Maping fields order is not preserved

[majkrzak]

Encoding the following mapping

encode [mapping ["a" .= (1 :: Int), "d" .= (2 :: Int), "c" .= (3 :: Int), "b" .= (4 :: Int)]]

results in:

a: 1
b: 4
c: 3
d: 2

[sjakobi]

Is this a feature request?

[majkrzak]

Depend how to treat it. According to https://yaml.org/spec/1.2.2/#3221-mapping-key-order mapping key order is not important, but from the point of readability is.

[sjakobi]

I agree that preserving the field order is desirable. I'd be happy to accept a PR that implements this.

I think we need to be a bit careful not to introduce a performance regression with this though:

HsYAML/src/Data/YAML/Internal.hs

Lines 40 to 41 in be60400

	-- | YAML mapping
	type Mapping loc = Map (Node loc) (Node loc)

A simple list of pairs might be too slow during lookups:

 type Mapping loc = [(Node loc, Node loc)]

So a type similar to Dhall.Map might be a better choice.