# Lazy Contracts in 30 lines

12 January 2015

## Instance Level Interface (Typeclass) Extension

Once again continuing previous lines of thought let’s consider validations beyond :pre and :post conditions. This time we’ll see how ClojureScript’s specify construct allows us to trivially build a form of lazy contracts similar to those described by Findler et al.. Note: the following approach can be duplicated with deftype or reify on Clojure JVM, but specify is more succinct.

Lazy contracts are useful because they allow us to lazily validate a large data structure. All the previous examples with :pre and :post validated eagerly which is quite impractical when dealing with large amounts of data.

For some time now ClojureScript has supported extension of individual values to protocols via specify. Again protocols are analogous to Java interfaces, Go interfaces, Haskell typeclasses, and Objective-C protocols. Clojure has had protocols since 1.2.0 and they are commonly leveraged via extend-type and extend-protocol. However specify is only currently available in ClojureScript as the semantics of JavaScript and the performance of modern JavaScript engines permit practical efficient implementation.

Imagine that we want to ensure that a vector contains only even numbers. Fortunately there are only two ways to get an updated vector in Clojure, conj and assoc. So given an existing vector we simply need to specify new implementations of the ICollection and IVector protocols. The only other protocols we care about are those involved in iteration - for simplicity’s sake we’re only going to bother with ISeqable, however for completeness you should implement IReduce and IKVReduce. They are trivial to do and involve the same simple delegation approach.

## Lazy Contracts

We will provide a function called add-contract* that takes 3 arguments: a vector, a Var and a map of source location information that informs us where the contract was asserted. This is important for debugging as this allows us to prune our search - the contract violation had to occur before the contract. In a mutable context this wouldn’t be very useful but because of immutability we can laser in on the origin of the issue.

As to why we take a Var for the second argument, the Var holds useful reflection information if we want to print verbose error messages. We can also just deref the function from the Var to get the validating function. A first attempt at add-contract* might look like the following:

(ns lazy-contracts.core)

(enable-console-print!)

([v cvar src-info] (add-contract* v cvar @cvar src-info))
([v cvar f src-info]
(specify v
ISeqable
(-seq [_]
(map #(do (assert (f %)) %) v))
ICollection
(-conj [_ x]
(assert (f x))
(add-contract* (-conj v x) cvar f src-info))
IVector
(-assoc-n [_ i x]
(assert (f x))
(add-contract* (-assoc-n v i x) cvar f src-info)))))


This won’t be very fun to use since the assertion doesn’t print an informative string. Let’s fix that by adding contract-fail-str:

(defn contract-fail-str [x {:keys [ns name]} src-info]
(str x " fails vector contract " (symbol (str ns) (str name))
" specified at " (:file src-info) ":" (:line src-info)))


Now let’s rewrite add-contract*:

(defn add-contract*
([v cvar src-info] (add-contract* v cvar @cvar src-info))
([v cvar f src-info]
(specify v
ISeqable
(-seq [_]
(letfn [(check [x]
(assert (f x)
(contract-fail-str x (meta cvar) src-info))
x)]
(map check v)))
ICollection
(-conj [_ x]
(assert (f x) (contract-fail-str x (meta cvar) src-info))
(add-contract* (-conj v x) cvar f src-info))
IVector
(-assoc-n [_ i x]
(assert (f x) (contract-fail-str x (meta cvar) src-info))
(add-contract* (-assoc-n v i x) cvar f src-info)))))


Much nicer!

Now the only problem is providing source location information. Fortunately this trivial through a little macro sugar:

In a lazy-contract.core macro file we write the following:

(ns blog.contracts.core)

(blog.contracts.core/add-contract*
~v ~cvar ~(select-keys (meta &form) [:file :line])))


That’s it!

Now we can start a REPL and try to create a contract constrained vector:

To quit, type: :cljs/quit
ClojureScript Node.js REPL server listening on 54977
ClojureScript:cljs.user> (require '[blog.contracts.core :as c :include-macros true])
ClojureScript:cljs.user> (c/add-contract [2 4 6] #'even?)
[2 4 6]


This works fine. But if we try this we’ll get a sensible error:

ClojureScript:cljs.user> (conj (c/add-contract [2 4 6] #'even?) 7)
Error: Assert failed: 7 fails vector contract cljs.core/even? ...


Voila!

We’ll also get a sensible error even in simple cases like this:

ClojureScript:cljs.user> (c/add-contract [1 2 3] #'even?)
Error: Assert failed: 1 fails vector contract cljs.core/even?
`

This is because printing needs to traverse the vector!

Again if a program with a lazy contract fails the error will always tell you precisely where the contract was asserted.

## Conclusion

Of the course the above could benefit from generalization but we already have a powerful lazy contract system without bothering with wrappers or having to dig into the implementation of any of the core data structures.

With very little effort ClojureScript programs can leverage powerful forms of runtime validation with precise provenance.

Happy hacking!