Handy F# snippets [closed]

There are already two questions about F#/functional snippets.

However what I'm looking for here are useful snippets, little 'helper' functions that are reusable. Or obscure but nifty patterns that you can never quite remember.

Something like:

open System.IO

let rec visitor dir filter= 
    seq { yield! Directory.GetFiles(dir, filter)
          for subdir in Directory.GetDirectories(dir) do 
              yield! visitor subdir filter} 

I'd like to make this a kind of handy reference page. As such there will be no right answer, but hopefully lots of good ones.

EDIT Tomas Petricek has created a site specifically for F# snippets http://fssnip.net/.

Perl style regex matching

let (=~) input pattern =
    System.Text.RegularExpressions.Regex.IsMatch(input, pattern)

It lets you match text using let test = "monkey" =~ "monk.+" notation.

Infix Operator

I got this from http://sandersn.com/blog//index.php/2009/10/22/infix-function-trick-for-f go to that page for more details.

If you know Haskell, you might find yourself missing infix sugar in F#:

// standard Haskell call has function first, then args just like F#. So obviously
// here there is a function that takes two strings: string -> string -> string 
startsWith "kevin" "k"

//Haskell infix operator via backQuotes. Sometimes makes a function read better.
"kevin" `startsWith` "K" 

While F# doesn't have a true 'infix' operator, the same thing can be accomplished almost as elegantly via a pipeline and a 'backpipeline' (who knew of such a thing??)

// F# 'infix' trick via pipelines
"kevin" |> startsWith <| "K"

Multi-Line Strings

This is pretty trivial, but it seems to be a feature of F# strings that is not widely known.

let sql = "select a,b,c \
           from table \
           where a = 1"

This produces:

val sql : string = "select a,b,c from table where a = 1"

When the F# compiler sees a back-slash followed by a carriage return inside a string literal, it will remove everything from the back-slash to the first non-space character on the next line. This allows you to have multi-line string literals that line up, without using a bunch of string concatenation.