Support older yaml versions (#981)

Fixes https://github.com/dhall-lang/dhall-haskell/issues/969

dhall-json/dhall-json.cabal

@@ -53,6 +53,8 @@ Library
     Exposed-Modules:
         Dhall.JSON
         Dhall.JSONToDhall
+    Other-Modules:
+        Dhall.JSON.Compat
     GHC-Options: -Wall
 
     if flag(yaml-pre-0_11)

dhall-json/src/Dhall/JSON.hs

@@ -196,11 +196,11 @@ import qualified Data.Vector
 import qualified Data.Yaml
 import qualified Dhall.Core
 import qualified Dhall.Import
+import qualified Dhall.JSON.Compat
 import qualified Dhall.Map
 import qualified Dhall.Parser
 import qualified Dhall.TypeCheck
 import qualified Options.Applicative
-import qualified Text.Libyaml
 
 {-| This is the exception type for errors that might arise when translating
     Dhall to JSON
@@ -907,24 +907,8 @@ jsonToYaml
 jsonToYaml json documents quoted = case (documents, json) of
   (True, Data.Yaml.Array elems)
     -> Data.ByteString.intercalate "\n---\n"
-       $ fmap (encodeYaml encodeOptions)
+       $ fmap encodeYaml
        $ Data.Vector.toList elems
-  _ -> encodeYaml encodeOptions json
+  _ -> encodeYaml json
   where
-    encodeYaml = Data.Yaml.encodeWith
-
-    customStyle = \s -> case () of
-        ()
-            | "\n" `Data.Text.isInfixOf` s -> ( noTag, literal )
-            | otherwise -> ( noTag, Text.Libyaml.SingleQuoted )
-        where
-            noTag = Text.Libyaml.NoTag
-            literal = Text.Libyaml.Literal
-
-    quotedOptions = Data.Yaml.setStringStyle
-                        customStyle
-                        Data.Yaml.defaultEncodeOptions
-
-    encodeOptions = if quoted
-        then quotedOptions
-        else Data.Yaml.defaultEncodeOptions
+    encodeYaml = Dhall.JSON.Compat.encodeYaml quoted

dhall-json/src/Dhall/JSON/Compat.hs

@@ -0,0 +1,37 @@
+{-# LANGUAGE CPP               #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Dhall.JSON.Compat where
+
+import Data.ByteString (ByteString)
+import Data.Yaml (Value)
+
+import qualified Data.Yaml
+#if MIN_VERSION_yaml(0,10,2)
+import qualified Data.Text
+import qualified Text.Libyaml
+#endif
+
+encodeYaml :: Bool -> Value -> ByteString
+encodeYaml _quoted =
+#if MIN_VERSION_yaml(0,10,2)
+    Data.Yaml.encodeWith encodeOptions
+  where
+    customStyle = \s -> case () of
+        ()
+            | "\n" `Data.Text.isInfixOf` s -> ( noTag, literal )
+            | otherwise -> ( noTag, Text.Libyaml.SingleQuoted )
+        where
+            noTag = Text.Libyaml.NoTag
+            literal = Text.Libyaml.Literal
+
+    quotedOptions = Data.Yaml.setStringStyle
+                        customStyle
+                        Data.Yaml.defaultEncodeOptions
+
+    encodeOptions = if _quoted
+        then quotedOptions
+        else Data.Yaml.defaultEncodeOptions
+#else
+    Data.Yaml.encode
+#endif