Add support for key transforming functions when generating/parsing JSON

[rs3vans]

Cheshire supports arbitrary functions that transform the map keys when going to/from JSON. This PR adds support for passing in such transformers (beyond the already-supported :keywords? option).

[weavejester]

I think you want the precondition to ensure that you cannot have a :key-fn and :keywords? at the same time. You might also want to mention it in the docstring.

[rs3vans]

Actually, no, you can have them both at the same time (as it says in the README). The idea was that they could be combined. Maybe that's too confusing?

[weavejester]

But as it's written, the code doesn't do that. The argument is either true for keywords, or a function otherwise. In addition, if you allow both options to be set at once, you need to define how that behaves.

[rs3vans]

Actually, check out line no. 16 in json.clj. The redefining of key-fn in the let bindings wraps the passed-in function so that its value is passed through the keyword function (but only if both key-fn and keywords? are both non-false). Cheshire's parse-string function takes either a key function, or true (meaning use keywords), so that's why I did it the way I did...

Also there's a test ("transform keys with keywords") for exactly this combination of options.

[weavejester]

Ah, I see. My apologies; I missed the line where you compose the two functions first.

I think that behaviour is okay then, but I'd change the way key-fn is defined to something like:

(comp (if keywords? keyword identity) (or key-fn identity))

Then we can just write:

(json/parse-string body-string key-fn)

You can do a similar thing in the tests. Instead of:

#(-> % name clojure.string/upper-case)

You can write:

(comp str/upper-case name)

Also remember to require clojure.string in the test namespace if you're going to use it.

[rs3vans]

Suggested changes made :)

[kardan]

Was looking for this feature, how is it moving along or yet to be decided? Thanks for the work!

[rs3vans]

@kardan You're welcome to use my repository until it gets merged here (if it ever does).

[weavejester]

The notification for the PR got lost or never made it to my inbox. I'll take a look over it.

[kardan]

Cool! You guys didn't seem to totally disagree, so thought it might slipped through the net.

[rs3vans]

@weavejester Any chance you want to merge this? It's been sitting around for a while. If not, just close it out - no hard feelings!

[weavejester]

Added a few comments. Sorry this keeps getting lost off the bottom of my TODO list.

[weavejester]

Once the comments are addressed, if you squash your commits, we should be good to merge.