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POUnderstanding the return value of spawn
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  1. POUnderstanding the return value of spawn
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    copied!<p>I'm getting started with Erlang, and could use a little help understanding the different results when applying the PID returned from <code>spawn/3</code> to the <code>process_info/1</code> method.</p> <p>Given this simple code where the <code>a/0</code> function is exported, which simply invokes <code>b/0</code>, which waits for a message:</p> <pre><code>-module(tester). -export([a/0]). a() -&gt; b(). b() -&gt; receive {Pid, test} -&gt; Pid ! alrighty_then end. </code></pre> <p>...please help me understand the reason for the different output from the shell:</p> <hr> <p><strong>Example 1:</strong></p> <p>Here, <code>current_function</code> of <code>Pid</code> is shown as being <code>tester:b/0</code>:</p> <pre><code>Pid = spawn(tester, a, []). process_info( Pid ). &gt; [{current_function,{tester,b,0}}, {initial_call,{tester,a,0}}, ... </code></pre> <hr> <p><strong>Example 2:</strong></p> <p>Here, <code>current_function</code> of <code>process_info/1</code> is shown as being <code>tester:a/0</code>:</p> <pre><code>process_info( spawn(tester, a, []) ). &gt; [{current_function,{tester,a,0}}, {initial_call,{tester,a,0}}, ... </code></pre> <hr> <p><strong>Example 3:</strong></p> <p>Here, <code>current_function</code> of <code>process_info/1</code> is shown as being <code>tester:a/0</code>, but the <code>current_function</code> of <code>Pid</code> is <code>tester:b/0</code>:</p> <pre><code>process_info( Pid = spawn(tester, a, []) ). &gt; [{current_function,{tester,a,0}}, {initial_call,{tester,a,0}}, ... process_info( Pid ). &gt; [{current_function,{tester,b,0}}, {initial_call,{tester,a,0}}, ... </code></pre> <hr> <p>I assume there's some asynchronous code happening in the background when <code>spawn/3</code> is invoked, but how does variable assignment and argument passing work (especially in the last example) such that <code>Pid</code> gets one value, and <code>process_info/1</code> gets another?</p> <p>Is there something special in Erlang that binds variable assignment in such cases, but no such binding is offered to argument passing?</p> <hr> <p><strong>EDIT:</strong></p> <p>If I use a function like this:</p> <pre><code>TestFunc = fun( P ) -&gt; P ! {self(), test}, flush() end. TestFunc( spawn(tester,a,[]) ). </code></pre> <p>...the message is returned properly from <code>tester:b/0</code>:</p> <pre><code>Shell got alrighty_then ok </code></pre> <p>But if I use a function like this:</p> <pre><code>TestFunc2 = fun( P ) -&gt; process_info( P ) end. TestFunc2( spawn(tester,a,[]) ). </code></pre> <p>...the <code>process_info/1</code> still shows <code>tester:a/0</code>:</p> <pre><code>[{current_function,{tester,a,0}}, {initial_call,{tester,a,0}}, ... </code></pre> <p>Not sure what to make of all this. Perhaps I just need to accept it as being above my pay grade!</p>
 

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