Using LINQ in F#?

There are kind of two aspects to LINQ. One is the language side, e.g. in C# there are (contextual) keywords like from and select and where you can use to author rather-SQL-like code that does queries over IEnumerables and whatnot. This is a relatively thin syntax sugar layer that transforms into LINQ library calls to Select and SelectMany and Where and whatnot.

Then there's the IQueryable aspect, which enables this same code to be reified as an IQueryable, which is effectively much like a quotation mechanism, in that it kind of reifies the syntax tree that constructed the query. And that is essential, because then different providers can plug in behavior to 'compile' the code in their own way, e.g. a SQL provider can get a 'whole query' view and compose a query with a good query plan that will run on the server (as opposed to the naive approach of treating a SQL database as an IEnumerable of rows and just loading all the rows in memory and filtering them in .NET inside your client app, which would perfom badly or simply fall down on large data).

The first aspect is just syntax sugar and whatnot. F# 2.0 (the F# version in VS2010) has no intrinsic support for LINQ, but the PowerPack has a LINQ bridge so that you can use F# quotations of normal F# Seq combinators as a way to express LINQ queries.

The second aspect is the more essential bit in terms of the overall technology, though. LINQ is about reifying queries so that programmers can declaratively specify intent, and then various providers can plug into the system and translate that intent into an efficient execution plan for the corresponding backing store of data. The .NET 3.5. expression tree types are 'the interface' to this essential aspect of LINQ, and so any given programming language just needs some way for programmers to write expressions that will generate these expression trees.

So I don't feel like the original question quite makes sense. The essential reason to use LINQ is because you want to query a SQL database (or an OData feed, or ...) in such a way that the query runs efficiently on the server (or does not needlessly use a million repetitive HTTP requests, or ...), and you want to do so in a way that does not require you to know much about the details of those technologies (the various LINQ back-end providers have all the individual domain-specific smarts). The syntax is mostly just syntax, and different languages may have different sugars (or sugaring mechanisms) to author LINQ queries in ways that are idiomatic to a given programming language.

I agree with @blucz answer, it's not recommended, but if you really want, it looks like this:

Import System and System.Linq

open System
open System.Linq

Wrap some LINQ extension methods

let select  f xs = Enumerable.Select(xs, new Func<_,_>(f))
let where   f xs = Enumerable.Where(xs, new Func<_,_>(f))
let orderBy f xs = Enumerable.OrderBy(xs, new Func<_,_>(f))

And use it

[1..100] 
|> where (fun x -> x > 2) 
|> select (fun x -> x * 2) 
|> printfn "%A"

You're right that for in-memory collections F# provides much of the behavior found in LINQ through the various collection modules, Seq, List, and the like. I believe LINQ's distinguishing feature is the plug-ability of query providers. While F# offers limited metaprogramming support through quotations, there's no trivial way that I'm aware of to translate these to something meaningful for disparate backing stores (PowerPack offers some support for this in F#).

To answer your question, if you intend to translate the queries instead of executing them against in-memory collections, LINQ is your best option. Otherwise, stick with the F# collection modules. By using LINQ when these suffice, you sacrifice partial function application, function chaining (piping), and you will write non-idiomatic code (there may be other pitfalls).

For people who find this thread. The newest answer is already a bit old.

In F# you can use query expressions when working a database. More details can be found here: https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/query-expressions

A simple example from that page:

open Microsoft.FSharp.Data.TypeProviders

type Northwind = 
    ODataService<"http://services.odata.org/Northwind/Northwind.svc">
let db = Northwind.GetDataContext()

// A query expression.
let query1 =
    query {
        for customer in db.Customers do
            select customer
    }

// Print results
query1
|> Seq.iter (fun customer -> printfn "Company: %s Contact: %s" 
    customer.CompanyName customer.ContactName)