State machine using recursive functions

July 8, 2013 at 7:58 AM

I had a log file containing sequences of operations descriptions like this:

/*
2013-06-21T15:01:46.222-04:00
. . .

and I needed to extract just the name (‘DBLoadSchema’) and date-time of each operation, plus their line number within the log file.

The ‘standard’ way would be to use a loop processing all the lines one by one, using some variables to keep track of the current state: ‘found /*’, ‘found operation name XXX’ etc. – a state machine.

The scripting language I use now is mostly F# – but using mutable variables to keep track of the state like that feels ‘wrong’ in a functional language like F# – a solution without mutable variables would be preferable.

Half-remembering something I read in a blog (or maybe was a book?) I came up with using separate functions – each representing a state of the state machine – that simply call each other to transition between states:

let rec processStart (log: StreamReader) lineNumber =
if log.EndOfStream then
Seq.empty
else
let lineNumber = lineNumber + 1
if line="/*" then
processStartComment log lineNumber
else
processStart log lineNumber
and processStartComment (log: StreamReader) lineNumber =
if log.EndOfStream then
Seq.empty
else
let lineNumber = lineNumber + 1
processProc (line.Trim()) log lineNumber
and processProc procName (log: StreamReader) lineNumber =
if log.EndOfStream then
Seq.empty
else
let lineNumber = lineNumber + 1
let dateTimeString = line.Trim()
let (ok, dateTime) = DateTime.TryParse(dateTimeString)
seq {
if ok then
yield (lineNumber, procName, dateTime)
yield! processStart log lineNumber
}

The source stream and the current line number are passed as a parameter to each function, and the state variables are parameters as well – in this case just the name of the operation, that is passed from ‘processStartComment’ to ‘processProc’. The functions return the result as a sequence of triplets line number, operation name, operation date-time. To process a log file simply open it and call ‘processStart’:

let filterLog (path: string) =
processStart log 1