[erlang-questions] Testing of nifs

[Dmitry Groshev]

Well, it's about hidden details. Erlang is pretty straightforward language; 
C is a lot more tricky to make right. So there is a lot of non obvious bugs 
that I can't foresee when I write unit tests; PropEr's *random* tests are 
way better in this sense, because you just describe the domain of your task, 
not particular cases. You can also test your NIF incrementally, writing 
property after property (or command after command if there is some state 
involved).
Speaking about my experience — I'm using PropEr to test this library 
https://github.com/band115/ecirca , that implements mutable circular array 
in Erlang (all calls to this array are constrained to it's creator, so I 
hope this mutability can't be a problem). There are no docs right now other 
than code; I will definitely fix it in a week or two when I'll have more 
time. I've tried to test it with EUnit when it was just plain array; but 
then I've added different types of array and realized that I should add like 
5 times more code to cover all cases, and there was a thought about adding 
different types of values (int16/32/64), so I realized than EUnit is 
definitely not the best tool, just because the space of inputs is so 
multidimensional (and I really wanted to cover as much of it as I can with 
tests — Erlang's VM is very fragile to NIFs after all). So I started to dig 
into PropEr's docs, wrote first commands/checks for statem, compiled a debug 
version of BEAM, made a habit to attach GDB to the process running tests 
(this way in case of segfault, and I had a lot of them, I can quickly find 
where I broke the code) — and right now I can be sure to some degree (which 
is proportional to amount of tests that you've asked PropEr to do) that this 
NIFs won't explode in my face tomorrow.
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