herb

Herbrandt-universum-style serialization of Haskell values.

• Like Aeson, but all structures have heads!
• Like Show, but no precedence issues or ties to Haskell syntax!
• Like Prolog, but capital case doesn't mean variables!

The grammar is super-simple to allow easy interoperability between programs.

Example

Let's make a small language:

{-# LANGUAGE -XOverloadedStrings #-}
{-# LANGUAGE -XDeriveGeneric #-}
{-# LANGUAGE -XDeriveAnyClass #-}

import Data.Herb
import Data.Herb.Instances

data Expr
  = Number Int
  | Var Ident
  | Apply Expr Expr
  deriving (Show, Generic, FromHerb, ToHerb)

(Some more general typeclasses are provided too, mainly the deriving via wrapper for using Show, auto-deriving from Generic as used above, and HKD support with FromHerb1 and ToHerb1.)

We can convert the data to Herb representation and serialize them:

>>> encodeHerb $ Apply (Var "sqrt") (Number 5)
"Apply(Var(sqrt),Number(5))"

There's a prettyprinter for showing unwieldy values like this symbolic representation of foldr (+) 0 [1..5]:

>>> encodeHerb $ foldr (Apply . Apply (Var "add") . Number) (Number 0) [1..5]
"Apply(Apply(Var(add),Number(1)),Apply(Apply(Var(add),Number(2)),Apply(Apply(Var(add),Number(3)),Apply(Apply(Var(add),Number(4)),Apply(Apply(Var(add),Number(5)),Number(0))))))"

...piping the above through herb-format gives a relatively readable output:

Apply(Apply(Var(add), Number(1)),
      Apply(Apply(Var(add), Number(2)),
            Apply(Apply(Var(add), Number(3)),
                  Apply(Apply(Var(add), Number(4)),
                        Apply(Apply(Var(add), Number(5)), Number(0))))))

There's also an Attoparsec-based parser (the parsers and helper functions are in Data.Herb.Parser):

>>> import Data.Herb.Parser
>>> import Data.Attoparsec.Text
>>> parseOnly herbFile "stuff(this,that) \n stuff(others)"
Right [Struct "stuff" [Atom "this",Atom "that"],Struct "stuff" [Atom "others"]]

...together with a parsing shortcut:

>>> decodeHerb "Apply(Var(sqrt),Number(2))" :: Either String Expr
Right (Apply (Var "sqrt") (Number 2))

And that's it; this isn't supposed to be complex.