[erlang-questions] call by value
Thu May 19 18:17:29 CEST 2011
On 19 May 2011, at 17:59, Paulo Sérgio Almeida wrote:
> On 5/19/11 4:41 PM, Ulf Wiger wrote:
>> You should ignore the issue of copying. Essentially, Erlang can be viewed as using copy-on-write semantics.
>> The key is whether the expression is evaluated or not (it is) before the function is called, and also whether the called function can modify the input parameters (it can't)*.
>> * Unless you use unholy functions like hipe_bifs:bytearray_update/3, which of course you don't. ;-)
> Unless you need to implement huge bloom filters with good performance. ;-)
Yeah, well, I was bitten by exactly that last week, as I prepared a code kata based on a bloom filter.
I had a bloom filter implementation that used hipe_bifs:bitarray_update/3 for performance and simplicity, but since I was messing with those, I used them also in the code calling on the bloom filter (which was to find all words of a given length).
The fun started when I then made a QLC generator out of the CPS-style word finder. It worked, but instead of returning all words of length N, it returned a list of several instances of the SAME word.
It took me a little while to realise that, as I was using destructive update when permuting the "character vector", the QLC iterator kept mutating the same binary the whole time, and simply returned a pointer to that one binary for each word found.
Mutation is evil…
Ulf Wiger, CTO, Erlang Solutions, Ltd.
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