Re: lazy-seq and somewhat greedy apply

[Jason Wolfe]

In case it's useful, here is a fully lazy variant of 'apply concat':

https://github.com/plumatic/plumbing/blob/master/src/plumbing/core.cljx#L179

-Jason

On Friday, June 3, 2016 at 2:26:43 AM UTC+8, Andy L wrote:
>
> Hi,
>
> I have a riddle I am not sure how to solve:
> ```
> user=> (defn just-a-demo-seq[] (println "in just-a-demo-seq") [1 2 3 4 5 6 
> 7 8 9])
> #'user/just-a-demo-seq
> user=> (def do-not-print-it (apply concat (repeatedly just-a-demo-seq)))
> in just-a-demo-seq
> in just-a-demo-seq
> in just-a-demo-seq
> in just-a-demo-seq
> #'user/do-not-print-it
> ```
>
> Question is why `just-a-demo-seq` is invoked 4 times.
>
> We spent some time trying to answer the question to the point of analyzing 
> RT.java and core.clj and pretty much ruled out `concat`. It seems that 
> `apply` os somewhat greedy exercising too many elements of the lazy 
> sequence.
>
> Any insight?
>
> Thanks,
> Andy
>

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Re: lazy-seq and somewhat greedy apply

[Andy L]

> 1- concat is defined to have 4 different arities: 0-arg, 1-arg, 2-arg,
> 3+args. In order to figure out which arity to use, apply has to realize at
> least as many elements as the minimum number of args required by the
> largest arity, which in this case is 3
> 2- apply has a small bug that causes it to realize one element more than
> necessary, in this case making it realize 4 elements rather than 3, here's
> a ticket with a patch fixing that issue:
> http://dev.clojure.org/jira/browse/CLJ-1583
>
>
Thx for the explanation. In my original code, I have a "lazy" sequence of a
size determined by specified in the content and the entire thing read of
I/O without a way of telling that I/O is closed. "read" function hangs and
blocks the code if performed too many times.

I redesigned my code to avoid that combination.

Best,
Andy

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Re: lazy-seq and somewhat greedy apply

[Nicola Mometto]

Two things at play here:

1- concat is defined to have 4 different arities: 0-arg, 1-arg, 2-arg, 3+args. 
In order to figure out which arity to use, apply has to realize at least as 
many elements as the minimum number of args required by the largest arity, 
which in this case is 3
2- apply has a small bug that causes it to realize one element more than 
necessary, in this case making it realize 4 elements rather than 3, here's a 
ticket with a patch fixing that issue: 
http://dev.clojure.org/jira/browse/CLJ-1583

Nicola

> On 2 Jun 2016, at 19:26, Andy L  wrote:
> 
> Hi,
> 
> I have a riddle I am not sure how to solve:
> ```
> user=> (defn just-a-demo-seq[] (println "in just-a-demo-seq") [1 2 3 4 5 6 7 
> 8 9])
> #'user/just-a-demo-seq
> user=> (def do-not-print-it (apply concat (repeatedly just-a-demo-seq)))
> in just-a-demo-seq
> in just-a-demo-seq
> in just-a-demo-seq
> in just-a-demo-seq
> #'user/do-not-print-it
> ```
> 
> Question is why `just-a-demo-seq` is invoked 4 times.
> 
> We spent some time trying to answer the question to the point of analyzing 
> RT.java and core.clj and pretty much ruled out `concat`. It seems that 
> `apply` os somewhat greedy exercising too many elements of the lazy sequence.
> 
> Any insight?
> 
> Thanks,
> Andy
> 
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