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<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="writing-a-contract-using-clauses">
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<h1>Writing a contract using clauses<a class="headerlink" href="#writing-a-contract-using-clauses" title="Permalink to this headline">¶</a></h1>
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<p>This tutorial will take you through restructuring the commercial paper contract to use clauses. You should have
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already completed “<a class="reference internal" href="tutorial-contract.html"><span class="doc">Writing a contract</span></a>”.</p>
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<p>Clauses are essentially micro-contracts which contain independent verification logic, and can be logically composed
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together to form a contract. Clauses are designed to enable re-use of common logic, for example issuing state objects
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is generally the same for all fungible contracts, so a common issuance clause can be inherited for each contract’s
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issue clause. This cuts down on scope for error, and improves consistency of behaviour. By splitting verification logic
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into smaller chunks, they can also be readily tested in isolation.</p>
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<p>Clauses can be composed of subclauses, for example the <code class="docutils literal"><span class="pre">AllClause</span></code> or <code class="docutils literal"><span class="pre">AnyClause</span></code> clauses take list of clauses
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that they delegate to. Clauses can also change the scope of states and commands being verified, for example grouping
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together fungible state objects and running a clause against each distinct group.</p>
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<p>The commercial paper contract has a <code class="docutils literal"><span class="pre">Group</span></code> outermost clause, which contains the <code class="docutils literal"><span class="pre">Issue</span></code>, <code class="docutils literal"><span class="pre">Move</span></code> and <code class="docutils literal"><span class="pre">Redeem</span></code>
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clauses. The result is a contract that looks something like this:</p>
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<blockquote>
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<div><ol class="arabic">
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<li><dl class="first docutils">
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<dt>Group input and output states together, and then apply the following clauses on each group:</dt>
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<dd><ol class="first last loweralpha simple">
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<li>If an <code class="docutils literal"><span class="pre">Issue</span></code> command is present, run appropriate tests and end processing this group.</li>
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<li>If a <code class="docutils literal"><span class="pre">Move</span></code> command is present, run appropriate tests and end processing this group.</li>
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<li>If a <code class="docutils literal"><span class="pre">Redeem</span></code> command is present, run appropriate tests and end processing this group.</li>
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</ol>
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</dd>
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</dl>
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</li>
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</ol>
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</div></blockquote>
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<div class="section" id="commercial-paper-class">
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<h2>Commercial paper class<a class="headerlink" href="#commercial-paper-class" title="Permalink to this headline">¶</a></h2>
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<p>To use the clause verification logic, the contract needs to call the <code class="docutils literal"><span class="pre">verifyClause</span></code> function, passing in the
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transaction, a clause to verify, and a collection of commands the clauses are expected to handle all of. This list of
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commands is important because <code class="docutils literal"><span class="pre">verifyClause</span></code> checks that none of the commands are left unprocessed at the end, and
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raises an error if they are. The top level clause would normally be a composite clause (such as <code class="docutils literal"><span class="pre">AnyComposition</span></code>,
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<code class="docutils literal"><span class="pre">AllComposition</span></code>, etc.) which contains further clauses. The following examples are trimmed to the modified class
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definition and added elements, for brevity:</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CommercialPaper</span> <span class="p">:</span> <span class="n">Contract</span> <span class="p">{</span>
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<span class="k">override</span> <span class="k">val</span> <span class="py">legalContractReference</span><span class="p">:</span> <span class="n">SecureHash</span> <span class="p">=</span> <span class="n">SecureHash</span><span class="p">.</span><span class="n">sha256</span><span class="p">(</span><span class="s">"https://en.wikipedia.org/wiki/Commercial_paper"</span><span class="p">)</span>
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<span class="k">override</span> <span class="k">fun</span> <span class="nf">verify</span><span class="p">(</span><span class="n">tx</span><span class="p">:</span> <span class="n">TransactionForContract</span><span class="p">)</span> <span class="p">=</span> <span class="n">verifyClause</span><span class="p">(</span><span class="n">tx</span><span class="p">,</span> <span class="n">Clauses</span><span class="p">.</span><span class="n">Group</span><span class="p">(),</span> <span class="n">tx</span><span class="p">.</span><span class="n">commands</span><span class="p">.</span><span class="n">select</span><span class="p"><</span><span class="n">Commands</span><span class="p">>())</span>
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</pre></div>
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</div>
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<div class="highlight-java"><div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CommercialPaper</span> <span class="kd">implements</span> <span class="n">Contract</span> <span class="o">{</span>
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<span class="nd">@Override</span>
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<span class="kd">public</span> <span class="n">SecureHash</span> <span class="nf">getLegalContractReference</span><span class="o">()</span> <span class="o">{</span>
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<span class="k">return</span> <span class="n">SecureHash</span><span class="o">.</span><span class="na">Companion</span><span class="o">.</span><span class="na">sha256</span><span class="o">(</span><span class="s">"https://en.wikipedia.org/wiki/Commercial_paper"</span><span class="o">);</span>
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<span class="o">}</span>
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<span class="nd">@Override</span>
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<span class="kd">public</span> <span class="kt">void</span> <span class="nf">verify</span><span class="o">(</span><span class="nd">@NotNull</span> <span class="n">TransactionForContract</span> <span class="n">tx</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">IllegalArgumentException</span> <span class="o">{</span>
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<span class="n">ClauseVerifier</span><span class="o">.</span><span class="na">verifyClause</span><span class="o">(</span><span class="n">tx</span><span class="o">,</span> <span class="k">new</span> <span class="n">Clauses</span><span class="o">.</span><span class="na">Group</span><span class="o">(),</span> <span class="n">extractCommands</span><span class="o">(</span><span class="n">tx</span><span class="o">));</span>
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<span class="o">}</span>
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</pre></div>
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</div>
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</div>
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</div>
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<div class="section" id="clauses">
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<h2>Clauses<a class="headerlink" href="#clauses" title="Permalink to this headline">¶</a></h2>
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<p>We’ll tackle the inner clauses that contain the bulk of the verification logic, first, and the clause which handles
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grouping of input/output states later. The clauses must extend the <code class="docutils literal"><span class="pre">Clause</span></code> abstract class, which defines
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the <code class="docutils literal"><span class="pre">verify</span></code> function, and the <code class="docutils literal"><span class="pre">requiredCommands</span></code> property used to determine the conditions under which a clause
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is triggered. Composite clauses should extend the <code class="docutils literal"><span class="pre">CompositeClause</span></code> abstract class, which extends <code class="docutils literal"><span class="pre">Clause</span></code> to
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add support for wrapping around multiple clauses.</p>
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<p>The <code class="docutils literal"><span class="pre">verify</span></code> function defined in the <code class="docutils literal"><span class="pre">Clause</span></code> interface is similar to the conventional <code class="docutils literal"><span class="pre">Contract</span></code> verification
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function, although it adds new parameters and returns the set of commands which it has processed. Normally this returned
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set is identical to the <code class="docutils literal"><span class="pre">requiredCommands</span></code> used to trigger the clause, however in some cases the clause may process
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further optional commands which it needs to report that it has handled.</p>
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<p>The <code class="docutils literal"><span class="pre">Move</span></code> clause for the commercial paper contract is relatively simple, so we will start there:</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Move</span><span class="p">:</span> <span class="n">Clause</span><span class="p"><</span><span class="n">State</span><span class="p">,</span> <span class="n">Commands</span><span class="p">,</span> <span class="n">Issued</span><span class="p"><</span><span class="n">Terms</span><span class="p">>>()</span> <span class="p">{</span>
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<span class="k">override</span> <span class="k">val</span> <span class="py">requiredCommands</span><span class="p">:</span> <span class="n">Set</span><span class="p"><</span><span class="n">Class</span><span class="p"><</span><span class="k">out</span> <span class="n">CommandData</span><span class="p">>></span>
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<span class="k">get</span><span class="p">()</span> <span class="p">=</span> <span class="n">setOf</span><span class="p">(</span><span class="n">Commands</span><span class="p">.</span><span class="n">Move</span><span class="o">::</span><span class="k">class</span><span class="p">.</span><span class="n">java</span><span class="p">)</span>
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<span class="k">override</span> <span class="k">fun</span> <span class="nf">verify</span><span class="p">(</span><span class="n">tx</span><span class="p">:</span> <span class="n">TransactionForContract</span><span class="p">,</span>
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<span class="n">inputs</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">State</span><span class="p">>,</span>
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<span class="n">outputs</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">State</span><span class="p">>,</span>
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<span class="n">commands</span><span class="p">:</span> <span class="n">List</span><span class="p"><</span><span class="n">AuthenticatedObject</span><span class="p"><</span><span class="n">Commands</span><span class="p">>>,</span>
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<span class="n">groupingKey</span><span class="p">:</span> <span class="n">Issued</span><span class="p"><</span><span class="n">Terms</span><span class="p">>?):</span> <span class="n">Set</span><span class="p"><</span><span class="n">Commands</span><span class="p">></span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">command</span> <span class="p">=</span> <span class="n">commands</span><span class="p">.</span><span class="n">requireSingleCommand</span><span class="p"><</span><span class="n">Commands</span><span class="p">.</span><span class="n">Move</span><span class="p">>()</span>
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<span class="k">val</span> <span class="py">input</span> <span class="p">=</span> <span class="n">inputs</span><span class="p">.</span><span class="n">single</span><span class="p">()</span>
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<span class="n">requireThat</span> <span class="p">{</span>
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<span class="s">"the transaction is signed by the owner of the CP"</span> <span class="k">by</span> <span class="p">(</span><span class="n">input</span><span class="p">.</span><span class="n">owner</span> <span class="k">in</span> <span class="n">command</span><span class="p">.</span><span class="n">signers</span><span class="p">)</span>
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<span class="s">"the state is propagated"</span> <span class="k">by</span> <span class="p">(</span><span class="n">outputs</span><span class="p">.</span><span class="n">size</span> <span class="p">==</span> <span class="m">1</span><span class="p">)</span>
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<span class="c1">// Don't need to check anything else, as if outputs.size == 1 then the output is equal to</span>
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<span class="c1">// the input ignoring the owner field due to the grouping.</span>
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<span class="p">}</span>
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<span class="k">return</span> <span class="n">setOf</span><span class="p">(</span><span class="n">command</span><span class="p">.</span><span class="n">value</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<div class="highlight-java"><div class="highlight"><pre><span></span><span class="kd">class</span> <span class="nc">Move</span> <span class="kd">extends</span> <span class="n">Clause</span><span class="o"><</span><span class="n">State</span><span class="o">,</span> <span class="n">Commands</span><span class="o">,</span> <span class="n">State</span><span class="o">></span> <span class="o">{</span>
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<span class="nd">@NotNull</span>
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<span class="nd">@Override</span>
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<span class="kd">public</span> <span class="n">Set</span><span class="o"><</span><span class="n">Class</span><span class="o"><?</span> <span class="kd">extends</span> <span class="n">CommandData</span><span class="o">>></span> <span class="nf">getRequiredCommands</span><span class="o">()</span> <span class="o">{</span>
|
|
<span class="k">return</span> <span class="n">Collections</span><span class="o">.</span><span class="na">singleton</span><span class="o">(</span><span class="n">Commands</span><span class="o">.</span><span class="na">Move</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
|
|
<span class="o">}</span>
|
|
|
|
<span class="nd">@NotNull</span>
|
|
<span class="nd">@Override</span>
|
|
<span class="kd">public</span> <span class="n">Set</span><span class="o"><</span><span class="n">Commands</span><span class="o">></span> <span class="nf">verify</span><span class="o">(</span><span class="nd">@NotNull</span> <span class="n">TransactionForContract</span> <span class="n">tx</span><span class="o">,</span>
|
|
<span class="nd">@NotNull</span> <span class="n">List</span><span class="o"><?</span> <span class="kd">extends</span> <span class="n">State</span><span class="o">></span> <span class="n">inputs</span><span class="o">,</span>
|
|
<span class="nd">@NotNull</span> <span class="n">List</span><span class="o"><?</span> <span class="kd">extends</span> <span class="n">State</span><span class="o">></span> <span class="n">outputs</span><span class="o">,</span>
|
|
<span class="nd">@NotNull</span> <span class="n">List</span><span class="o"><?</span> <span class="kd">extends</span> <span class="n">AuthenticatedObject</span><span class="o"><?</span> <span class="kd">extends</span> <span class="n">Commands</span><span class="o">>></span> <span class="n">commands</span><span class="o">,</span>
|
|
<span class="nd">@NotNull</span> <span class="n">State</span> <span class="n">groupingKey</span><span class="o">)</span> <span class="o">{</span>
|
|
<span class="n">AuthenticatedObject</span><span class="o"><</span><span class="n">Commands</span><span class="o">.</span><span class="na">Move</span><span class="o">></span> <span class="n">cmd</span> <span class="o">=</span> <span class="n">requireSingleCommand</span><span class="o">(</span><span class="n">tx</span><span class="o">.</span><span class="na">getCommands</span><span class="o">(),</span> <span class="n">Commands</span><span class="o">.</span><span class="na">Move</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
|
|
<span class="c1">// There should be only a single input due to aggregation above</span>
|
|
<span class="n">State</span> <span class="n">input</span> <span class="o">=</span> <span class="n">single</span><span class="o">(</span><span class="n">inputs</span><span class="o">);</span>
|
|
|
|
<span class="k">if</span> <span class="o">(!</span><span class="n">cmd</span><span class="o">.</span><span class="na">getSigners</span><span class="o">().</span><span class="na">contains</span><span class="o">(</span><span class="n">input</span><span class="o">.</span><span class="na">getOwner</span><span class="o">()))</span>
|
|
<span class="k">throw</span> <span class="k">new</span> <span class="n">IllegalStateException</span><span class="o">(</span><span class="s">"Failed requirement: the transaction is signed by the owner of the CP"</span><span class="o">);</span>
|
|
|
|
<span class="c1">// Check the output CP state is the same as the input state, ignoring the owner field.</span>
|
|
<span class="k">if</span> <span class="o">(</span><span class="n">outputs</span><span class="o">.</span><span class="na">size</span><span class="o">()</span> <span class="o">!=</span> <span class="mi">1</span><span class="o">)</span> <span class="o">{</span>
|
|
<span class="k">throw</span> <span class="k">new</span> <span class="n">IllegalStateException</span><span class="o">(</span><span class="s">"the state is propagated"</span><span class="o">);</span>
|
|
<span class="o">}</span>
|
|
<span class="c1">// Don't need to check anything else, as if outputs.size == 1 then the output is equal to</span>
|
|
<span class="c1">// the input ignoring the owner field due to the grouping.</span>
|
|
<span class="k">return</span> <span class="n">Collections</span><span class="o">.</span><span class="na">singleton</span><span class="o">(</span><span class="n">cmd</span><span class="o">.</span><span class="na">getValue</span><span class="o">());</span>
|
|
<span class="o">}</span>
|
|
<span class="o">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="group-clause">
|
|
<h2>Group Clause<a class="headerlink" href="#group-clause" title="Permalink to this headline">¶</a></h2>
|
|
<p>We need to wrap the move clause (as well as the issue and redeem clauses - see the relevant contract code for their
|
|
full specifications) in an outer clause that understands how to group contract states and objects. For this we extend
|
|
the standard <code class="docutils literal"><span class="pre">GroupClauseVerifier</span></code> and specify how to group input/output states, as well as the top-level to run on
|
|
each group. As with the top level clause on a contract, this is normally a composite clause that delegates to subclauses.</p>
|
|
<div class="codeset container">
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Group</span> <span class="p">:</span> <span class="n">GroupClauseVerifier</span><span class="p"><</span><span class="n">State</span><span class="p">,</span> <span class="n">Commands</span><span class="p">,</span> <span class="n">Issued</span><span class="p"><</span><span class="n">Terms</span><span class="p">>>(</span>
|
|
<span class="n">AnyComposition</span><span class="p">(</span>
|
|
<span class="n">Redeem</span><span class="p">(),</span>
|
|
<span class="n">Move</span><span class="p">(),</span>
|
|
<span class="n">Issue</span><span class="p">()))</span> <span class="p">{</span>
|
|
<span class="k">override</span> <span class="k">fun</span> <span class="nf">groupStates</span><span class="p">(</span><span class="n">tx</span><span class="p">:</span> <span class="n">TransactionForContract</span><span class="p">):</span> <span class="n">List</span><span class="p"><</span><span class="n">TransactionForContract</span><span class="p">.</span><span class="n">InOutGroup</span><span class="p"><</span><span class="n">State</span><span class="p">,</span> <span class="n">Issued</span><span class="p"><</span><span class="n">Terms</span><span class="p">>>></span>
|
|
<span class="p">=</span> <span class="n">tx</span><span class="p">.</span><span class="n">groupStates</span><span class="p"><</span><span class="n">State</span><span class="p">,</span> <span class="n">Issued</span><span class="p"><</span><span class="n">Terms</span><span class="p">>></span> <span class="p">{</span> <span class="n">it</span><span class="p">.</span><span class="n">token</span> <span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
<div class="highlight-java"><div class="highlight"><pre><span></span><span class="kd">class</span> <span class="nc">Group</span> <span class="kd">extends</span> <span class="n">GroupClauseVerifier</span><span class="o"><</span><span class="n">State</span><span class="o">,</span> <span class="n">Commands</span><span class="o">,</span> <span class="n">State</span><span class="o">></span> <span class="o">{</span>
|
|
<span class="kd">public</span> <span class="nf">Group</span><span class="o">()</span> <span class="o">{</span>
|
|
<span class="kd">super</span><span class="o">(</span><span class="k">new</span> <span class="n">AnyComposition</span><span class="o"><>(</span>
|
|
<span class="k">new</span> <span class="n">Clauses</span><span class="o">.</span><span class="na">Redeem</span><span class="o">(),</span>
|
|
<span class="k">new</span> <span class="n">Clauses</span><span class="o">.</span><span class="na">Move</span><span class="o">(),</span>
|
|
<span class="k">new</span> <span class="n">Clauses</span><span class="o">.</span><span class="na">Issue</span><span class="o">()</span>
|
|
<span class="o">));</span>
|
|
<span class="o">}</span>
|
|
|
|
<span class="nd">@NotNull</span>
|
|
<span class="nd">@Override</span>
|
|
<span class="kd">public</span> <span class="n">List</span><span class="o"><</span><span class="n">InOutGroup</span><span class="o"><</span><span class="n">State</span><span class="o">,</span> <span class="n">State</span><span class="o">>></span> <span class="nf">groupStates</span><span class="o">(</span><span class="nd">@NotNull</span> <span class="n">TransactionForContract</span> <span class="n">tx</span><span class="o">)</span> <span class="o">{</span>
|
|
<span class="k">return</span> <span class="n">tx</span><span class="o">.</span><span class="na">groupStates</span><span class="o">(</span><span class="n">State</span><span class="o">.</span><span class="na">class</span><span class="o">,</span> <span class="n">State</span><span class="o">::</span><span class="n">withoutOwner</span><span class="o">);</span>
|
|
<span class="o">}</span>
|
|
<span class="o">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>For the <code class="docutils literal"><span class="pre">CommercialPaper</span></code> contract, this is the top level clause for the contract, and is passed directly into
|
|
<code class="docutils literal"><span class="pre">verifyClause</span></code> (see the example code at the top of this tutorial).</p>
|
|
</div>
|
|
<div class="section" id="summary">
|
|
<h2>Summary<a class="headerlink" href="#summary" title="Permalink to this headline">¶</a></h2>
|
|
<p>In summary the top level contract <code class="docutils literal"><span class="pre">CommercialPaper</span></code> specifies a single grouping clause of type
|
|
<code class="docutils literal"><span class="pre">CommercialPaper.Clauses.Group</span></code> which in turn specifies <code class="docutils literal"><span class="pre">GroupClause</span></code> implementations for each type of command
|
|
(<code class="docutils literal"><span class="pre">Redeem</span></code>, <code class="docutils literal"><span class="pre">Move</span></code> and <code class="docutils literal"><span class="pre">Issue</span></code>). This reflects the flow of verification: In order to verify a <code class="docutils literal"><span class="pre">CommercialPaper</span></code>
|
|
we first group states, check which commands are specified, and run command-specific verification logic accordingly.</p>
|
|
</div>
|
|
<div class="section" id="debugging">
|
|
<h2>Debugging<a class="headerlink" href="#debugging" title="Permalink to this headline">¶</a></h2>
|
|
<p>Debugging clauses which have been composed together can be complicated due to the difficulty in knowing which clauses
|
|
have been matched, whether specific clauses failed to match or passed verification, etc. There is “trace” level
|
|
logging code in the clause verifier which evaluates which clauses will be matched and logs them, before actually
|
|
performing the validation. To enable this, ensure trace level logging is enabled on the <code class="docutils literal"><span class="pre">Clause</span></code> interface.</p>
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