Add formatDateFieldValue for arbitrarily formatting date metadata

lib/Hakyll/Web/Template/Context.hs

@@ -21,6 +21,7 @@ module Hakyll.Web.Template.Context
     , snippetField
     , dateField
     , dateFieldWith
+    , formatDateFieldValue
     , getItemUTC
     , getItemModificationTime
     , modificationTimeField
@@ -294,6 +295,29 @@ dateFieldWith locale key format = field key $ \i -> do
     time <- getItemUTC locale $ itemIdentifier i
     return $ formatTime locale format time
 
+-- | Replace metadata date format with a new formatted date or do nothing
+formatDateFieldValue :: String    -- ^ Field name
+                     -> String    -- ^ Current format string
+                     -> String    -- ^ New format string
+                     -> Context a -- ^ Resulting context
+formatDateFieldValue name currFmt newFmt = Context $ \k _ i ->
+  if k == name
+  then (do value <- getMetadataField (itemIdentifier i) k
+           maybe empty (\v -> do
+                           let mSDate = parseAndFormat newFmt v
+                           case mSDate of
+                             (Just sDate) -> (return . StringField) sDate
+                             Nothing  -> empty
+                       ) value
+       )
+  else empty
+  where
+    parseAndFormat :: String -> String -> Maybe String
+    parseAndFormat fmt' v' = do
+      let timeV = parseTimeM True defaultTimeLocale currFmt v' :: Maybe UTCTime
+      case timeV of
+        (Just t) -> Just $ formatTime defaultTimeLocale fmt' t
+        Nothing   -> Nothing
 
 --------------------------------------------------------------------------------
 -- | Parser to try to extract and parse the time from the @published@

tests/Hakyll/Web/Template/Context/Tests.hs

@@ -50,6 +50,12 @@ testDateField = do
         "posts/2019/05/10/tomorrow.md" "date" $
             dateField "date" "%B %e, %Y"
     date4 @=? "May 10, 2019"
+
+    date5 <- testContextDone store provider
+      "posts/2018-09-28-fall-prep.md" "modified" $
+      formatDateFieldValue "modified" "%Y-%m-%d %H:%M:%S" "%B %e, %Y - %r"
+    date5 @=? "September 29, 2018 - 06:58:15 PM"
+
     cleanTestEnv
 
 

tests/data/posts/2018-09-28-fall-prep.md

@@ -0,0 +1,4 @@
+---
+modified: 2018-09-29 18:58:15
+---
+Getting ready for fall season