[erlang-questions] variable exported from case in OTP 18

Martin Koroudjiev <>
Thu Oct 1 07:05:59 CEST 2015

Thanks all! I all makes sense. I know the example is quite naive but was
trying to show code that produces the warning.


On 9/29/2015 2:58 PM, zxq9 wrote:
> On 2015年9月29日 火曜日 20:42:19 zxq9 wrote:
>> On 2015年9月29日 火曜日 11:06:48 Martin Koroudjiev wrote:
>>> Hello,
>>> I am confused why this code generates warning with the warn_export_vars
>>> option:
>> This has been discussed several times (here, for example: http://erlang.org/pipermail/erlang-questions/2014-March/078017.html), and because of the weirdness of case scope being one way (its a scope semantically and conceptually, but its not really) and list comprehensions being another (it really is a separate scope, but that's not how some other languages work) this has been made into a warning.
> Martin,
> By the way -- something I should have mentioned is that this is almost never a problem in practice because the normal way to deal with lots of declarations within a `case` (usually because you have a large chain of case statements within a single function, so lots of bindings lay around in scope or collide) is to break most of this stuff out into separate functions.
> For example, your original code is something I don't think anyone would write:
>> test(Mode) ->
>>     case Mode of
>>         r -> {ok, FP} = file:open("warn.erl", [read]);
>>         w -> {ok, FP} = file:open("warn.erl", [write])
>>     end,
>>     file:close(FP).
> Would be more like:
> test(Mode) ->
>     {ok, FD} = file:open("warn.erl", [mode(Mode)]),
>     ok = do_stuff(FD),
>     file:close(FD).
> mode(r) -> read;
> mode(w) -> write.
> This bothered me initially because it is hard to imagine this being more natural any other way -- and its pretty weird to need 'r' or 'w' instead of 'read' or 'write' to begin with. But contrived examples are difficult to compare to real situations sometimes.
> -Craig
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