Headline

Logging in Haskell with non-monadic code

Characteristics

Starting from a straightforward family of functions for cutting salaries, the concern of logging the salary changes is incorporated into the functions such that the function results are enriched by the log entries for salary changes. This code is relatively verbose and implies poor abstraction. In particular, functionality for composing logs is scattered all over the functions. Ultimately, such a problem must be addressed with monads.

Illustration

Salary changes can be tracked in logs as follows:

type Log = [LogEntry]

data LogEntry = 
     LogEntry {
       name :: String, 
       oldSalary :: Float,
       newSalary :: Float 
     }
  deriving (Show)

Here are a few entries resulting from a salary cut for the sample company:

[LogEntry {name = "Craig", oldSalary = 123456.0, newSalary = 61728.0},
 LogEntry {name = "Erik", oldSalary = 12345.0, newSalary = 6172.5},
 LogEntry {name = "Ralf", oldSalary = 1234.0, newSalary = 617.0},
 LogEntry {name = "Ray", oldSalary = 234567.0, newSalary = 117283.5},
 LogEntry {name = "Klaus", oldSalary = 23456.0, newSalary = 11728.0},
 LogEntry {name = "Karl", oldSalary = 2345.0, newSalary = 1172.5},
 LogEntry {name = "Joe", oldSalary = 2344.0, newSalary = 1172.0}]

Given a log, the median of salary deltas can be computed as follows:

log2median :: Log -> Float
log2median = median . log2deltas

log2deltas :: Log -> [Float]
log2deltas = sort . map delta
  where
    delta entry = newSalary entry - oldSalary entry

The above log reduces to the following median:

-6172.5

Feature:Cut is implemented in logging-enabled fashion as follows:

cut :: Company -> (Company, Log)
cut (Company n ds) = (Company n ds', log)
  where
   (ds', logs) = unzip (map cutD ds)
   log = concat logs
   cutD :: Department -> (Department, Log)
   cutD (Department n m ds es)
     = (Department n m' ds' es', log)
     where
       (m',log1) = cutE m
       (ds', logs2) = unzip (map cutD ds)
       (es', logs3) = unzip (map cutE es)
       log = concat ([log1]++logs2++logs3)
       cutE :: Employee -> (Employee, Log)
       cutE (Employee n a s) = (e', log)
         where
           e' = Employee n a s'
           s' = s/2
           log = [ LogEntry { 
                     name = n,
                     oldSalary = s,
                     newSalary = s'
                 } ]

Thus, all functions return a regular data item (i.e., some part of the company) and a corresponding log. When lists of company parts are processed with map, then the lists of results must be unzipped (to go from a list of pairs to a pair of lists). In the function for departments, multiple logs arise for parts a department; these intermediate logs must be composed.

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:haskellWriter for a variation on the present contribution, which leverages a writer monad, though, for conciseness and proper abstraction.

Architecture

There are these Haskell modules:

The contribution relies on the hackage package hstats.

Usage

See Contribution:haskellComposition.

The "Makefile" provides an extra "install" target for installing the hackage package hstats.

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