[Haskell-cafe] Writer + log each computation to stdout

Bryan Vicknair bryanvick at gmail.com
Mon Nov 25 22:54:20 UTC 2013


I have a bunch of little database IO functions.  Each does something to the
database, and returns a log string describing what it did, and possibly a
meaningful result from the database.

  query  :: IO (String, a)
  update :: a -> IO (String, ())

...and a few functions that orchestrate all the little functions into doing
useful work.

  syncWeek :: Week -> IO ()
  syncAll  : : IO ()

I don't want the individual functions to know what is done with the log string
describing what they did.  Top-level orchestrating functions should make that
decision, which can be one of:

1) Collect and print all to a log once all computations are done.
2) Print to stdout *as each computation is run*.
3) Ignore them.

Here is my understanding of how common monads would handle these requirements:

Writer: 1 and 3 are easy.  This is what I originally attempted to use, but I
        couldn't figure out how to accomplish #2.
Reader: 2 and 3 can be accomplished if each function reads a shouldLog config
        variable from the reader and does a putStrLn depending on the value.
        Very ugly, as now each function has to know how to log output.
State:  Not sure, but the Writer docs in the transformers package points to
        this monad as maybe solving requirement #2 above.

The use case is that when I call the top-level functions from a command line
script, I want to see logging happening in real-time to stdout, but I may call
the same top-level functions from a larger application that may be logging to
somewhere other than stdout, and may call the top-level functions from yet
another larger application which doesn't want anything to be logged.

How can I glue together a bunch of smaller computations, which may call
each other, and decide at a higher level what to do with the logging result of
each computation?  Seems like a perfect fit for Writer, except for the
requirement to be able to print to stdout at each step.

Bryan Vicknair

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