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copied!<p>Debugging Erlang code can be tricky at times, especially dealing with <code>badmatch</code> errors. In general, two good guidelines to keep are:</p> <ul> <li>Keep functions short</li> <li>Use return values directly if you can, instead of binding temporary variables (this will give you the benefit of getting <code>function_clause</code> errors etc which are way more informative)</li> </ul> <p>That being said, using the debuggers are usually required to quickly get to the bottom of errors. I recommend to use the command line debugger, <code>dbg</code>, instead of the graphical one, <code>debugger</code> (it's way faster when you know how to use it, and you don't have to context switch from the Erlang shell to a GUI).</p> <p>Given the sample expression you provided, the case is often that you have more than just variables being assigned to other variables (which is absolutely unnecessary in Erlang):</p> <pre><code>run(X, Y) -> X = something(whatever), Y = other:do(more_data), </code></pre> <p>Debugging a <code>badmatch</code> error here is aided by using the command line debugger:</p> <pre><code>1> dbg:tracer(). % Start the CLI debugger {ok,<0.55.0>} 2> dbg:p(all, c). % Trace all processes, only calls {ok,[{matched,nonode@nohost,29}]} 3> dbg:tpl(my_module, something, x). % tpl = trace local functions as well {ok,[{matched,nonode@nohost,1},{saved,x}]} 4> dbg:tp(other, do, x). % tp = trace exported functions {ok,[{matched,nonode@nohost,1},{saved,x}]} 5> dbg:tp(my_module, run, x). % x means print exceptions {ok,[{matched,nonode@nohost,1},{saved,x}]} % (and normal return values) </code></pre> <p>Look for <code>{matched,_,1}</code> in the return value... if this would have been <code>0</code> instead of <code>1</code> (or more) that would have meant that no functions matched the pattern. Full documentation for the <code>dbg</code> module can be found <a href="http://erldocs.com/R14B03/runtime_tools/dbg.html" rel="noreferrer">here</a>.</p> <p>Given that both <code>something/1</code> and <code>other:do/1</code> always returns ok, the following could happen:</p> <pre><code>6> my_module:run(ok, ok). (<0.72.0>) call my_module:run(ok,ok) (<0.72.0>) call my_module:something(whatever) (<0.72.0>) returned from my_module:something/1 -> ok (<0.72.0>) call other:do(more_data) (<0.72.0>) returned from other:do/1 -> ok (<0.72.0>) returned from my_module:run/2 -> ok ok </code></pre> <p>Here we can see the whole call procedure, and what return values were given. If we call it with something we know will fail:</p> <pre><code>7> my_module:run(error, error). ** exception error: no match of right hand side value ok (<0.72.0>) call my_module:run(error,error) (<0.72.0>) call my_module:something(whatever) (<0.72.0>) returned from my_module:something/1 -> ok (<0.72.0>) exception_from {my_module,run,2} {error,{badmatch,ok}} </code></pre> <p>Here we can see that we got a <code>badmatch</code> exception, <code>something/1</code> was called, but never <code>other:do/1</code> so we can deduce that the badmatch happened before that call.</p> <p>Getting proficient with the command line debugger will save you a lot of time, wether you debug simple (but tricky!) <code>badmatch</code> errors or something much more complex.</p> <p>Hopefully, all this will be easier when Erlang R15 comes out with line numbers in exceptions!</p>

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