Filter predicates and side-effects

[James Elliott]

I notice that the documentation for filter and filterv are emphatic that the predicate you are using be free of side effects. Why is that so? For example, I would like to use filterv to collect a list of lighting effects which respond false to a protocol method which asks whether they have finished. In the implementation of that method, the effects are free to update their internal state based on the current show state and time, when deciding whether or not they are finished, and so I can’t guarantee there will be no side effects while filtering the effects. Does that mean I can’t use filterv? If not, why not? Looking at the source, I see no obvious problems.

[Raymond Huang]

I believe the reason it's not recommended is because it returns a lazy chunked sequence. This means that it's not guaranteed to eagerly perform your side-effects, unless you wrap it in `doall`.

On Wed, Sep 2, 2015 at 8:48 AM, James Elliott <brun...@gmail.com> wrote:

I notice that the documentation for filter and filterv are emphatic that the predicate you are using be free of side effects. Why is that so? For example, I would like to use filterv to collect a list of lighting effects which respond false to a protocol method which asks whether they have finished. In the implementation of that method, the effects are free to update their internal state based on the current show state and time, when deciding whether or not they are finished, and so I can’t guarantee there will be no side effects while filtering the effects. Does that mean I can’t use filterv? If not, why not? Looking at the source, I see no obvious problems.

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[James Elliott]

Indeed, that would make sense for filter, but since filterv immediately puts them all into a vector, I was particularly puzzled by the warning on its doc string.

In my case I am not relying on the side effects for anything, I merely cannot guarantee there are none (and even if chunking were an issue, the list is immediately fully consumed). So I tentatively think I am ok, but it is still an unsettling warning.

[Michael Blume]

Don't call filter for side effects, don't rely on when side effects will or will not happen, but yeah, your filter predicate can have side effects.