module Util.Hakyll.Context (
  updateFieldWith,
  defaultConstField,
  ifField,
) where

import Hakyll
import Data.Maybe (fromMaybe)
import Control.Applicative (empty)


{- |
  Shortcut function for getting an Items Metadata
  This is used as a helper function in a lot of places.
-}
getItemMetadata :: Item a -> Compiler Metadata
getItemMetadata = getMetadata . itemIdentifier

{- |
-}
updateFieldWith :: String -> String -> (String -> String) -> Item a -> Compiler String
updateFieldWith field defaultPreviousValue f = 
  fmap updateField . getMetadata . itemIdentifier
    where
      updateField :: Metadata -> String
      updateField = f . fromMaybe defaultPreviousValue . lookupString field

{- |
  This function takes a field name, and a default String value.
  If the field is found, it will leave it be, else inject the default value.
  Similar to the behaviour of (fromMaybe)
-}
defaultConstField :: String -> String -> Context String
defaultConstField fieldString defaultValue =
  field fieldString (fmap f . getItemMetadata)
    where
      f :: Metadata -> String
      f = fromMaybe defaultValue . lookupString fieldString

{-
-}
ifField :: String -> (Item a -> Compiler ContextField) -> Context a
ifField key value = Context $ \k _ i -> if k == key then value i else empty