Add new --omitEmpty option (#842)

... as proposed in https://github.com/dhall-lang/dhall-kubernetes/issues/46#issuecomment-468530601

`--omitEmpty` is the same as `--omitNull` except it also omits fields that
are empty records.  To be precise, it omits fields whose transitive fields
are all `null` (and an empty record is a special case of a record whose
transitive fields are all `null` since it is vacuously true when there are
no fields).

This allows Dhall configurations that target YAML to avoid having to nest
sub-records inside of an `Optional` value.  Omitting unnecessary `Optional`
layers reduces the number of default values that the configuration format
needs to thread around.

dhall-json/dhall-json.cabal

@@ -63,6 +63,7 @@ Executable dhall-to-yaml
     Main-Is: Main.hs
     Build-Depends:
         base                                   ,
+        aeson                                  ,
         bytestring                       < 0.11,
         dhall                                  ,
         dhall-json                             ,

dhall-json/dhall-to-json/Main.hs

@@ -6,6 +6,7 @@ module Main where
 import Control.Applicative ((<|>))
 import Control.Exception (SomeException)
 import Control.Monad (when)
+import Data.Aeson (Value)
 import Data.Monoid ((<>))
 import Data.Version (showVersion)
 import Dhall.JSON (Conversion)
@@ -28,7 +29,7 @@ import qualified System.IO
 data Options = Options
     { explain    :: Bool
     , pretty     :: Bool
-    , omitNull   :: Bool
+    , omission   :: Value -> Value
     , version    :: Bool
     , conversion :: Conversion
     }
@@ -38,7 +39,7 @@ parseOptions =
         Options
     <$> parseExplain
     <*> parsePretty
-    <*> parseOmitNull
+    <*> Dhall.JSON.parseOmission
     <*> parseVersion
     <*> Dhall.JSON.parseConversion
   where
@@ -68,12 +69,6 @@ parseOptions =
         defaultBehavior =
             pure False
 
-    parseOmitNull =
-        Options.Applicative.switch
-            (   Options.Applicative.long "omitNull"
-            <>  Options.Applicative.help "Omit record fields that are null"
-            )
-
     parseVersion =
         Options.Applicative.switch
             (   Options.Applicative.long "version"
@@ -111,11 +106,9 @@ main = do
 
         let explaining = if explain then Dhall.detailed else id
 
-        let omittingNull = if omitNull then Dhall.JSON.omitNull else id
-
         stdin <- Data.Text.IO.getContents
 
-        json <- omittingNull <$> explaining (Dhall.JSON.codeToValue conversion "(stdin)" stdin)
+        json <- omission <$> explaining (Dhall.JSON.codeToValue conversion "(stdin)" stdin)
 
         Data.ByteString.Char8.putStrLn $ Data.ByteString.Lazy.toStrict $ encode json
 

dhall-json/dhall-to-yaml/Main.hs

@@ -4,6 +4,7 @@
 module Main where
 
 import Control.Exception (SomeException)
+import Data.Aeson (Value)
 import Data.Monoid ((<>))
 import Dhall.JSON (Conversion)
 import Options.Applicative (Parser, ParserInfo)
@@ -22,7 +23,7 @@ import qualified System.IO
 
 data Options = Options
     { explain    :: Bool
-    , omitNull   :: Bool
+    , omission   :: Value -> Value
     , documents  :: Bool
     , conversion :: Conversion
     }
@@ -31,7 +32,7 @@ parseOptions :: Parser Options
 parseOptions =
         Options
     <$> parseExplain
-    <*> parseOmitNull
+    <*> Dhall.JSON.parseOmission
     <*> parseDocuments
     <*> Dhall.JSON.parseConversion
   where
@@ -41,12 +42,6 @@ parseOptions =
             <>  Options.Applicative.help "Explain error messages in detail"
             )
 
-    parseOmitNull =
-        Options.Applicative.switch
-            (   Options.Applicative.long "omitNull"
-            <>  Options.Applicative.help "Omit record fields that are null"
-            )
-
     parseDocuments =
         Options.Applicative.switch
             (   Options.Applicative.long "documents"
@@ -70,11 +65,9 @@ main = do
     handle $ do
         let explaining = if explain then Dhall.detailed else id
 
-        let omittingNull = if omitNull then Dhall.JSON.omitNull else id
-
         stdin <- Data.Text.IO.getContents
 
-        json <- omittingNull <$> explaining (Dhall.JSON.codeToValue conversion "(stdin)" stdin)
+        json <- omission <$> explaining (Dhall.JSON.codeToValue conversion "(stdin)" stdin)
 
         let yaml = case (documents, json) of
               (True, Data.Yaml.Array elems)

dhall-json/src/Dhall/JSON.hs

@@ -161,6 +161,8 @@ module Dhall.JSON (
     -- * Dhall to JSON
       dhallToJSON
     , omitNull
+    , omitEmpty
+    , parseOmission
     , Conversion(..)
     , convertToHomogeneousMaps
     , parseConversion
@@ -329,8 +331,9 @@ toOrderedList =
 
 -- | Omit record fields that are @null@
 omitNull :: Value -> Value
-omitNull (Object object) =
-    Object (fmap omitNull (Data.HashMap.Strict.filter (/= Null) object))
+omitNull (Object object) = Object fields
+  where
+    fields =Data.HashMap.Strict.filter (/= Null) (fmap omitNull object)
 omitNull (Array array) =
     Array (fmap omitNull array)
 omitNull (String string) =
@@ -342,6 +345,40 @@ omitNull (Bool bool) =
 omitNull Null =
     Null
 
+{-| Omit record fields that are @null@ or records whose transitive fields are
+    all null
+-}
+omitEmpty :: Value -> Value
+omitEmpty (Object object) =
+    if null fields then Null else Object fields
+  where
+    fields = Data.HashMap.Strict.filter (/= Null) (fmap omitEmpty object)
+omitEmpty (Array array) =
+    Array (fmap omitEmpty array)
+omitEmpty (String string) =
+    String string
+omitEmpty (Number number) =
+    Number number
+omitEmpty (Bool bool) =
+    Bool bool
+omitEmpty Null =
+    Null
+
+-- | Parser for command-line options related to omitting fields
+parseOmission :: Parser (Value -> Value)
+parseOmission =
+        Options.Applicative.flag'
+            omitNull
+            (   Options.Applicative.long "omitNull"
+            <>  Options.Applicative.help "Omit record fields that are null"
+            )
+    <|> Options.Applicative.flag'
+            omitEmpty
+            (   Options.Applicative.long "omitEmpty"
+            <>  Options.Applicative.help "Omit record fields that are null or empty records"
+            )
+    <|> pure id
+
 {-| Specify whether or not to convert association lists of type
     @List { mapKey: Text, mapValue : v }@ to records
 -}
@@ -691,6 +728,7 @@ convertToHomogeneousMaps (Conversion {..}) e0 = loop (Dhall.Core.normalize e0)
         Dhall.Core.Embed a ->
             Dhall.Core.Embed a
 
+-- | Parser for command-line options related to homogeneous map support
 parseConversion :: Parser Conversion
 parseConversion =
         conversion