Serialize a map to YAML with a specific order

Question

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?

`object . map (\(k,v) -> yourTypeToText k .= v ) . sort . toList` — tomferon, Feb 02 '14 at 15:23
Have you verified that? Objects are unordered (hash)maps themselves. — Joachim Breitner, Feb 02 '14 at 16:14

score 2 · Accepted Answer · answered Jan 10 '21 at 22:51

2

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"
+        ]

answered Jan 10 '21 at 22:51

Joachim Breitner

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This works at the top level, but nested data structures don't have their fields ordered by the comparator :( – Guy Gastineau Jul 29 '21 at 13:39
Ugh, really? That sounds like a bug to me. – Joachim Breitner Aug 09 '21 at 15:09
Oh, I forgot about this comment. No, it does work. I was just horribly mistaken Thank you for reminding me to debunk my own comment. – Guy Gastineau Aug 14 '21 at 14:23

Joachim Breitner · Answer 2 · 2015-03-12T22:24:52.537

1

With yaml, or aeson on which it is based, it is currently not easily possible, says the yaml author, but he has started some experimental support for it in the form of the Data.Yaml.Builder module.

edited Mar 12 '15 at 22:24

answered Feb 02 '14 at 15:22

Joachim Breitner

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Serialize a map to YAML with a specific order

2 Answers2