prefer String instance of ToYaml over [Char]

[bergey]

No description provided.

[snoyberg]

I'd be very nervous about including OVERLAPPING like this and would like to see more motivation before including it.

[bergey]

Sorry for the lack of explanation. The current String instance overlaps with the general [a] instance. As I understand it, without some guidance, GHC won't pick between them; my attempts to call toYaml on String do not compile. I think OVERLAPPING is less intrusive than OVERLAPPABLE on the [a] instance.

[snoyberg]

The String instance was added in bde18b5. Maybe @robbiemcmichael can weigh in here?

[robbiemcmichael]

This one's my fault for not adding a test that called toYaml on a String.

I was under the impression that the constraint on

instance ToYaml a => ToYaml [a] where
    toYaml = array . map toYaml

would mean that the instances aren't overlapping since there's no ToYaml instance for Char, but that doesn't seem to be the case.

I had a look at how the list instance works for Show. They define an extra function showList with a default defined in the type class which gets overridden with a special definition for the Char instance. That trick looks like it might work here but it would require defining an instance of ToYaml for Char. Although Char doesn't really have a corresponding YAML type, we could just choose to convert it to a YAML string containing just a single character.

I don't know if that mess is more or less desirable than using OVERLAPPING so I'd probably rather someone else made that call.

[bergey]

I think adding a list method to ToYaml would also meet my needs.

My understanding is that GHC looks for an instance that matches right of the =>, and if it finds one, searches for the constraints from the left of the =>. I think it never backtracks.

[snoyberg]

I don't use this particular API myself. But my leaning is to avoid usage of String at all, as well as the associated lists [(a, b)]. That said: I'd be fine with OVERLAPPING. Can both of you check that this PR actually works as you'd expect?

[bergey]

Below are a few tests that I believe cover the instance resolution. I have a somewhat larger project that uses YamlBuilder to output hpack package.yaml files, where it is also behaving as expected.

import Data.Yaml.Builder
λ BS.putStrLn $ toByteString ([1..4] :: [Int])
- 1
- 2
- 3
- 4

λ BS.putStrLn $ toByteString "a single string"
a single string

λ BS.putStrLn $ toByteString (words "several strings in a list")
- several
- strings
- in
- a
- list

[snoyberg]

OK, I'm fine with including this change as-is. Would you mind adding a ChangeLog entry and minor version bump?