Why OO Matters (in F#)

I kind of agree and also prefer to use objects in some cases.

Eirik Tsarpalis' blog

F# is a functional-first programming language that comes with a substantial object-oriented feature set. It is so feature-complete in fact, that almost any C# class can be ported over to F# code with little substantial alteration.

However significant, this subset of the language is seeing limited appreciation from the community, which I suspect is partly fuelled by the known criticisms of OOP and partly by a desire to be different than C#. After all, this is a functional-first language so we can just replace all our classes with functions. There is also the opinion that OOP in F# merely serves as a compatibility layer for .NET, so it’s really only there to cover those unfortunate scenarios of having to use a library that accepts interfaces.

Enabling Abstraction

One of the most important aspects of maintaining a nontrivial codebase is controlling complexity. Complexity can be contained by partitioning code into logically…

View original post 1,596 more words

You’re better off using Exceptions

Worth reading, for sure.

Eirik Tsarpalis' blog

Exception handling is an error management paradigm that has often been met with criticism. Such criticisms typically revolve around scoping considerations, exceptions-as-control-flow abuse or even the assertion that exceptions are really just a type safe version of goto. To an extent, these seem like valid concerns but it is not within the scope of this article to address those per se.

Such concerns resonate particularly well within FP communities, often taken to the extreme: we should reject exceptions altogether, since code that throws is necessarily impure. In the F# community, this opinion is in part realized by advocating alternatives like result types and railway-oriented programming. In essence, these approaches follow the Either monad found in Haskell, but often intentionally avoiding the use of do notation/computation expressions (since that’s just interpreted exception semantics).

The TL;DR version of the approach is that we define a union type for results that looks…

View original post 920 more words

F# Advent Calendar in English 2016

Update (11/28/2016): Dear writers, this year we are going to make an e-book from #FsAdvent posts and to raise money for a good cause (read more here). If you do not allow to use your post, please let me know.

F# Advent Calendar is an adorable idea, which has become a tradition:

Each year we’ve had an incredible Advent full of F# and Christmas spirit.

Advent 2016 is coming, do not to lose your chance to reserve a spot and spread your thoughts and love to F# with the community. Have you done something special this year? Do you have any unique experience you are willing to share? Have your project incredibly evolved this year? Are there any good ideas for the post, but you didn’t have time to write it? The time has come – it is right now! You have a chance to share your story with the globe! Join F# Advent Calendar and hurry up!


Rules are very simple:

  1. Choose F# related topic for your blog post and reserve the date on Twitter or leave a comment to this post. Please note that you do not have to announce the topic until the date.
  2. Prepare a blog post in English
  3. Publish your post on a specified date (according to the calendar)
  4. Post the link to your post on Twitter with hashtags #fsharp and #FsAdvent.


Date Author Post Title
Nov 27 (Sun) Anthony Lloyd Functional Event Sourcing meets The Elm Architecture
Nov 27 (Sun) Rick Pack  F# Data Visualization for a Neophyte – CDC Open Data
Nov 28 (Mon) Mark Seemann Easy domain modelling with types
Nov 28 (Mon) Bill Berry Introduction to Azure IoT with Fsharp
Nov 29 (Tue) Mikhail Shilkov Introducing Stream Processing in F#
Nov 29 (Tue) josesoyo Using OpenCL on F# via FSCL
Nov 30 (Wed) Alfonso Garcia-Caro Tree Shaking with Fable
Nov 30 (Wed) Jeremy Bellows NeuralFish – Evolving Neural Topologies in F#
Dec 01 (Thu) Steffen Forkmann Context matters
Dec 01 (Thu) Ramón Soto Mathiesen Semantic Versioning .NET libraries and NuGet packages
Dec 02 (Fri) Chet Husk A Host of Issues: Migrating from UserVoice To GitHub Issues with Canopy and OctoKit.net
Dec 02 (Fri) Kristian Schmidt F# in Production
Dec 03 (Sat) Henrik Feldt A gentle introduction to programming networked services on linux
Dec 04 (Sun) Jérémie Chassaing fck: Fake Construction Kit
Dec 04 (Sun) Phillip Trelford X-Platform Development With Xamarin.Forms And F#
Dec 05 (Mon) Scott Wlaschin Functional approaches to dependency injection
Dec 05 (Mon) Rachel Reese Using type providers to post slack messages
Dec 06 (Tue) Daniel Bachler Using F# on both the frontend and the backend
Dec 06 (Tue) Mark Heath  Asterisk Game in F# and WPF
Dec 07 (Wed) Edgar Sánchez Describing images in your favorite human language
Dec 07 (Wed) András Jankó Distributed web applications in F# with WebSharper
Dec 08 (Thu) Hussam Abu-Libdeh How F# delighted this newbie while experimenting with distributed systems
Dec 08 (Thu) Jonathan Leaver RdKafka for F# Microservices
Dec 09 (Fri) Doug Bruce Build Yourself a Robo-Advisor in F#. Part I : Domain Modelling
Dec 09 (Fri) Gergely Kalapos myTrades.Analytics – a netstandard compatible .NET technical analysis library written in F#
Dec 10 (Sat) Mårten Rånge F# Advent – December 10, 2016 – Implementing a persistent hash map
Dec 10 (Sat) Erik Schierboom Parsing text in F#
Dec 11 (Sun) Matt Olson An Introduction To Testing Suave WebParts
Dec 11 (Sun) Jeremy Abbott Merry Monad Xmas
Dec 12 (Mon) Pierre Irrmann Some advice to F# beginners
Dec 12 (Mon) Pierre-Luc Maheu Scripting in F# using Fake and Paket
Dec 13 (Tue) Roman Provazník Building F# community in Czech Republic
Dec 13 (Tue) Isak Sky Working with SQL syntax trees in F#
Dec 14 (Wed) Troy Kershaw The AsyncArrow
Dec 15 (Thu) Reed Copsey, Jr. Christmas Trees in WPF, 2016 Edition 
Dec 15 (Thu) Dustin Moris Gorski Running Suave in ASP.NET Core (and on top of Kestrel)
Dec 16 (Fri) Michael Newton Advent 2016 (It has a pretty d3 force graph at the end, FParsec examples and other good stuff)
Dec 17 (Sat) Kunjan Dalal Epic Adventure using F#
Dec 17 (Sat) Jan Schiefer Accessing native C libraries on Linux with F# and .Net Core
Dec 19 (Mon) Tomasz Heimowski Creating a tutorial from Git repo
Dec 19 (Mon) Tomasz Jaskuλa Data structures and algorithms – helping Santa Claus find his road to San Francisco
Dec 20 (Tue) Тэ дрэвэт утвикλэрэн Feeding On F#
Dec 20 (Tue) Vagif Abilov reF#actoring: rewriting an actor in F#
Dec 21 (Wed) Riccardo Terrell The Traveling Santa Problem… a Neural Network solution
Dec 21 (Wed) Elastic Inc Solidifying Releases with FAKE (F# MAKE)
Dec 22 (Thu) David Podhola Hello fable-import-sharepoint
Dec 23 (Fri) Carsten König Advent with a Star
Dec 23 (Fri) kekyo2 About Expandable F# Compiler project
Dec 24 (Sat) Roman Nevolin The magic of type providers
Dec 24 (Sat) Ross McKinlay + Andrea McAts When Playstation meets F#, PSX |> Pi
Dec 25 (Sun) Jamie Dixon Age and Sex Analysis Of Microsoft USA MVPs
Dec 25 (Sun) Leif Battermann Purity in an impure language with the free monad – by example of a Tic-Tac-Toe backend with CQRS and event sourcing
Dec 26 (Mon) Tamizh Vendan Implementing Two-Factor Authentication in Suave
Dec 26 (Mon) Marcel Schwark Implementing Pong in a functional manner with Fable
Dec 27 (Tue) Jon Wood Getting Up and Running with FsLab for Data Science 
Dec 29 (Thu) Eriawan Kusumawardho F# Advent: F# 4.0 function first constructor
Dec 30 (Fri) Paulmichael Blasucci Tips & Tricks to Improve Your F# Library’s Public API
Dec 31 (Sat) Adam Granicz Simple reactive scenarios with WebSharper
Jan 01 (Sun) F# Software Foundation Welcome to 2017! A retrospective of the past year

TypeShape: Practical Generic Programming in F#


Eirik Tsarpalis' blog

Last week I announced a new library, TypeShape, with claims that it provides a practical way of doing generic programming in F#. I’m following up with this blog post to elaborate why I believe this to be genuinely useful, and how it could benefit the day-to-day life of the working .NET developer.

The pain of Reflection

Almost everybody who has worked with .NET will at some point need to dabble in the murky ways of reflection. Reflection is needed in scenaria where we need to access data in an indirect fashion, or where circumvention of the type system is necessary.

For example, assume that we have defined the following static method

Assume now that we would like invoke that method, with a value whose type cannot be known at compile time. In other words, we want to define a function

which takes an input of type obj and invokes the generic method…

View original post 558 more words

Wire – Writing one of the fastest .NET serializers

Worth to try

Roger Johansson Blog

First of all, there is no such thing as “the fastest” serializer, it is all contextual.
But under some conditions, I would however argue that Wire is, by far, the fastest of all the .NET serializers out there.

Given the following POCO type.

Round tripping one million objects of this type, that is, serializing and then deserializing a million objects using Wire with all optimizations on, completes in about 550 milliseconds on my personal laptop.

Doing the same using MS Bond, which is the second fastest serializer in the benchmark, takes about 830 milliseconds, and this is while being very generous to Bond as it has some very specific prerequisites.
Protobuf.NET which is the third serializer on this benchmark completes in about 1360 milliseconds.

Other serializers that was included in the same benchmark was Jil, NetSerializer, FS Pickler, Json.NET and .NET BinaryFormatter.


View original post 1,847 more words

F# Type Providers Development Tips (not Tricks)

There are several tips that you probably would like to know if you plan to create a new F# Type Provider(TP) or to contribute to the existing one.

Tip #1: Use FSharp.TypeProviders.StarterPack

Latest version of ProvidedTypes SDK is stored in fsprojects/FSharp.TypeProviders.StarterPack repository and you have to use this version in most cases.

Community agreed to use this repository as master version for the SDK. You may find SDK files committed to other repositories, some of them (like files in FSharp.Data repository) may even be modified, contain new features or latest fixes. Once changes are tested and generalized enough, they will for sure be contributed back to FSharp.TypeProviders.StarterPack. So it is better to use a version from the official repository to be on the safe side.

Tip #2: Use Paket dependency manager

Paket allows you to reference files directly from GitHub and easily keep them up-to-date together with other NuGet dependencies.

All you need is to add two lines to your paket.dependecies files (that tell Paket to download files from GitHub to paket-files folder)

github fsprojects/FSharp.TypeProviders.StarterPack src/ProvidedTypes.fsi
github fsprojects/FSharp.TypeProviders.StarterPack src/ProvidedTypes.fs

and two lines to paket.references file (to tell Paket to insert files into corresponding fsproj files)


You can see how it works in ExcelProvider: paket.dependencies + paket.references

Tip #3: Create two solution files (*.sln)

You should not store TP project and projects that use TP in one solution, because when you reference TP dll, IDE/Intellisense loads this assembly and locks file on a disk. After that, you will not be able to rebuild your TP anymore until you close IDE.

So, it does not work in a long run and you have to separate your code to two solutions. The first one (let’s say SwaggerProvider.sln) will contain TP source code, tests for parser and for all components that don’t call TP directly. The second one (let’s say SwaggerProvider.TestsAndDocs.sln) will contain tests, which use TP directly and docs that may also use TP dll.


Tip #4: Automate build (using FAKE)

This tip is quite generic and you have to do it for all projects, but it becomes extremely useful for type providers. It’s tedious to open a new solution (IDE instance), when you want to test latest changes – it is much easier to have a build script that rebuilds and tests everything in one click.

The good start point here is fsprojects/ProjectScaffold that contains most useful generic build automation steps.

Tip #5: Yes, you can debug Type Providers

Debugging of TP does not look an easy task at first sight (and it is really slow in practice), but it is real.

You can start two instances of your IDE. The first one, for a solution with TP code and the second one for code that uses compiled type provider (here is important to check that the second one really references dll compiled by the first one). For the second IDE you can use TestAndDocs solution from Tip #4 or a simple *.fsx script that calls your TP. The last step is to set break point in the first IDE instance and to attach to the second IDE instance process.

This allows you to debug, but you will not be able to modify the code of TP. After each modification, you will need to close 2nd IDE, rebuild dll and repeat all these steps once again.

However, you can automate all manual steps:

  • Open project properties of TP project.
  • Open Debug tab.
  • Select start external program checkpoint
  • Enter path to your VS devenv.exe (For example: “C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv.exe”)
  • Put path to your tests project (file) in command line arguments (For example “d:\GitHub\SwaggerProvider\SwaggerProvider.TestsAndDocs.sln”)

After that, when you press F5 from your TP project, VS will automatically build your TP, start your instance of VS, attach to a new VS process and open your tests solution in it.


Tip #6: Write tests using FSharp.Compiler.Service

Usefulness of this tip really depends on the TP you are working on, but if you are lucky enough (like I am) and you have an access to a large collection of schemes for your type provider, you can automate testing of compilation of provided types.

There is an awesome resource called APIs.guru (Wikipedia for WEB APIs) that provides an access to several hundred Swagger schemes of real-world Web APIs. For sure, SwaggerProvider uses these schemes to test schema parser and it is relatively easy to do.

But we can go further and check that provided types are “compilable” by F# compiler (there is no collisions in type names, property names, field names and method names).

The one way to do this is to use “Hosted Compiler” feature of F# Compiler Services. This allows us to create a simple script that just instantiates TP for each schema and asks F# Compiler to compile this generated script.

Here is source code from tests:

[<Test; TestCaseSource("JsonSchemasSource")>]
let ``Compile TP`` url =
  let tempFile = Path.GetTempFileName()
  let fs = Path.ChangeExtension(tempFile, ".fs")
  let dll = Path.ChangeExtension(tempFile, ".dll")

  File.WriteAllText(fs, sprintf """
  module TestModule
  open SwaggerProvider
  type ProvidedSwagger = SwaggerProvider<"%s">
  let instance = ProvidedSwagger()
  """ url)

  let errors, exitCode =
      (["fsc.exe"; "-o"; dll; "-a"; fs] @ asms))

  [tempFile; fs; dll]
  |> List.filter File.Exists
  |> List.iter File.Delete

  if exitCode <> 0 then
    let strs = errors |> Array.map(fun x->x.ToString())
    failwithf "Error:\n%s" (String.Join("\n", strs )) 


Tip #7: Handling 3rd party NuGet dependencies

You face a dilemma when your TP needs 3rd party dlls: “How to deliver these dlls to a user?” This is a dilemma, because your IDE will not be able to resolve 3rd party dependencies without your participation if you just add them as dependencies to your NuGet package.

This is the case, for example, when your schema is in JSON format and you decided to use JSON.NET to parse it, or in Yaml format and you want to use YamlDotNet.

Option 1. Pack all dependencies inside your NuGet package.

The simplest solution is to put all required dlls in the folder with your TP dll in NuGet package. In this case, there is nothing to do with assembly resolution and it is definitely a good option to start from.

For example, FSharp.Configuration uses this option and packs SharpYaml.dll inside.

BUT, you have to remember that in this case you limit your users to the exact version of dll packed with your TP. If they reference a newer version from NuGet, it may lead to a run-time error.

Option 2. Uses AssemblyResolve event handler.

If I am not wrong, this solution was firstly developed in RProvider.

Actually, you split your TP into three assemblies:

  • TP.dll that setups AssemblyResolve event handler that helps IDE find assemblies on a hard drive and tells the compiler that TP will be in TP.DesignTime.dll (but doesn’t reference this assembly directly).
  • TP.DesignTime.dll that contains implementation of TP.
  • TP.Runtime.dll that contains the code that should exist in run-time and may be used by provided methods.

TP.dll references TP.Runtime.dll but uses nothing from it. Such dependencies do not break intellisense (because it does not need this assembly), but in the same time your provided code will be able to call code from TP.Runtime.dll.

Read more about this in RProvider Developer Notes (SwaggerProvider uses the same approach).


Tip #8: Use experience of other TP developers

Google Cloud Vision API from .NET\F# (OAuth2 with ServiceAccount.json)

Google Cloud Platform provides a wide range of APIs, one of which is Cloud Vision API that allows you to detect faces in images, extract sentiments, detect landmark, OCR and etc.

One of available annotators is “Logo Detection” that allows you to find company logo in your image and recognize it.

.NET is not the part of mainstream Google Cloud SDK. Google maintains google-api-dotnet-client that should allow you to authenticate to and call all available services. API design looks slightly not intuitive for .NET world (at least from my point of view).

I spent some time on Google/SO/Github trying to understand how to use OAuth2 in server-to-server authentication scenario with ServiceAccount.json file generated by Google API Manager.


You cannot use this API without billing account, so you have to put your credit card info, if you want to play with this API.

Also, note that you need to have two NuGet packages Google.Apis.Vision.v1Google.Apis.Oauth2.v2 (and a lot of their dependencies)

So, here is the full sample:

#load "Scripts/load-references-debug.fsx"

open System.IO
open Google.Apis.Auth.OAuth2
open Google.Apis.Services
open Google.Apis.Vision.v1
open Google.Apis.Vision.v1.Data

// OAuth2 authentication using service account JSON file
let credentials =
    let jsonServiceAccount = @"d:\ServiceAccount.json"
    use stream = new FileStream(jsonServiceAccount, 
                         FileMode.Open, FileAccess.Read)

let visionService = // Google Cloud Vision Service
        ApplicationName = "my-cool-app",
        HttpClientInitializer = credentials)
    |> VisionService

// Logo detection request for one image
let createRequest content = 
  let wrap (xs:'a list) = System.Collections.Generic.List(xs)
    Requests = wrap
        Features = wrap [Feature(Type = "LOGO_DETECTION")],
        Image = Image(Content = content))
  |> visionService.Images.Annotate

let call fileName = // Call and interpret results
    let request =
        File.ReadAllBytes fileName
        |> System.Convert.ToBase64String
        |> createRequest
    let response = request.Execute()

    [ for x in response.Responses do
        for y in x.LogoAnnotations do
          yield y.Description
    ] |> List.toArray

let x = call "D:\\fsharp256.png"
// val x : string [] = [|"F#"|]

FsShelter: a Storm shell for F#

I think, therefore I spam.

About a year ago Prolucid adopted Apache Storm as our platform of choice for event stream processing and F# as our language of choice for all of our “cloud” development.

FsStorm was an essential part that let us iterate, scale and deliver quickly, but even from the earliest days it was obvious that the developer experience could be improved. Unfortunately, it meant a complete rewrite of FsStorm:

  • FsStorm DSL is a really thin layer on top of Nimbus API model:
    • has explicit IDs when describing components in a topology
    • uses strings in all the names
    • matching of inputs/outputs is not guaranteed
  • FsStorm uses Json AST as it’s public API:
    • messages, tuples, configuration
    • serialization mechanism is hard-baked into the API

We’ve worked around some of the problems, usually by writing more code.

It actually makes sense that Storm itself doesn’t care about the type of the tuples/fields. It runs on JVM, which is very much typed…

View original post 220 more words

F#ools’ Day Party – April 1, 2016

AprilFools6301st April is coming, Mark Gray and I propose F# community to celebrate this day with funny posts about F#.

All you need is to write a blog post that incorporates the idea of April fool in the content or find out a new extraordinary use of F# and destroy the myth that “F# is for Math and Science”.

Rules are even simpler than in FsAdvent:

  1. Ping @sergey-tihon on Twitter and I will put your name in the table below
  2. Tweet link to your post with  tag on April 1 (+/- 1 day).

Happy F#ools’ Day,
Let’s joke together.

Author Post Title
 Pierre Irrmann  Making program behaviour explicit
 Ross McKinlay
 Phillip Trelford  Silverlight Resurrection
 Vicenç García-Altés  A Premier League team will win the Champions League this year