How to write a functional file "scanner" - f#

Question

First let me apologize for the scale of this problem but I'm really trying to think functionally and this is one of the more challenging problems I have had to work with.
I wanted to get some suggestions on how I might handle a problem I have in a functional manner, particularly in F#. I am writing a program to go through a list of directories and using a list of regex patterns to filter the list of files retrieved from the directories and using a second list of regex patterns to find matches in the text of the retreived files. I want this thing to return the filename, line index, column index, pattern and matched value for each piece of text that matches a given regex pattern. Also, exceptions need to be recorded and there are 3 possible exceptions scenarios: can't open the directory, can't open the file, reading content from the file failed. The final requirement of this is the the volume of files "scanned" for matches could be very large so this whole thing needs to be lazy. I'm not too worried about a "pure" functional solution as much as I'm interested in a "good" solution that reads well and performs well. One final challenge is to make it interop with C# because I would like to use the winform tools to attach this algorithm to a ui. Here is my first attempt and hopefully this will clarify the problem:
open System.Text.RegularExpressions
open System.IO
type Reader<'t, 'a> = 't -> 'a //=M['a], result varies
let returnM x _ = x
let map f m = fun t -> t |> m |> f
let apply f m = fun t -> t |> m |> (t |> f)
let bind f m = fun t -> t |> (t |> m |> f)
let Scanner dirs =
returnM dirs
|> apply (fun dirExHandler ->
Seq.collect (fun directory ->
try
Directory.GetFiles(directory, "*", SearchOption.AllDirectories)
with | e ->
dirExHandler e directory
Array.empty))
|> map (fun filenames ->
returnM filenames
|> apply (fun (filenamepatterns, lineExHandler, fileExHandler) ->
Seq.filter (fun filename ->
filenamepatterns |> Seq.exists (fun pattern ->
let regex = new Regex(pattern)
regex.IsMatch(filename)))
>> Seq.map (fun filename ->
let fileinfo = new FileInfo(filename)
try
use reader = fileinfo.OpenText()
Seq.unfold (fun ((reader : StreamReader), index) ->
if not reader.EndOfStream then
try
let line = reader.ReadLine()
Some((line, index), (reader, index + 1))
with | e ->
lineExHandler e filename index
None
else
None) (reader, 0)
|> (fun lines -> (filename, lines))
with | e ->
fileExHandler e filename
(filename, Seq.empty))
>> (fun files ->
returnM files
|> apply (fun contentpatterns ->
Seq.collect (fun file ->
let filename, lines = file
lines |>
Seq.collect (fun line ->
let content, index = line
contentpatterns
|> Seq.collect (fun pattern ->
let regex = new Regex(pattern)
regex.Matches(content)
|> (Seq.cast<Match>
>> Seq.map (fun contentmatch ->
(filename,
index,
contentmatch.Index,
pattern,
contentmatch.Value))))))))))
Thanks for any input.
Updated -- here is any updated solution based on feedback I received:
open System.Text.RegularExpressions
open System.IO
type ScannerConfiguration = {
FileNamePatterns : seq<string>
ContentPatterns : seq<string>
FileExceptionHandler : exn -> string -> unit
LineExceptionHandler : exn -> string -> int -> unit
DirectoryExceptionHandler : exn -> string -> unit }
let scanner specifiedDirectories (configuration : ScannerConfiguration) = seq {
let ToCachedRegexList = Seq.map (fun pattern -> new Regex(pattern)) >> Seq.cache
let contentRegexes = configuration.ContentPatterns |> ToCachedRegexList
let filenameRegexes = configuration.FileNamePatterns |> ToCachedRegexList
let getLines exHandler reader =
Seq.unfold (fun ((reader : StreamReader), index) ->
if not reader.EndOfStream then
try
let line = reader.ReadLine()
Some((line, index), (reader, index + 1))
with | e -> exHandler e index; None
else
None) (reader, 0)
for specifiedDirectory in specifiedDirectories do
let files =
try Directory.GetFiles(specifiedDirectory, "*", SearchOption.AllDirectories)
with e -> configuration.DirectoryExceptionHandler e specifiedDirectory; [||]
for file in files do
if filenameRegexes |> Seq.exists (fun (regex : Regex) -> regex.IsMatch(file)) then
let lines =
let fileinfo = new FileInfo(file)
try
use reader = fileinfo.OpenText()
reader |> getLines (fun e index -> configuration.LineExceptionHandler e file index)
with | e -> configuration.FileExceptionHandler e file; Seq.empty
for line in lines do
let content, index = line
for contentregex in contentRegexes do
for mmatch in content |> contentregex.Matches do
yield (file, index, mmatch.Index, contentregex.ToString(), mmatch.Value) }
Again, any input is welcome.

Answer 1

I think that the best approach is to start with the simplest solution and then extend it. Your current approach seems to be quite hard to read to me for two reasons:
The code uses a lot of combinators and function compositions in patterns that are not too common in F#. Some of the processing can be more easily written using sequence expressions.
The code is all written as a single function, but it is fairly complex and would be more readable if it was separated into multiple functions.
I would probably start by splitting the code in a function that tests a single file (say fileMatches) and a function that walks over the files and calls fileMatches. The main iteration can be quite nicely written using F# sequence expressions:
// Checks whether a file name matches a filename pattern
// and a content matches a content pattern.
let fileMatches fileNamePatterns contentPatterns
(fileExHandler, lineExHandler) file =
// TODO: This can be imlemented using
// File.ReadLines which returns a sequence.
// Iterates over all the files and calls 'fileMatches'.
let scanner specifiedDirectories fileNamePatterns contentPatterns
(dirExHandler, fileExHandler, lineExHandler) = seq {
// Iterate over all the specified directories.
for specifiedDir in specifiedDirectories do
// Find all files in the directories (and handle exceptions).
let files =
try Directory.GetFiles(specifiedDir, "*", SearchOption.AllDirectories)
with e -> dirExHandler e specifiedDir; [||]
// Iterate over all files and report those that match.
for file in files do
if fileMatches fileNamePatterns contentPatterns
(fileExHandler, lineExHandler) file then
// Matches! Return this file as part of the result.
yield file }
The function is still quite complicated, because you need to pass a lot of parameters around. Wrapping the parameters in a simple type or a record could be a good idea:
type ScannerArguments =
{ FileNamePatterns:string
ContentPatterns:string
FileExceptionHandler:exn -> string -> unit
LineExceptionHandler:exn -> string -> unit
DirectoryExceptionHandler:exn -> string -> unit }
Then you can define both fileMatches and scanner as functions that take just two parameters, which will make your code a lot more readable. Something like:
// Iterates over all the files and calls 'fileMatches'.
let scanner specifiedDirectories (args:ScannerArguments) = seq {
for specifiedDir in specifiedDirectories do
let files =
try Directory.GetFiles(specifiedDir, "*", SearchOption.AllDirectories)
with e -> args.DirectoryExceptionHandler e specifiedDir; [||]
for file in files do
// No need to propagate all arguments explicitly to other functions.
if fileMatches args file then yield file }