mszmurlo
Does the compiler optimize contructs with `_` or with `_variable`
Hi.
I was wondering if the compiler makes any optimization for constructs like below.
# Let’s define `my_func`
def my_func(:ok, param) do some_thing_with(param) end
def my_func(:error, _param) do some_thing_else() end
# and then call my_func
my_func(:error, some_var)
Does the compiler copy the value of some_var but then simply doesn’t use it or does it not even bother to copy it ?
Same question if I write _ = some_var. Does the compiler simply discard the line as it does nothing or does it do something that uses CPU ?
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benwilson512
Hey @mszmurlo values passed to functions are not copied. Values in Elixir are immutable, so no copying needs to occur when a value is passed to a function.
When you say _ = some_var do you mean a variable of _ = some_function_call(x) ? In the case of _ = some_var or even x = some_var no work happens at all and I believe the whole thing is optimized out. Again values are immutable so the runtime doesn’t have two things to deal with when you do x = some_var it just has one thing.
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garrison
Not your exact question but the binary efficiency guide has a section which mentions that unused variables in binary matches are optimized out regardless of whether they start with an underscore. I’d imagine this is true in general (that the underscore doesn’t matter), though in practice of course the Elixir compiler will complain either way.
mszmurlo
Yes, this sounds very reasonable! Thanks for your enlightenment.
No, I really meant _ = some_var. To give the context of my question, I was building a template compiler based on EEX. I’m creating dynamically functions with the following signature MyApp.Module.SubModule.template_name(assigns) out of template files module/sub_module/template_name.eex. When the template doesn’t use the assigns bindings I got a warning saying that assigns is not used. I spent hours trying different things, playing with the @compile directive but the only way I found to get rid of the warning was to actually use assigns by adding at the end of the template <% _ = assigns %>. I was wondering if it generated any kind of overhead, but obviously, it doesn’t
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