsec
one.
Writing complex type-level Haskell programs in 2025 is unintuitive. I intend to
provide guides on writing Symparsec parsers that walk through type-level
programming design patterns and solutions. Please ping me on the issues tab if
you read this, and can’t find any such guides!
Contributing
I would gladly accept further combinators or other suggestions. Please add an
issue or pull request, or contact me via email or whatever (I’m raehik
everywhere).
License
Provided under the MIT license. See LICENSE
for license text.