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Add the relevant dependency to your project:
Leiningen: [com.taoensso/truss "x-y-z"] ; or
deps.edn: com.taoensso/sruss {:mvn/version "x-y-z"}And setup your namespace imports:
(ns my-ns (:require [taoensso.truss :as truss :refer [have have?]]))The main way to use Truss is with the have macro.
You give it a predicate, and an argument that you believe should satisfy the predicate.
For example:
(defn greet
"Given a string username, prints a greeting message."
[username]
(println "hello" (have string? username)))In this case the predicate is string? and argument is username:
- If
(string? username)is truthy: the invariant succeeds and(have ...)returns the given username. - If
(string? username)is falsey: the invariant fails and a detailed error is thrown to help you debug.
That's the basic idea.
These (have <pred> <arg>) annotations are standard Clojure forms that both documents the intention of the code in a way that cannot go stale, and provides a runtime check that throws a detailed error message on any unexpected violation.
Everything else documented here is either:
- Advice on how best to use Truss, or
- Details on features for convenience or advanced situations
You use Truss to formalize assumptions that you have about your data (e.g. function arguments, intermediate values, or current application state at some point in your execution flow).
So any time you find yourself making implementation choices based on implicit information (e.g. the state your application should be in if this code is running) - that's when you might want to reach for Truss instead of a comment or Clojure assertion.
Use Truss assertions like salt in good cooking; a little can go a long way.
While most users will only need to use the base have macro, a few variations are provided for convenience:
| Macro | On success | On failure | Subject to elision? | Comment |
|---|---|---|---|---|
| have | Returns given arg/s | Throws | Yes | Most common |
| have! | Returns given arg/s | Throws | No | As above, without elision |
| have? | Returns true | Throws | Yes | Useful in pre/post conditions |
| have!? | Returns true | Throws | No | As above, without elision |
In all cases:
- The basic syntax is identical
- The behaviour on failure is identical
What varies is the return value, and whether elision is possible.
All examples are from
/examples/truss_examples.cljc
Truss's sweet spot is often in longer, complex code (difficult to show here). So these examples are mostly examples of syntax, not use case. In particular, they mostly focus on simple argument type assertions since those are the easiest to understand.
In practice, you'll often find more value from assertions about your application state or intermediate let values within a larger piece of code.
A Truss (have <pred> <arg>) form will either throw or return the given argument. This lets you use these forms within other expressions and within let bindings, etc.
;; You can add an assertion inline
(println (have string? "foo"))
;; Or you can add an assertion to your bindings
(let [s (have string? "foo")]
(println s))
;; Anything that fails the predicate will throw an error
(have string? 42) ; =>
;; Invariant failed at truss-examples[41,1]: (string? 42)
;; {:dt #inst "2023-07-31T09:58:07.927-00:00",
;; :pred clojure.core/string?,
;; :arg {:form 42, :value 42, :type java.lang.Long},
;; :env {:elidable? true, :*assert* true},
;; :loc
;; {:ns truss-examples,
;; :line 41,
;; :column 1,
;; :file "examples/truss_examples.cljc"}}
;; Truss also automatically traps and handles exceptions
(have string? (/ 1 0)) ; =>
;; Invariant failed at truss-examples[54,1]: (string? (/ 1 0))
;;
;; Error evaluating arg: Divide by zero
;; {:dt #inst "2023-07-31T09:59:06.149-00:00",
;; :pred clojure.core/string?,
;; :arg
;; {:form (/ 1 0),
;; :value truss/undefined-arg,
;; :type truss/undefined-arg},
;; :env {:elidable? true, :*assert* true},
;; :loc
;; {:ns truss-examples,
;; :line 54,
;; :column 1,
;; :file "examples/truss_examples.cljc"},
;; :err
;; #error
;; {:cause "Divide by zero"
;; :via
;; [{:type java.lang.ArithmeticException
;; :message "Divide by zero"
;; :at [clojure.lang.Numbers divide "Numbers.java" 190]}]
;; :trace
;; [<...>]}};; You can assert against multipe args at once
(let [[x y z] (have string? "foo" "bar" "baz")]
(str x y z)) ; => "foobarbaz"
;; This won't compromise error message clarity
(let [[x y z] (have string? "foo" 42 "baz")]
(str x y z)) ; =>
;; Invariant failed at truss-examples[89,15]: (string? 42)
;; {:dt #inst "2023-07-31T10:01:00.991-00:00",
;; :pred clojure.core/string?,
;; :arg {:form 42, :value 42, :type java.lang.Long},
;; :env {:elidable? true, :*assert* true},
;; :loc
;; {:ns truss-examples,
;; :line 89,
;; :column 15,
;; :file "examples/truss_examples.cljc"}}You can attach arbitrary debug data to be displayed on violations:
(defn my-handler [ring-req x y]
(let [[x y] (have integer? x y :data {:ring-req ring-req})]
(* x y)))
(my-handler {:foo :bar} 5 nil) ; =>
;; Invariant failed at truss-examples[107,15]: (integer? y)
;; {:dt #inst "2023-07-31T10:02:03.415-00:00",
;; :pred clojure.core/integer?,
;; :arg {:form y, :value nil, :type nil},
;; :env {:elidable? true, :*assert* true},
;; :loc
;; {:ns truss-examples,
;; :line 107,
;; :column 15,
;; :file "examples/truss_examples.cljc"},
;; :data {:dynamic nil, :arg {:ring-req {:foo :bar}}}}And you can attach shared debug data at the binding level:
(defn wrap-ring-dynamic-assertion-data
"Returns Ring handler wrapped so that assertion violation errors in handler
will include `(data-fn <ring-req>)` as debug data."
[data-fn ring-handler-fn]
(fn [ring-req]
(truss/with-data (data-fn ring-req)
(ring-handler-fn ring-req))))
(defn ring-handler [ring-req]
(have? string? 42) ; Will always fail
{:status 200 :body "Done"})
(def wrapped-ring-handler
(wrap-ring-dynamic-assertion-data
;; Include Ring session with all handler's assertion errors:
(fn data-fn [ring-req] {:ring-session (:session ring-req)})
ring-handler))
(wrapped-ring-handler
{:method :get :uri "/" :session {:user-name "Stu"}}) ; =>
;; Invariant failed at truss-examples[136,3]: (string? 42)
;; {:dt #inst "2023-07-31T10:02:41.459-00:00",
;; :pred clojure.core/string?,
;; :arg {:form 42, :value 42, :type java.lang.Long},
;; :env {:elidable? true, :*assert* true},
;; :loc
;; {:ns truss-examples,
;; :line 136,
;; :column 3,
;; :file "examples/truss_examples.cljc"},
;; :data {:dynamic {:ring-session {:user-name "Stu"}}, :arg nil}};;; Compare
(have vector? [:a :b :c]) ; => [:a :b :c]
(have keyword? :in [:a :b :c]) ; => [:a :b :c]Just make sure to use the have? variant which always returns a truthy val on success:
(defn square [n]
;; Note the use of `have?` instead of `have`
{:pre [(have? #(or (nil? %) (integer? %)) n)]
:post [(have? integer? %)]}
(let [n (or n 1)]
(* n n)))
(square 5) ; => 25
(square nil) ; => 1Truss offers some shorthands for your convenience. These are all optional: the same effect can always be achieved with an equivalent predicate fn:
;; A predicate can be anything
(have #(and (integer? %) (odd? %) (> % 5)) 7) ; => 7
;; Omit the predicate as a shorthand for #(not (nil? %))
(have "foo") ; => "foo"
(have nil) ; => Error
;;; There's a number of other optional shorthands
;; Combine predicates (or)
(have [:or nil? string?] "foo") ; => "foo"
;; Combine predicates (and)
(have [:and integer? even? pos?] 6) ; => 6
;; Element of (checks for set containment)
(have [:el #{:a :b :c :d nil}] :b) ; => :b
(have [:el #{:a :b :c :d nil}] nil) ; => nil
(have [:el #{:a :b :c :d nil}] :e) ; => Error
;; Superset
(have [:set>= #{:a :b}] #{:a :b :c}) ; => #{:a :b :c}
;; Key superset
(have [:ks>= #{:a :b}] {:a "A" :b nil :c "C"}) ; => {:a "A" :b nil :c "C"}
;; Non-nil keys
(have [:ks-nnil? #{:a :b}] {:a "A" :b nil :c "C"}) ; => ErrorNo need for any special syntax or concepts, just define a function as you'd like:
;; A custom predicate:
(defn pos-int? [x] (and (integer? x) (pos? x)))
(defn have-person
"Returns given arg if it's a valid `person`, otherwise throws an error"
[person]
(truss/with-data {:person person} ; (Optional) setup some extra debug data
(have? map? person)
(have? [:ks>= #{:age :name}] person)
(have? [:or nil? pos-int?] (:age person)))
person ; Return input if nothing's thrown
)
(have-person {:name "Steve" :age 33}) ; => {:name "Steve", :age 33}
(have-person {:name "Alice" :age "33"}) ; => Error