[Haskell-cafe] New version of itself

Daniel Díaz diaz.carrete at gmail.com
Sat Jun 4 15:00:59 UTC 2016

This is a bit of a tangent, but one can use the FoldM 
type from the foldl <http://hackage.haskell.org/package/foldl>package to 
build composable logger-like things that perform effects.

The semigroupoids package contains the Extend 
typeclass, which is basically "Comonad without extract". If we make FoldM 
an instance of Extend, we can "single-step" a FoldM with the function:

     import Data.Functor.Extend (duplicated 
     import qualified Control.Foldl as L

     singleStep :: i -> L.FoldM IO i r -> IO (L.FoldM IO i r)
     singleStep i :: flip L.foldM [i] . duplicated

Which is basically a logging function.

Also, since FoldM is an instance of Applicative, you can combine a list of 
loggers using sequenceA_ 

Another way to define a logger type would be with Cofree 
the free package. Something like:

     type Logger = Cofree ((->) String) (IO ()) 

Which would also be an Applicative. One could then define stateful loggers 
using Cofree's unfold <http://One then could define loggers using unfold.>.

I think the Cofree version has the advantage that you can be sure the 
logger's state only depends on the messages it receives (even as it emits 
IO actions on each step). The FoldM version, meanwhile, could perform some 
devious IO action to update its state.

On Saturday, June 4, 2016 at 3:24:57 PM UTC+2, martin wrote:
> Hello all, 
> I find myself frequentlly writing types like this 
> data Logger a l = Lgr { 
>   runLogger :: a -> Log l -> (Log l, Logger a l) 
>   } 
> The purpose is to give a logger a chance to carry an internal state. It 
> could e.g. choose to log only every n 
> invocations. To do this it must keep track of how many times it has been 
> called. I want to leave such things private to 
> the Logger. 
> (1) I know that this is not an unusal thing to do, but it has an OO feel 
> to it. Is there a more functional alternative 
> to it. Should I just not worry? 
> (2) I can write a combinator which creates a Logger from a list of 
> Loggers. Since each Logger potentially returns a new 
> version of itself, I must always re-assemble the combined logger from all 
> the returned new versions. I am worried that 
> this is a costly operation, particularly when most Loggers just return 
> themselves unaltered. I don't have any hard 
> numbers about the performance penalty though. 
> These Loggers are used in a discrete-event-simulation and they will get 
> called many times (once for each event), but 
> only occastionally actually write to the Log. 
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