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

ASP.NET MVC with Simple Windows Authorization

A lot of enterprises use Active Directory (AD) to manage user accounts and Security Groups to manage access to resources.

So (I think) that there is a common task when you want to create some internal resource that will provide certain functionality for your team, but you do not want to expose your data outside. We can easily enable Windows authentication, however usually we also need to add an authorization(limit access to certain groups)

The task is simple, but I do not know why it is so hard to find manual for this. Steps are as follows:

  • Enable Windows authentication in web.config
  • Add WindowsTokenRoleProvider that transforms all Security Groups to ASP.NET Roles
  • Configure Authorization rules based on roles
  • Disable anonymous authentication for IIS Express.

Changes in Web.config:

<?xml version="1.0" encoding="utf-8"?>
    <authentication mode="Windows" />
      <allow roles="DOMAIN\MyTeam" />
      <deny users="*"/>
    <roleManager cacheRolesInCookie="false" defaultProvider="WindowsProvider" enabled="true">
        <clear />
        <add name="WindowsProvider" type="System.Web.Security.WindowsTokenRoleProvider" applicationName="/" />

Changes in project file:

<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="">
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <IISExpressSSLPort />
    <IISExpressUseClassicPipelineMode />
    <UseGlobalApplicationHostFile />

P.S. You can use security groups to restrict access to Controllers/Views based on the roles (AuthorizeAttribute)

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 (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 |> 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

About JavaScript memory leaks. Again.

Worth to read, at least, if you are not JS dev

Anton Gorbikov's Blog

Web applications are getting more and more complex from year to year. Couple of years ago nobody cared about the memory leaks on the web pages (yeah, they were really a set of web pages, but not web applications). Even if you forget to clean up some memory – it’s not expected, that user will spend a lot of time on the same page. There were a lot of navigation links, which removed information about the entire page from memory and loaded new page.

But now you cannot relate on this behavior, because now web sites turned into web applications. User loads one small HTML file, one script file and some other stuff (like CSS and images) and that’s it. Making requests from browser to servers user can stay on on the “page” for ages. In the worst scenario you will receive a report from production user: “Oh, your application…

View original post 2,020 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