Back to Questions
874
views2
comments
Abuse of _ in Pattern Matching?
f#

I'm trying to manually extract tokens without Lex, but I'm a little confused about the _ operator in pattern matching. I am trying to use _ as a parameter to a function call and the VS2008 compile is barking at me. Can someone explain to me what I am doing wrong?

let rec tokens s =

match s with

| '<' :: '#' :: rest -> "openSymbol" :: tokens(rest)

| '#' :: '>' :: rest -> "closeSymbol" :: tokens(rest)

| ',' :: rest -> "comma" :: tokens(rest)

| '+' :: rest -> "plus" :: tokens(rest)

| '-' :: rest -> "plus" :: tokens(rest)

| _ :: rest ->

if isDigit(_) then "digit" :: tokens(rest)

else "Nothing" :: tokens(rest)

| [] -> []

let isDigit c =

match c with

| '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' -> true

| _ -> false

// [link:stackoverflow.com]

// Somewhat modified

let rec explode str =

let len = String.length str in

if len=0 then [] else

(str.[0] :: explode (str.[1..(len-1)]))

let theExpression = "<#35,~10#>+<#3,4#>"

let anotherExpression = explode(theExpression)

// Should now have a rudimentary list of tokens

// I know the integers should aren't being fully extracted in the function tokens but

// that is another matter...

let result = tokens(anotherExpression)

.

_ is not an identifier, it's a pattern that means "don't care". If you want to call isDigit on the first token, then you need to change e.g.

| _ :: rest -> if isDigit(_) ...

to

| t :: rest -> if isDigit(t) ...

that is, name the thing you are matching, so you can pass it as an argument to a function.

(Note also that you'll have to define isDigit before you can call it, and that you could use System.Char.IsDigit rather than define your own.)

By on 4/24/2010 3:21 PM ()

Thanks brianmcn!!!

Your answer works really well!

By on 4/25/2010 12:37 PM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper