While defining contracts local to the constrained function is nice (see defconstrainedfn for more information), very often you will find yourself in possession of an existing function that is not constrained:
<div class=”gist”> <div class=”gist-file”> <div class=”gist-data gist-syntax”> <div class=”gist-highlight”> <pre><div class=”line” id=”LC3”><span class=”p”>(</span><span class=”nf”>sqr</span> <span class=”mi”>0</span><span class=”p”>)</span></div><div class=”line” id=”LC4”><span class=”c1”>;=> 0</span></div></pre> </div> </div> </div> </div>
In this case, core.contracts provides the provide-contracts macro to define contracts and apply them dynamically to existing functions:
<div class=”gist”> <div class=”gist-file”> <div class=”gist-data gist-syntax”> <div class=”gist-highlight”> <pre><div class=”line” id=”LC1”><span class=”p”>(</span><span class=”nf”>provide-contracts</span> </div><div class=”line” id=”LC2”> <span class=”p”>[</span><span class=”nv”>sqr</span> <span class=”s”>”Constraints for squaring”</span> </div><div class=”line” id=”LC3”> <span class=”p”>[</span><span class=”nv”>x</span><span class=”p”>]</span> <span class=”p”>[</span><span class=”nv”>number?</span> <span class=”p”>(</span><span class=”nb”>not= </span><span class=”mi”>0</span> <span class=”nv”>x</span><span class=”p”>)</span> <span class=”nv”>=></span> <span class=”nv”>number?</span> <span class=”nv”>pos?</span><span class=”p”>]])</span></div><div class=”line” id=”LC4”><br/></div><div class=”line” id=”LC5”><span class=”p”>(</span><span class=”nf”>sqr</span> <span class=”mi”>0</span><span class=”p”>)</span></div><div class=”line” id=”LC6”><span class=”c1”>; java.lang.AssertionError: </span></div><div class=”line” id=”LC7”><span class=”c1”>; Assert failed: (not= 0 n)</span></div></pre> </div> </div> </div> </div>
As shown, the sqr function is dynamically modified with the contract defined in the body of provide-contracts. This macro can take any number of vectors where each corresponds to a contract for a given function; including multiple arities embedded within. core.contracts also allows you to include existing named contracts (see defcontract for more information) instead of the contract specification vector, as shown below:
<div class=”gist”> <div class=”gist-file”> <div class=”gist-data gist-syntax”> <div class=”gist-highlight”> <pre><div class=”line” id=”LC1”><span class=”p”>(</span><span class=”nf”>defcontract</span> <span class=”nv”>sqr-contract</span></div><div class=”line” id=”LC2”> <span class=”s”>”Defines the constraints on squaring.”</span></div><div class=”line” id=”LC3”> <span class=”p”>[</span><span class=”nv”>n</span><span class=”p”>]</span> <span class=”p”>[</span><span class=”nv”>number?</span> <span class=”p”>(</span><span class=”nb”>not= </span><span class=”mi”>0</span> <span class=”nv”>n</span><span class=”p”>)</span> <span class=”nv”>=></span> <span class=”nv”>pos?</span> <span class=”nv”>number?</span><span class=”p”>])</span></div><div class=”line” id=”LC4”><br/></div><div class=”line” id=”LC5”><span class=”p”>(</span><span class=”nf”>sqr</span> <span class=”mi”>0</span><span class=”p”>)</span></div><div class=”line” id=”LC6”><span class=”c1”>;=> 0</span></div><div class=”line” id=”LC7”><br/></div><div class=”line” id=”LC8”><span class=”p”>(</span><span class=”nf”>provide-contracts</span></div><div class=”line” id=”LC9”> <span class=”p”>[</span><span class=”nv”>sqr</span> <span class=”s”>”Apply the contract for squaring”</span> </div><div class=”line” id=”LC10”> <span class=”nv”>sqr-contract</span><span class=”p”>])</span></div><div class=”line” id=”LC11”><br/></div><div class=”line” id=”LC12”><span class=”p”>(</span><span class=”nf”>sqr</span> <span class=”mi”>0</span><span class=”p”>)</span></div><div class=”line” id=”LC13”><span class=”c1”>; java.lang.AssertionError: </span></div><div class=”line” id=”LC14”><span class=”c1”>; Assert failed: (not= 0 n)</span></div></pre> </div> </div> </div> </div>
provide-contracts gives you a lot of flexibilty in how to separate functions from their contracts and likewise apply them in domain-specific ways.