Fix FromJSON instance for better error msgs

[nitinprakash96]

Before:

ghci> let s = "[{ \"radius\": 2, \"density\": \"x\"}]"
ghci> eitherDecodeStrict s :: Either String GrowthSpread
Left "Error in $: parsing Growth failed, expected Object, but encountered Array

The error msg above is misleading because we weren't trying to parse Growth datatype but GrowthSpread instead.

After:

ghci> let s = "[{ \"radius\": 2, \"density\": \"x\"}]"
ghci> eitherDecodeStrict s :: Either String GrowthSpread
Left "Error in $: parsing GrowthSpread failed, expected Object, but encountered Array"

[nitinprakash96]

A couple of things that I've noticed:

• names in withObject, withText etc do not correspond 1-1 with the datatype. This can be misleading when dealing with error msgs like in the description of the PR.
• Manually written FromJSON instances should have manually written ToJSON instances. This enables round trip tests.
  Here is an example of not doing so:

ghci> let s = "{\"radius\": 2, \"density\": 0.3}"
ghci> eitherDecodeStrict s :: Either String GrowthSpread
Right (GrowthSpread {spreadRadius = 2, spreadDensity = 0.3})
ghci>
ghci> let spread = GrowthSpread 2 0.3
ghci> encode spread
"{\"spreadDensity\":0.3,\"spreadRadius\":2}" -- different than what the input was

Having written it manually gives us more control over the JSON in question. Is there a reason why we do not do so?

[byorgey]

@nitinprakash96 , no, there's no reason to have a manually written FromJSON instance but an autogenerated ToJSON instance. If we can't round-trip the encoding then that is definitely a bug. In fact I just fixed another such bug recently, see #1870. I also added a ticket to test round-tripping of JSON instances in our CI, since currently there's nothing to catch things like this: #1869. Currently, we don't really use the ToJSON instance for anything like GrowthSpread in a scenario description (we only read them from the data files in data/scenarios). But we will eventually need them once we implement #50.

[nitinprakash96]

Makes sense. Let me see if I can get round tripping to work in the test suite.