Custom formatter for ex-info thrown by cljs.spec

[olivergeorge]

Currently it's quite tricky to get good output from cljs.spec. Reason being that the ex-info thrown includes the full spec commentary in ex-message. This includes the serialised val which failed. In my case this can fill the console many times over.

Alex Miller recommended catching these exceptions and using the ex-data to present the spec nicely. (e.g. don't print the ex-message to the console). This would be fine except it's difficult to catch all errors due to the nature of how UI code executes (react render loop, re-frame dispatch queue, callbacks)

How about automatically detecting and pretty formatting ex-info errors thrown by cljs.spec?

[darwin]

I like your idea. But how do you think cljs-devtools could implement this without this feature being framework-specific?

[darwin]

Ok, thinking about this more. We could install window.onerror handler as we do with sanity hints[1] and do some magic there. But if "framework" catches the exceptions, we are out of luck.

[1]

cljs-devtools/src/lib/devtools/sanity_hints.cljs

Line 123 in 868cf1e

(defn install-type-error-enhancer []

[olivergeorge]

That sounds like a good approach.

I guess the other possibility is to extend IFormat for ExceptionInfo and use ex-data to enable special treatment.

[darwin]

To keep the ball rolling, I have just tested ExceptionInfo behaviour[1].

Currently cljs-devtools has pretty nice behaviour, it presents the error printed with IPrintWithWriter protocol[2]. The implementation is questionable because they are re-implementing clojurescript map printer in there, but that is not essential to our issue at hand.

I think nothing is stopping you overriding IPrintWithWriter on ExceptionInfo and doing something better there (e.g. abbreviating long messages).

But I agree that we could do even better job with IFormat. We could reuse string abbreviation logic already implemented[3] in devtools and present other parts as expandable data.

[1] binaryage/cljs-devtools-sample@6422ec1
[2] https://github.com/clojure/clojurescript/blob/e2db5d9ff8cb6a099ebc2a8cd379385bf4649b38/src/main/cljs/cljs/core.cljs#L10283-L10286
[3]

cljs-devtools/src/lib/devtools/formatters/markup.cljs

Line 89 in 9c2eda4

(defn <string> [string]

[darwin]

btw. here is how it looks currently:

[darwin]

Actually this ExceptionInfo is not a fully working CLJS type, so cljs-devtools do not treat it correctly under all circumstances.

I went down the rabbit hole and proposed a solution upstream:
http://dev.clojure.org/jira/browse/CLJS-1722

Let's wait for its resolution before moving forward.

[olivergeorge]

Interesting. Thanks. (I'd love to know what Bruce is thinking about presenting spec errors. Can't help but think he'll be the trendsetter.)

[arichiardi]

This would be a killer feature!