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Error Handling

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Matt Huebert edited this page Jul 24, 2017 · 31 revisions

Philosophy

Maria provides a safe playground for experimentation. Therefore, errors of all sorts must be caught and translated into humane, beginner-friendly language. These errors should be worded gently and should suggest ways to solve the error.

Those interested in how Maria wraps errors should read src/maria/messages.cljs and related namespaces. 🤓 👍🏾

😇 Specific errors we could handle better 😇

When it comes to the fight against opaque error messages, we are stronger together. 💪🏾 Please feel free to add to this list!

Accidentally using single quotes rather than double quotes

expressions with single-quoted terms, like (circle 'hello') slip through undeclared-var detection and result in "invalid is too many arguments!"

  • M: looks like ' is a valid character inside a symbol, so errors will be unpredictable -- caused by passing a symbol where something else is expected. We could try and generate a warning whenever a symbol ends in ' (maybe we can do this check in the magic-tree read or emit phase, otherwise we have to traverse every form). Or combine better overall argument validation (eg. more clojure.spec) with a hook at the end which checks invalid args for patterns like this.

Multiple overloads of a function with the same arity

See https://github.com/yogthos/clojure-error-message-catalog/blob/master/cljs/duplicate-case-test-constant.md for an overview of the error.

We currently catch and reword the initial error, but that is only one-third of the total error message. We still are getting an unwanted follow-up looking like:

Duplicate case test constant '1' on line 1  at line 1 

Error: Duplicate case test constant '1' on line 1  at line 1 
    at new cljs$core$ExceptionInfo (http://localhost:3449/js/compiled/out-user-dev/cljs/core.js:35636:10)
    at Function.cljs.core.ex_info.cljs$core$IFn$_invoke$arity$3 (http://localhost:3449/js/compiled/out-user-dev/cljs/core.js:35712:9)
    at cljs$core$ex_info (http://localhost:3449/js/compiled/out-user-dev/cljs/core.js:35698:26)
    at Function.cljs.analyzer.error.cljs$core$IFn$_invoke$arity$3 (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:1073:26)
    at cljs$analyzer$error (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:1059:28)
    at cljs$analyzer$macroexpand_1 (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:5291:27)
    at Function.cljs.analyzer.analyze_seq.cljs$core$IFn$_invoke$arity$4 (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:5365:41)
    at cljs$analyzer$analyze_seq (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:5337:34)
    at cljs$analyzer$analyze_form (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:5508:34)
    at cljs$analyzer$analyze_STAR_ (http://localhost:3449/js/compiled/out-user-dev/cljs/analyzer.js:5549:38)
{:type :overload-arity, :extra {:name twice}}

- M: this always produces a nice warning map - evaluate `(fn ([x] 1) ([x] 2))` to see just this map without the additional `defn`-related message - we simply need to add this to `maria.messages/reformat-warning`

Undetected misuse of function as integer

(inc dec) causes ""function cljs$core$dec(x){\nreturn (x - (1));\n}1""

  • M: no runtime validation of args. maybe using clojure.spec with instrument can help - added #71.

Wrong order of args in HOF causes no response

See https://github.com/yogthos/clojure-error-message-catalog/blob/master/clj/dont-know-how-to-create-iseq-from.md

(map [1 2 3] inc) causes console error 😿, no result 😿, and a broken environment. 💩!

  • M: lazy seqs... we should be catching errors in format-value, and pass them to our error code, as lazy seqs do not realize until rendered, and even then only one slice at a time. Added #70.

We don't detect misuse of keyword lookups

(:a "B") causes "nil" output, even though we'd like to detect attempted use of key lookups on non-maps and suggest alternatives. See https://github.com/yogthos/clojure-error-message-catalog/blob/master/clj/wrong-num-args-passed-to-keyword.md

Similarly, (:first-name "Bob", :last-name "Bobberton") causes "(intermediate value).cljs$core$IFn$_invoke$arity$3 is not a function\n\nThe function first-name in the expression above needs a different number of arguments.""

Even number of forms in let

We misreport the cause of this. See https://github.com/yogthos/clojure-error-message-catalog/blob/master/clj/let-requires-even-number-forms.md

  • M: looks like all "Could not compile" errors are currently interpreted as 'It looks like you're declaring a function' which is getting in the way.

Destructuring errors

(defn destructure-fn [[a b] num] a) then (destructure-fn 3 [1 2]) causes "nth not supported on this type function Number() { [native code] }"

We should catch the attempt to destructure and point to it as a possible cause when this error occurs. See https://github.com/yogthos/clojure-error-message-catalog/blob/master/clj/nth-not-supported-on-this-type.md

  • M: not sure if we can make sense of this without moving down the stack and showing the location in the destructuring form where the mismatch is occurring. In general, traversing the stack with source lookup and location underline would be a nice thing to have.
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