Bryan Vicknair wrote:

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

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.

What an excellent case for extensible effects! Many thanks to Ben Foppa for making the package available on hackage, as extensible-effects. Extensible Effects designed specifically to separate a client (the requestor of an action) from a handler, the executor of an action. A logging client merely requests a string to be logged. It has no idea what happens when that request is executed. Therefore, the same client can be used with several handlers, even within the same expression. One can also build various interceptors, such as logIntercept, which is similar to Unix' tee. The following file illustrates how to solve the problem with all three scenarios. http://okmij.org/ftp/ The extensible effects package actually defines a very similar function, calls trace. For illustration and clarity, the file develops the Logging effect from scratch. After all, Extensible Effects encourages developing effects to suit the problem at hand. Defining a new effect is as simple as -- The request to log a string -- The request carries the String to log. It seems no particular -- reply is needed for the request (other than the simple acknowledgment), -- so we set the reply to be unit. data Log v = Log String (() -> v) deriving (Typeable, Functor) Clients use the function logit :: Member Log r => [Char] -> Eff r () (this signature, as all signatures in the code, are inferred). The signature says that the function request Log effect to be done. It does not concern itself how this effect its to be done. The file shows three handlers: runLogCollect, runLogTrace, runLogIgnore. There is also another handler, logIntercept, that like Unix 'tee', prints out the log message immediately while still propagating it. It can be used with any of the three handlers.