CODEWITHSUNDEEP
regexf#functional-programming
Nick Gilbert
Nick Gilbert

Reputation: 4240

Using Regex in F# to Filter Strings

I'm trying to use regex to filter all the strings in a list of strings. Here is my function which takes a string and filters it

let filterWord wordToFilter = 
    Regex.Replace(wordToFilter, "[^a-zA-Z0-9/!\'?.-]", "");

Since I want to apply that function to every element in my list of strings, it seems that using List.map would make sense. Here is my attempt at using map 

let filteredWords = unfilteredWords |> List.map(fun x -> filterWord(x));

I was hoping that line would apply my filter function to every string in my list (unfilteredWords is a list of strings) but instead I get a syntax error saying 

"Type mismatch. Expecting a
    string [] -> 'a    
but given a
    'b list -> 'c list    
The type 'string []' does not match the type ''a list'"

And have no idea why. Here is the full code

open System;
open System.IO;
open System.Text.RegularExpressions;

(*Tests if an element is in a list*)
let isInList elementToFind listToCheck = 
    List.fold(fun a b -> a || b = elementToFind) false listToCheck;

(*Takes a string and filters it down to common text characters*)
let filterWord wordToFilter = 
    Regex.Replace(wordToFilter, "[^a-zA-Z0-9/!\'?.-]", "");

(*Main method of the program*)
[<EntryPoint>]
let main argsv =
    let input = File.ReadAllText("Alice in Wonderland.txt"); //Reads all the text into a single string
    let unfilteredWords = input.Split(' ');
    let filteredWords = unfilteredWords |> List.map(fun x -> filterWord(x));
    0;

Any help would be appreciated, thanks!

EDIT: Apparently changing the value of unfilteredWords to a hard coded string array fixes it. Problem is I have no idea how to make it work with split.

Upvotes: 3

Views: 2250

Answers (2)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627128

Here is a fix: use Array.map instead of List.map.

Note that you can use F# .Split [|' '|] instead of .Split(' ').

let input = "### $$$ Alice In Wonderland ,,,,";
let unfilteredWords = input.Split [|' '|]
let filteredWords = unfilteredWords |> Array.map(fun x -> filterWord(x));
printfn "%A" filteredWords;

See IDEONE demo

The output for the sample string in my demo is

[|""; ""; "Alice"; "In"; "Wonderland"; ""|]

Upvotes: 2

khebbie
khebbie

Reputation: 2503

Seeing this question: F# Using List.map on an array of strings

I would guess you should use Array.map or Seq.map instead of List.map

Upvotes: -1

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