SQL and NoSQL tools as you know and love them with F#

by DotNetNerd 6. May 2013 14:31

toolsWhile looking at how you can do your common scenarios in F# an obvious thing to look at is data access. I already covered type providers which are fantastic for consuming data. However type providers won't help you when you need to create data or in other ways interact with your SQL database, MongoDB, RavenDB or any other NoSQL solution you might be using. For this you will most likely want to use some of the libraries you already know and love - there is no reason to reinvent everything.

Working with existing libraries is mostly trivial, but F# likes types that are immutable which straight out of the box doesn't play well with serialization and mapping. This means that ORM's and libraries for MongoDB and RavenDB will throw exceptions when they try to construct your F# types. This basically boils down to the fact that code like the sample below won’t work, because the serializer can not instantiate the type and fill in the values.

open System.Xml.Serialization
let ser = new XmlSerializer(typeof<Person>)
let ms = new System.IO.MemoryStream()
ser.Serialize(ms, r)
ms.Seek(0L, System.IO.SeekOrigin.Begin) |> ignore
let r2 = ser.Deserialize(ms)

In the first versions of F# this meant that you had to write default constructors and setters for properties. Besides being tedious it also meant introducing mutability in your F# code. Since F# 3.0 you can use an attribute called CLIMutable, which emits a default constructor and setters for properties in IL code - presto everything works as expected and you avoid introducing mutability in your own code. A simple sample could be done like this.

type Person = { Name : string; Age : int }

In the book Building Web, Cloud, and Mobile Solutions with F# this is covered nicely with examples in MongoDB and RavenDB. I also wanted to test this with Dapper, which is my Micro ORM of choice these days along with Peta Poco. It litterally worked in the first try, as almost nothing ever does. So I pushed the envelope a little and tried letting Dapper return a dynamic object and BOOM! The error message let me know that F# didn't know how to dynamically access .Name on my Person object. Lucky me I quickly found a solution to this after googled pointed me toward the answer on Stack Overflow - as I was not the first developer to run into this problem.

To use dynamic types you can use the ImpromptuInterface.FSharp library, which can easilly be installed using Nuget: Install-Package ImpromptuInterface.FSharp

ImpromptuInterface gives you an ? operator that you can use to do a dynamic lookup, which let me to the resulting code.

use conn = new SqlCeConnection("Data Source=C:\Databases\MyDB.sdf;Persist Security Info=False;")

    let result =
        conn.Query("SELECT * FROM People")
        |> List.ofSeq

    for person in result do
        printfn "%s" person?Name

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My name is Christian Holm Diget, and I work as an independent consultant, in Denmark, where I write code, give advice on architecture and help with training. On the side I get to do a bit of speaking and help with miscellaneous community events.

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