334 lines
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334 lines
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<div class="section" id="contracts">
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<h1>Contracts<a class="headerlink" href="#contracts" title="Permalink to this headline">¶</a></h1>
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<p>Some quick examples for techniques with Contracts.</p>
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<hr class="docutils" />
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<div class="section" id="return-a-value-from-a-state-changing-method">
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<h2>Return a Value from a State-Changing Method<a class="headerlink" href="#return-a-value-from-a-state-changing-method" title="Permalink to this headline">¶</a></h2>
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<p>Since a state-changing (non-constant) function can take a non-trivial
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amount of time to mine, the response is a transaction, and cannot
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return values directly.</p>
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<p>Using events, we can simulate a return value from a non-constant function.</p>
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<div class="literal-block-wrapper docutils container" id="id1">
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<div class="code-block-caption"><span class="caption-text"><em>Solidity</em></span><a class="headerlink" href="#id1" title="Permalink to this code">¶</a></div>
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<div class="highlight-javascript notranslate"><div class="highlight"><pre><span></span><span class="nx">contract</span> <span class="nx">Example</span> <span class="p">{</span>
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<span class="nx">event</span> <span class="nx">Return</span><span class="p">(</span><span class="nx">uint256</span><span class="p">);</span>
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<span class="nx">uint256</span> <span class="nx">_accum</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
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<span class="kd">function</span> <span class="nx">increment</span><span class="p">()</span> <span class="nx">returns</span> <span class="p">(</span><span class="nx">uint256</span> <span class="nx">sum</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">_accum</span><span class="o">++</span><span class="p">;</span>
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<span class="nx">Returns</span><span class="p">(</span><span class="nx">_accum</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>
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<div class="literal-block-wrapper docutils container" id="id2">
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<div class="code-block-caption"><span class="caption-text"><em>JavaScript</em></span><a class="headerlink" href="#id2" title="Permalink to this code">¶</a></div>
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<div class="highlight-javascript notranslate"><div class="highlight"><pre><span></span><span class="kr">const</span> <span class="nx">assert</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">'assert'</span><span class="p">)</span>
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<span class="kr">const</span> <span class="p">{</span>
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<span class="nx">Contract</span><span class="p">,</span>
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<span class="nx">Wallet</span><span class="p">,</span>
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<span class="nx">getDefaultProvider</span>
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<span class="p">}</span> <span class="o">=</span> <span class="nx">require</span><span class="p">(</span><span class="s1">'ethers'</span><span class="p">)</span>
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<span class="kr">const</span> <span class="nx">provider</span> <span class="o">=</span> <span class="nx">getDefaultProvider</span><span class="p">(</span><span class="s1">'ropsten'</span><span class="p">)</span>
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<span class="kr">const</span> <span class="nx">wallet</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Wallet</span><span class="p">(</span><span class="nx">privateKey</span><span class="p">,</span> <span class="nx">provider</span><span class="p">)</span>
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<span class="kr">const</span> <span class="nx">abi</span> <span class="o">=</span> <span class="p">[</span>
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<span class="s2">"event Return(uint256)"</span><span class="p">,</span>
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<span class="s2">"function increment() returns (uint256 sum)"</span>
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<span class="p">]</span>
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<span class="kr">const</span> <span class="nx">contractAddress</span> <span class="o">=</span> <span class="s2">"0x..."</span>
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<span class="kr">const</span> <span class="nx">contract</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Contract</span><span class="p">(</span><span class="nx">contractAddress</span><span class="p">,</span> <span class="nx">abi</span><span class="p">)</span>
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<span class="nx">async</span> <span class="kd">function</span> <span class="nx">increment</span><span class="p">()</span> <span class="p">{</span>
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<span class="c1">// Call the contract, getting back the transaction</span>
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<span class="kd">let</span> <span class="nx">tx</span> <span class="o">=</span> <span class="nx">await</span> <span class="nx">contract</span><span class="p">.</span><span class="nx">increment</span><span class="p">()</span>
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<span class="c1">// Wait for the transaction to have 2 confirmations.</span>
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<span class="c1">// See the note below on "Economic Value" for considerations</span>
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<span class="c1">// regarding the number of suggested confirmations</span>
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<span class="kd">let</span> <span class="nx">receipt</span> <span class="o">=</span> <span class="nx">await</span> <span class="nx">tx</span><span class="p">.</span><span class="nx">wait</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
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<span class="c1">// The receipt will have an "events" Array, which will have</span>
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<span class="c1">// the emitted event from the Contract. The "Return(uint256)"</span>
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<span class="c1">// call is the last event.</span>
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<span class="kd">let</span> <span class="nx">sumEvent</span> <span class="o">=</span> <span class="nx">receipt</span><span class="p">.</span><span class="nx">events</span><span class="p">.</span><span class="nx">pop</span><span class="p">()</span>
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<span class="c1">// Not necessary; these are just for the purpose of this</span>
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<span class="c1">// example</span>
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<span class="nx">assert</span><span class="p">.</span><span class="nx">equal</span><span class="p">(</span><span class="nx">sumEvent</span><span class="p">.</span><span class="nx">event</span><span class="p">,</span> <span class="s1">'Return'</span><span class="p">)</span>
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<span class="nx">assert</span><span class="p">.</span><span class="nx">equal</span><span class="p">(</span><span class="nx">sumEvent</span><span class="p">.</span><span class="nx">eventSignature</span><span class="p">,</span> <span class="s1">'Return(uint256)'</span><span class="p">)</span>
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<span class="c1">// The sum is the first (and in this case only) parameter</span>
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<span class="c1">// in the "Return(uint256 sum)" event</span>
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<span class="kd">let</span> <span class="nx">sum</span> <span class="o">=</span> <span class="nx">sumEvent</span><span class="p">.</span><span class="nx">args</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
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<span class="k">return</span> <span class="nx">sum</span>
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<span class="p">}</span>
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<span class="nx">increment</span><span class="p">.</span><span class="nx">then</span><span class="p">((</span><span class="nx">value</span><span class="p">)</span> <span class="p">=></span> <span class="p">{</span>
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<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">value</span><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>
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</div>
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<hr class="docutils" />
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<div class="section" id="economic-incentives-and-economic-value">
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<h2>Economic Incentives and Economic Value<a class="headerlink" href="#economic-incentives-and-economic-value" title="Permalink to this headline">¶</a></h2>
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<p>A very important aspect of Smart Contracts is consideration of the
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Economic Value being protected; even a completely logically correct
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Smart Contract can fall victim to misaligned economic incentives.</p>
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<p>Imagine a city with an average parking ticket cost of $60, but the
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parking enforcement is not effective, allowing a person to park anytime
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and anywhere with only an average of 3 tickets per year. If
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the average cost to pay for parking is $10, and you park 100 times per year,
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the $180 in fines compared to the $1,000 in paying for parking is actually
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incentivizing illegal parking and disincentivizing paying legitimate parking.</p>
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<p>This is a fairly involved topic, which we will write an article for
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and then link to from here, along with a related topic, “Miner
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Front-Running”.</p>
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<hr class="docutils" />
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<p>
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© Copyright 2016, Richard Moore <me@ricmoo.com>
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