Add maybeField

This field allows to easily introduce optional keys which can then be
conditionally rendered in templates.

src/Hakyll/Web/Template/Context.hs

@@ -5,6 +5,7 @@ module Hakyll.Web.Template.Context
     , Context (..)
     , field
     , constField
+    , maybeField
     , listField
     , listFieldWith
     , functionField
@@ -103,6 +104,21 @@ constField :: String -> String -> Context a
 constField key = field key . const . return
 
 
+maybeField' :: String
+           -> (Item a -> Compiler (Maybe ContextField))
+           -> Context a
+maybeField' key value = Context $ \k _ i -> if k == key
+    then value i >>= maybe empty return
+    else empty
+
+--------------------------------------------------------------------------------
+-- | A 'field' which will only contain value for 'Just's. This
+--   is mostly useful along with conditional template construct @$if(key)$@.
+maybeField :: String
+           -> (Item a -> Compiler (Maybe String))
+           -> Context a
+maybeField key value = maybeField' key (fmap (fmap StringField) . value)
+
 --------------------------------------------------------------------------------
 listField :: String -> Context a -> Compiler [Item a] -> Context b
 listField key c xs = listFieldWith key c (const xs)