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haskellserializationyamlaeson

Serialize a map to YAML with a specific order

I use the yaml library to serialize a value of type Map String t (or some type t). The order in the resulting output is rather random, which is suboptimal, as the file should be human readable.

Is there a way to control the serialization order of a map? Or, probably closer to the core of the problem, an aeson Object? If not, what are suitable workarounds?

Solution

  • Since 0.8.13, the yaml package contains the Data.Yaml.Prettty module. This allows you to configure how to pretty-print yaml documents, including ordering fields using setConfCompare

    In one of my own projects, I started using it with a change like this:

     writeTipToiYaml :: FilePath -> TipToiYAML -> IO ()
-writeTipToiYaml out tty = encodeFile out tty
+writeTipToiYaml out tty =
+    SBC.writeFile out (encodePretty opts tty)
+  where
+    opts = setConfCompare (compare `on` fieldIndex) defConfig
+    fieldIndex s = fromMaybe (length fields) $ s `elemIndex` fields
+    fields = map T.pack
+        [ "product-id"
+        , "comment"
+        , "welcome"
+        , "media-path"
+        , "gme-lang"
+        , "init"
+        , "scripts"
+        , "language"
+        , "speak"
+        , "scriptcodes"
+        ]