2010/6/24 Laurent PETIT <laurent.pe...@gmail.com>:
> Hi,
>
> 2010/6/17 William Wadsworth <will.wadsworth...@gmail.com>:
>> Hi.
>>
>> I have just started learning Clojure and I am really enjoying
>> myself. However, I am still getting to grips with the workings of its
>> concurrency model, so please excuse me if my question seems too
>> obvious.
>>
>> I have some code that reads data from a file, parses it and generates
>> a CSV file. The flow is as follows:
>>
>> <code>
>> (use 'clojure.contrib.duck-streams)
>> (use 'clojure.contrib.pprint)
>>
>> (defn parse-line [s]
>> (cl-format nil "~{\"~A\"~^,~}" (into [] (.split s ";"))))
>>
>> (let [input-seq (for [line (read-lines "input.dat")]
>> (parse-line line))]
>> (write-lines "out.CSV" input-seq))
>> </code>
>>
>> Now the question:
>>
>> I would like to generate log entries for any malformed lines occurring
>> in the input file. Once of the ways might be to do this in parse-line
>> using clojure.contrib.logging as follows:
>>
>> <code>
>> (use 'clojure.contrib.logging)
>>
>> (defn parse-line [s]
>> (let [cs (into [] (.split s ";"))]
>> (if (= (count cs) 5)
>> (cl-format nil "~{\"~A\"~^,~}" cs)
>> (error "Incorrect number of columns in file."))))
>> </code>
>>
>> Since FOR returns a lazy sequence of the results, is this function
>> safe? (Seeing that it is not side-effect free?)
>
> You have to acknowledge that the calls to parse-line will not
> necessarily occur during the call to for, but maybe not before calling
> write-lines, maybe partly if chunked-seqs are in play (sequences
> preloading items in chunks of 32 items for example). And, if there is
> an exception that is not thrown in write-lines, your logs may miss the
> error logs for some of the remaining items not yet consumed by
> write-lines.
s/if there is an exception that is not thrown in write-lines/if there is
an exception that *is* thrown in write-lines/
>
>> BTW:
>> clojure.contrib.logging indicates that it can use an agent for
>> logging. Is my assumption that invoking the logger (with or without
>> an agent) within a function would still mean that the function has
>> side-effects correct?
>
> Technically speaking, yes. Imagine there's an I/O exception thrown
> from the logging call (disk full, no write access, network access,
> etc.), then your function would not be "safe" anymore.
>
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