Binder.hs

{-# LANGUAGE OverloadedStrings, GeneralizedNewtypeDeriving #-}
-- |
-- Module: Data.Greskell.Binder
-- Description: Binder monad to make binding between Gremlin variables and JSON values
-- Maintainer: Toshio Ito <debug.ito@gmail.com>
--
-- 
module Data.Greskell.Binder
       ( -- * Types
         Binder,
         Binding,
         -- * Actions
         newBind,
         newAsLabel,
         -- * Runners
         runBinder
       ) where

import Control.Monad.Trans.State (State)
import qualified Control.Monad.Trans.State as State
import Data.Aeson (Value, ToJSON(toJSON), Object)
import Data.Monoid ((<>))
import qualified Data.HashMap.Strict as HM
import qualified Data.Text as T
import qualified Data.Text.Lazy as TL

import Data.Greskell.AsLabel (AsLabel(..))
import Data.Greskell.Greskell (unsafeGreskellLazy, Greskell)

-- | State in the 'Binder'.
data BinderS =
  BinderS
  { varIndex :: PlaceHolderIndex,
    varBindings :: [Value],
    asLabelIndex :: PlaceHolderIndex
  }
  deriving (Show,Eq)

initBinderS :: BinderS
initBinderS =
  BinderS
  { varIndex = 0,
    varBindings = [],
    asLabelIndex = 0
  }

-- $setup
--
-- >>> import Control.Applicative ((<$>), (<*>))
-- >>> import Data.Greskell.Greskell (toGremlin)
-- >>> import Data.List (sortBy)
-- >>> import Data.Ord (comparing)
-- >>> import qualified Data.HashMap.Strict as HashMap

-- | A Monad that manages binding variables and labels to values.
--
-- >>> let binder = (,) <$> newBind (10 :: Int) <*> newBind "hoge"
-- >>> let ((var_int, var_str), binding) = runBinder binder
-- >>> toGremlin var_int
-- "__v0"
-- >>> toGremlin var_str
-- "__v1"
-- >>> sortBy (comparing fst) $ HashMap.toList binding
-- [("__v0",Number 10.0),("__v1",String "hoge")]
newtype Binder a = Binder { unBinder :: State BinderS a }
                   deriving (Functor, Applicative, Monad)

-- | Binding between Gremlin variable names and JSON values.
type Binding = Object

-- | Create a new Gremlin variable bound to the given value.
--
-- The value @v@ is kept in the monadic context. The returned
-- 'Greskell' is a Gremlin variable pointing to the @v@. The Gremlin
-- variable is guaranteed to be unique in the current monadic context.
newBind :: ToJSON v
        => v -- ^ bound value
        -> Binder (Greskell v) -- ^ variable
newBind val = Binder $ do
  state <- State.get
  let next_index = varIndex state
      values = varBindings state
  State.put $ state { varIndex = succ next_index,
                      varBindings = values ++ [toJSON val]
                    }
  return $ unsafePlaceHolder next_index

-- | Execute the given 'Binder' monad to obtain 'Binding'.
runBinder :: Binder a -> (a, Binding)
runBinder binder = (ret, binding)
  where
    (ret, state) = State.runState (unBinder binder) initBinderS
    values = varBindings state
    binding = HM.fromList $ zip (map toPlaceHolderVariableStrict [0 ..]) $ values
    toPlaceHolderVariableStrict = TL.toStrict . toPlaceHolderVariable

-- | __This type is only for internal use.__
type PlaceHolderIndex = Int

-- | __This function is only for internal use.__
--
-- Unsafely create a placeholder variable of arbitrary type with the
-- given index.
unsafePlaceHolder :: PlaceHolderIndex -> Greskell a
unsafePlaceHolder = unsafeGreskellLazy . toPlaceHolderVariable

-- | __This function is only for internal use.__
--
-- Create placeholder variable string from the index.
toPlaceHolderVariable :: PlaceHolderIndex -> TL.Text
toPlaceHolderVariable i = TL.pack ("__v" ++ show i)

-- | Create a new 'AsLabel'.
--
-- The returned 'AsLabel' is guaranteed to be unique in the current
-- monadic context.
--
-- @since 0.2.2.0
newAsLabel :: Binder (AsLabel a)
newAsLabel = Binder $ do
  state <- State.get
  let label_index = asLabelIndex state
      label = "__a" ++ show label_index
  State.put $ state { asLabelIndex = succ label_index }
  return $ AsLabel $ T.pack label