Trammel is a Clojure providing contracts programming (sometimes called “Design by Contract” or DbC) capabilities. Features of Trammel currently include:

• Contracts on functions
• Composable contracts
• Post-definition function contract application
• defrecord and deftype invariants
• Reference type invariants (i.e. Agents, Atoms, Refs, and Vars)
• Ability to turn-off checks in a production setting
• Support for Clojure versions 1.3 and 1.4

Trammel is inspired by Eiffel and Racket.

Absorb

You can use Trammel in your Leiningen and Cake projects with the following :dependencies directive in your project.clj file:

    [trammel "0.7.0]

For Maven-driven projects, use the following slice of XML in your pom.xml’s <dependencies> section:

<dependency>
  <groupId>org.clojure</groupId>
  <artifactId>trammel</artifactId>
  <version>0.7.0</version>
</dependency>

Enjoy!

Places

• Source code
• Ticket system
• Examples and documentation – in progress
• Dream Date - an example project using Trammel – in progress

Changes from version 0.6.0

• Better assertion error reporting
• Ability to add constraint descriptions used in assertion reporting
• Constrained defrecords no longer support default field values
• Constrained defrecords no longer generate keyword args ctor function
• Reference type invariants

Reference type invariants work in the following ways.

Atoms

Atoms are created directly with their invariants:

(def a (constrained-atom 0
         "only numbers allowed"
         [number?]))

@a 
;=> 0

And checked on change:

(swap! a inc)
;=> 1

Invariant violations are reported right away:

(swap! a str)
; Pre-condition failure: only numbers allowed 

(compare-and-set! a 0 "a")
; Pre-condition failure: only numbers allowed 

Refs

Refs are created directly with their invariants:

(def r (constrained-ref 0
         "only numbers allowed"
         [number?]))

And also checked on change, within a transaction:

(dosync (alter r inc))
;=> 1

(dosync (alter r str))
; Pre-condition failure: only numbers allowed 

Vars

Vars are created directly with their invariants:

(defconstrainedvar ^:dynamic foo 0
  "only numbers allowed in Var foo"
  [number?])

Var invariants are checked on rebinding, such as with binding:

(binding [foo :a] [foo])
; Pre-condition failure: only numbers allowed 

Agents

Agents are created directly with their invariants:

(def ag (constrained-agent 0
         "only numbers allowed"
         [number?]))

And are checked on send and send-off

(send ag str)
(agent-error ag)

However, the invariant violations are reported consistently with the agent setup. In this case, the errors are accessible via the agent-error function.

Plans

The following capabilities are under design, development, or consideration for future versions of Trammel:

• Contracts for higher-order functions
• Better error messages
• defconstraint – with ability to relax requires and tighten ensures
• Modify macros to also allow regular Clojure constraint maps
• Allow other stand-alones: true/false, numbers, characters, regexes
• Marrying test.generative with Trammel
• More documentation and examples

More planning is needed around capabilities not listed nor thought of.