ReadMe.md

   1 ### Main differences with respect to `ParsleyHaskell`
   2
   3 - Primitive grammar combinators are extensible, including the optimization pass for which they are the current top-level combinator.
   4
   5 - Error messages are based upon the farthest input position reached (not yet implemented in `ParsleyHaskell`) and there is a preliminary support for error messages based upon [labeled failures](https://dl.acm.org/doi/10.1145/2851613.2851750).
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   7 - Minimal input length checks ("horizon" checks) required for a successful parsing are statically computed using a [polyfix](http://okmij.org/ftp/Computation/fixed-point-combinators.html#Poly-variadic) to see beyond calls to subroutines, which is not (yet) possible in `ParsleyHaskell`.
   8
   9 - Symantics are used for grammars productions instead of GHC plugins: `lift-plugin`, `idioms-plugin` or `parsley-garnish` for users. Those provide convenient syntaxic-sugar (by quoting an Haskell expression as itself and its `TemplateHaskell` equivalent), but I do not understand them that much and do not feel confortable to maintain them in case their authors abandon them.
  10
  11 - Fresh `TemplateHaskell` names are used directly (when observing sharing, introducing join-points, etc.) instead of a detour depending upon `dependent-map`.
  12
  13 - Code is a common published under the copyleft license `AGPL-3.0-or-later`, instead of the more liberal `BSD-3-Clause` of `ParsleyHaskell`.
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  15 - Testing grammars have their generated machines and `TemplateHaskell` splices followed by golden tests.
  16
  17 ### Main goals
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  19 - For me to better understand [ParsleyHaskell](https://github.com/j-mie6/ParsleyHaskell), and find a manageable balance between simplicity of the codebase and features of the parser. And by doing so, challenging and showcasing symantic techniques.
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  21 - To support the parsing of tree-like data structures instead of only string-like data structures. Eg. to validate XML using RelaxNG in [symantic-xml](https://hackage.haskell.org/package/symantic-xml) or to perform routing of HTTP requests in [symantic-http-server](http://hackage.haskell.org/package/symantic-http-server). This is currently done in those packages using `megaparsec`, but `megaparsec` is not conceived for such input, and is less principled when it comes to optimizing, like merging alternatives.
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  23 ### Implementation techniques
  24
  25 #### Typed Tagless-Final
  26 The syntax of grammars are term-level combinators defined in type-classes,
  27 and their semantics are data-types having instances of those type-classes.
  28 And the same technique is applied for machine instructions and grammar productions.
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  30 For automatic deriving, `DefaultSignatures` are supplied, see `Symantic.Syntaxes.Derive`.
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  32 For pattern-matching, data-families indexed by the syntaxic type-class are supplied,
  33 see `Symantic.Semantics.Data`.