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.
> 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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