Headline

Logging in Haskell with the Writer monad

Characteristics

Salary changes are logged in a Language:Haskell-based implementation with the help of a writer monad. Compared to a non-monadic implementation, the code is more concise. Details of logging are localized such that they only surface in the context of code that actually changes salaries.

Illustration

See Contribution:haskellLogging for a simpler, non-monadic implementation.

The present, monadic implementation differs only with regard to the cut function:

cut :: Company -> Writer Log Company
cut (Company n ds) =
  do
     ds' <- mapM cutD ds
     return (Company n ds')
  where
    cutD :: Department -> Writer Log Department
    cutD (Department n m ds es) =
      do
         m' <- cutE m
         ds' <- mapM cutD ds
         es' <- mapM cutE es
         return (Department n m' ds' es')
      where
        cutE :: Employee -> Writer Log Employee
        cutE (Employee n a s) =
          do 
             let s' = s/2
             let log = [ LogEntry { 
                           name = n,
                           oldSalary = s,
                           newSalary = s'
                       } ]
             tell log
             return (Employee n a s')

Thus, the family of functions uses a writer monad in the result types. The sub-traversals are all composed by monadic bind (possibly expressed in do-notation). The function for processing departments totally abstracts from the fact that logging is involved. In fact, that function could be defined to be parametrically polymorphic in the monad at hand.

Relationships

  • See Contribution:haskellComposition for the corresponding contribution that does not yet involve logging. The data model is preserved in the present contribution, but the functions for cutting salaries had to be rewritten since the logging concern crosscuts the function.
  • See Contribution:haskellLogging for a variation on the present contribution which does not yet use monadic style.

Architecture

See Contribution:haskellLogging.

In addition, a trivialized implementation of the standard writer monad is included into the contribution.

Usage

See Contribution:haskellLogging.

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