The document discusses the challenges and improvements in error handling within the Clojure programming language, specifically with regard to the evolution of error messages from versions 1.9.0 to 1.10.0 and the integration of various tools and libraries for better developer experience. It highlights the use of Clojure.spec for validation and error reporting, along with third-party tools like kibit and eastwood. The content emphasizes the importance of fast failure during the development process and proposes community collaboration to enhance error formatting and handling.

1. Fun With Errors?
Clojure Finland Meetup, MOW@Tampere
Tommi Reiman
tommi@metosin.
@ikitommi

2. Who is interested in errors?
Developers <-- you, me!, the girl/guy next to you?
Programs
Systems
End Users

3. Clojure & Error Messages
Bare-bones before 1.9.0
Really bad with 1.9.0 (clojure.spec)
Kinda ok in 1.10.0 (data + helpers)

4. 1.9.0
(ns kikka
(require '[clojure.string]))
; CompilerException clojure.lang.ExceptionInfo: Call to clojure.
; did not conform to spec: In: [1] val: ((require (quote [clojur
; fails spec: :clojure.core.specs.alpha/ns-form at: [:args] pred
; (cat :docstring (? string?) :attr-map (? map?) :clauses :cloju
; Extra input #:clojure.spec.alpha{:problems [{:path [:args], :r
; "Extra input", :pred (clojure.spec.alpha/cat :docstring (cloju
; clojure.core/string?) :attr-map (clojure.spec.alpha/? clojure.
; :clojure.core.specs.alpha/ns-clauses), :val ((require (quote [
; :via [:clojure.core.specs.alpha/ns-form], :in [1]}], :spec #ob
; "clojure.spec.alpha$regex_spec_impl$reify__2436@3c18dcbf"], :v
; (kikka (require (quote [clojure.string]))), :args (kikka (requ
; [clojure.string])))}, compiling:(/Users/tommi/projects/metosin

5. 1.10.0
(ns kikka
(require '[clojure.string]))
; Syntax error macroexpanding clojure.core/ns at
; (spec.cljc:141:1). ((require (quote [clojure.string]))) -
; failed: Extra input spec: :clojure.core.specs.alpha/ns-form

6. New helpers in 1.10.0
Throwable->map , ex-triage , ex-str
clojure.main/repl with :caught option
https://clojure.org/reference/repl_and_main

7. New ex-data in 1.10.0
:clojure.error/phase - phase indicator
:clojure.error/source - le name (no path)
:clojure.error/line - integer line number
:clojure.error/column - integer column number
:clojure.error/symbol - symbol being
expanded/compiled/invoked
:clojure.error/class - cause exception symbol
:clojure.error/cause - cause exception message
:clojure.error/spec - explain-data for a spec error

9. ETA

10. ELM

11. Clojure 3rd party thingies

12. IDEs & Linters
Kibit (https://github.com/jonase/kibit)
Eastwood (https://github.com/jonase/eastwood)
Cursive (https://cursive-ide.com/)
Joker (https://github.com/candid82/joker)
CLJ-Kondo (https://github.com/borkdude/clj-
kondo)

13. Pretty (developers)
https://github.com/AvisoNovate/pretty

14. Schema
https://github.com/plumatic/schema
(app {:request-method :post
:uri "/api/plus"
:query-params {"x" "1", "y" "-7"}})
; {:status 500,
; :body {:schema {:total "(constrained Int PositiveInt)"},
; :errors {:total "(not (PositiveInt -6))"},
; :type :reitit.coercion/response-coercion,
; :coercion :schema,
; :value {:total -6},
; :in [:response :body]}}

15. clojure.spec
(require '[clojure.spec.alpha :as s])
(s/def ::city string?)
(s/def ::state string?)
(s/def ::place (s/keys :req-un [::city ::state]))
(s/explain-data ::place {:city "Denver", :state :CO})
;(:problems ({:path [:state],
; :pred clojure.core/string?,
; :val :CO,
; :via [::place ::state],
; :in [:state]}),
; :spec :spec-tools.data-spec-test/place,
; :value {:city "Denver", :state :CO} }

16. Expound 1/2
https://github.com/bhb/expound
(require '[expound.alpha :as expound])
(expound/expound ::place {:city "Denver", :state :CO})
;; -- Spec failed --------------------
;;
;; {:city ..., :state :CO}
;; ^^^
;;
;; should satisfy
;;
;; string?
;;
;; -------------------------
;; Detected 1 error

17. Expound 2/2
Ships with custom pretty printer

18. Spell-spec
https://github.com/bhauman/spell-spec
spell-spec.alpha/strict-keys

20. Metosin Public Corner
Integrating spell-spec with spec-tools for closed
validation of spec'd con gurations (with
@bhauman-grade error reporting)
metosin/reitit is a playground of joyful things,
including data, performance and... error messages!
Some Schema too?

21. Ingredients
Fipp, fast Idiomatic Pretty-Printer
https://github.com/brandonbloom/ pp
Expound & Spell-Spec
Lovely colors from rebel-readline
https://github.com/bhauman/rebel-readline
Our libraries (spec-tools & friends)

22. Principles
Data-oriented libraries & tools
Design to Fail-fast
Good developer experience
Lead by Example
Towards clj-commons error formatter?

23. Fail Fast
During static analysis (e.g. linter)
At compile-time (macros, defs)
At creation-time
At development-time (schema/spec annos)
At runtime ( )

24. On Component 1/2
Clean separation of creation and request-time
Support declarative validation at creation-time
Correct Initialization, Fast Runtime
(defn coerce-request-interceptor
"Interceptor for pluggable request coercion.
Expects a :coercion of type `reitit.coercion/Coercion`
and :parameters, otherwise does not mount."
[]
{:name ::coerce-request
:spec ::rs/parameters
:compile (fn [{:keys [coercion parameters]} opts]
...)})

25. On Component 2/2
Schema is great at validating function args:
clojure.spec has fdef and s/assert
(require '[schema.core :as s])
(s/defn ^:always-validate interceptor-x
[opts :- {:string? s/Bool}]
...)
(coerce-request-interceptor {})
; Syntax error (ExceptionInfo) compiling at (test.cljc:150:1).
; Input to interceptor-x does not match schema:
;
; [(named {:string? missing-required-key} opts)]

26. On Error
No formatting here, just emit a unique id for the
error and all the needed info for print the error:
(when-let [problems (validate-route-data routes spec)]
(exception/fail!
::invalid-route-data
{:problems problems}))

27. On (Router) Creation
Before there is a way to hook a custom exception
handler into a running REPL, we just catch the
exceptions ourself on the public api and use a
con gured formatter for the errors:
(defn router [raw-routes opts]
(try
...
(catch #?(:clj Exception, :cljs js/Error) e
(throw ((get opts :exception identity) e)))))

28. Formatting (DEMO)

29. Formatting Guide
Respect the user
Use lame colors, it's not xmas
Just simple tips (or none)
Link to documentation (if needed)

30. On route con ict 1/2
(require '[reitit.core :as r])
(require '[reitit.dev.pretty :as pretty])
(r/router
[["/ping"]
["/:user-id/orders"]
["/bulk/:bulk-id"]
["/public/*path"]
["/:version/status"]]
{:exception pretty/exception})

31. On route con ict 2/2

32. Invalid Route Data 1/2
(require '[reitit.spec :as spec])
(require '[clojure.spec.alpha :as s])
(s/def ::role #{:admin :user})
(s/def ::roles (s/coll-of ::role :into #{}))
(r/router
["/api/admin" {::roles #{:adminz}}]
{:validate spec/validate
:exception pretty/exception})

33. Invalid Route Data 2/2

34. All solved?

35. Challenges
clojure.spec is open by design
enables growth (and typos!)
clojure.spec is macros (Spec2 adds functions)
clojure.spec doesn't support rt-transformations
Spec2 is xing some of the things, WIP
(Remember Schema, anyone?)

36. Batteries to clojure.spec ?
Ability to deep-merge (map) specs
Ability to close (map) specs
s/select in Spec2 looks promising (WIP, TBD):
(s/select
::user
[::first ::last ::addr
{::addr [::street ::city ::state ::zip]}])

37. While waiting
spec-tools.spell will have functions to close specs
Work nicely with data-specs (DEMO)
Might work with normal specs using Coercion
https://cljdoc.org/d/metosin/spec-
tools/0.9.1/doc/spec-coercion
Will integrate to reitit if that ever works
st/defn to help validating component options?

39. Final Words
Clojure is a Dynamic language -> Fail Fast(!!!)
Fast linters coming ( joker & clj-kondo )
Ongoing work to make reitit awesome
Fipp, Expound, spell-spec & spec-tools
Community should build a kick-ass error
formatter & rules for Clojure, Made in ?
Join #reitit & #clj-commons in Slack
join@metosin.

40. Thanks.