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