Just had a chance to play around with spec. Looks like this is going to 
destroy a lot of problem space :) Thanks.

Probably a bug:
      (s/exercise
        (s/and
         set?
         (s/coll-of
          (s/with-gen keyword?
            #(gen/elements [:s1 :s2 :s3]))
          #{})))
;->
([#{} #{}] [#{} #{}] [#{} #{}] [#{:o-85:1ywl} #{:o-85:1ywl}] [#{} #{}] [#{} 
#{}] [#{} #{}] 
[#{:_Qi.Qj?dtMZh_s*3.x.sTxm9-E.NHr!?b5f0Ir2.u.+bof*-P.r.m_y**e0ntq.W+*.+?Urxe+Xp+/Q}
 
#{:_Qi.Qj?dtMZh_s*3.x.sTxm9-E.NHr!?b5f0Ir2.u.+bof*-P.r.m_y**e0ntq.W+*.+?Urxe+Xp+/Q}]
 
[#{} #{}] [#{} #{}])

Reverse argument order to s/and and it works.

On Monday, May 23, 2016 at 4:12:29 PM UTC+2, Rich Hickey wrote:
>
> Introducing clojure.spec 
>
> I'm happy to introduce today clojure.spec, a new core library and support 
> for data and function specifications in Clojure. 
>
> Better communication 
>
> Clojure is a dynamic language, and thus far we have relied on 
> documentation or external libraries to explain the use and behavior of 
> functions and libraries. But documentation is difficult to produce, is 
> frequently not maintained, cannot be automatically checked and varies 
> greatly in quality. Specs are expressive and precise. Including spec in 
> Clojure creates a lingua franca with which we can state how our programs 
> work and how to use them. 
>
> More leverage and power 
>
> A key advantage of specifications over documentation is the leverage they 
> provide. In particular, specs can be utilized by programs in ways that docs 
> cannot. Defining specs takes effort, and spec aims to maximize the return 
> you get from making that effort. spec gives you tools for leveraging specs 
> in documentation, validation, error reporting, destructuring, 
> instrumentation, test-data generation and generative testing. 
>
> Improved developer experience 
>
> Error messages from macros are a perennial challenge for new (and 
> experienced) users of Clojure. specs can be used to conform data in macros 
> instead of using a custom parser. And Clojure's macro expansion will 
> automatically use specs, when present, to explain errors to users. This 
> should result in a greatly improved experience for users when errors occur. 
>
> More robust software 
>
> Clojure has always been about simplifying the development of robust 
> software. In all languages, dynamic or not, tests are essential to quality 
> - too many critical properties are not captured by common type systems. 
> spec has been designed from the ground up to directly support generative 
> testing via test.check https://github.com/clojure/test.check. When you 
> use spec you get generative tests for free. 
>
> Taken together, I think the features of spec demonstrate the ongoing 
> advantages of a powerful dynamic language like Clojure for building robust 
> software - superior expressivity, instrumentation-enhanced REPL-driven 
> development, sophisticated testing and more flexible systems. I encourage 
> you to read the spec rationale and overview  http://clojure.org/about/spec. 
> Look for spec's inclusion in the next alpha release of Clojure, within a 
> day or so. 
>
> Note that spec is still alpha, and some details are likely to change. 
> Feedback welcome. 
>
> I hope you find spec useful and powerful! 
>
> Rich 
>
>

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