B9 produces not only VM-Images but also text documents such as configuration
files required by virtual machines. This module is about creating and merging
files containing parsable syntactic structures, such as most configuration files

Imagine you would want to create a cloud-init 'user-data' file from a set
of 'user-data' snippets which each are valid 'user-data' files in yaml syntax
and e.g. a 'writefiles' section. Now the goal is, for b9 to be able to merge
these snippets into one, such that all writefiles sections are combined into
a single writefile section. Another example is OTP/Erlang sys.config files.
This type class is the greatest commonon denominator of types describing a
syntax that can be parsed, concatenated e.g. like in the above example and
rendered. The actual concatenation operation is the append from Monoid,
i.e. like monoid but without the need for an empty element.

module B9.Content.AST ( ConcatableSyntax (..)
                      , ASTish(..)
                      , AST(..)
                      , CanRender(..)
                      ) where

import qualified Data.ByteString as B
import Data.Semigroup
import Data.Data
import Control.Applicative
import Control.Monad.IO.Class
import Control.Monad.Reader

import B9.Content.StringTemplate

import Test.QuickCheck
import B9.QCUtil

-- | Types of values that can be parsed/rendered from/to 'ByteString's. This
-- class is used as basis for the 'ASTish' class.
class (Semigroup a) => ConcatableSyntax a where
  -- Parse a bytestring into an 'a', and return @Left errorMessage@ or @Right a@
  decodeSyntax :: FilePath -- ^ An arbitrary string for error messages that corresponds
                          -- to a possible input file.
               -> B.ByteString -- ^ The raw input to parse
               -> Either String a
  -- Generate a string representation of @a@
  encodeSyntax :: a -> B.ByteString

instance ConcatableSyntax B.ByteString where
  decodeSyntax _ = Right
  encodeSyntax   = id

-- | Describe how to create structured content that has a tree-like syntactic
-- structure, e.g. yaml, JSON and erlang-proplists. The first parameter defines
-- a /context/ into which the 'AST' is embeded,
-- e.g. B9.Content.Generator.Content'. The second parameter defines a specifix
-- syntax, e.g 'B9.Content.ErlangPropList' that the 'AST' value generates.
data AST c a = ASTObj [(String, AST c a)] -- ^ Create an object similar to a
                                          -- Json object.
             | ASTArr [AST c a] -- ^ An array.
             | ASTMerge [AST c a] -- ^ Merge the nested elements, this is a very
                                  -- powerful tool that allows to combine
                                  -- several inputs in a smart and safe way,
                                  -- e.g. by merging the values of the same
                                  -- fields in yaml objects.
             | ASTEmbed c -- Embed some pure content.
             | ASTString String -- A string literal.
             | ASTParse SourceFile -- An 'AST' obtained from parsing a source
                                   -- file that contains a string corresponding
                                   -- to the type parameter @a@, e.g. 'YamlObject's
             | AST a -- Embed a literal @a@.
  deriving (Read, Show, Typeable, Data, Eq)

-- | Types of values that describe content, that can be created from an 'AST'.
class (ConcatableSyntax a) => ASTish a where
  fromAST :: (CanRender c
            ,Applicative m
            ,Monad m
            ,MonadIO m
            ,MonadReader Environment m)
          => AST c a
          -> m a

-- | Types of values that can be /rendered/ into a 'ByteString'
class CanRender c where
  render :: (Functor m
           ,Applicative m
           ,MonadIO m
           ,MonadReader Environment m)
         => c
         -> m B.ByteString

instance (Arbitrary c, Arbitrary a) => Arbitrary (AST c a) where
  arbitrary = oneof [ASTObj <$> smaller (listOf ((,)
                                                 <$> arbitrary
                                                 <*> arbitrary))
                    ,ASTArr <$> smaller (listOf arbitrary)
                    ,ASTMerge <$> sized
                                    (\s -> resize (max 2 s)
                                                  (listOf (halfSize arbitrary)))
                    ,ASTEmbed <$> smaller arbitrary
                    ,ASTString <$> arbitrary
                    ,ASTParse <$> smaller arbitrary
                    ,AST <$> smaller arbitrary