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F# 3.0 on AppHarbor

November 1, 2012 6 comments

The online Analytics service for Rowing in Motion has some intense data processing requirements. A typical logfile that users may want to work with for a 90minutes training session is about 5 megabytes in compressed size. The in-memory models we need to work with for data analysis need to encompass millions of data points and can easily exceed 30mb of memory when fully unfold.

It’s pretty clear we could not offer a good user experience when processing all this data locally on a device, so we decided to build the data analysis software as an online service. There are some other benefits to this model too, especially in the space of data archival and historical comparisons. F# excels at expressing our calculation models in a short and concise manner and makes parallellizing these calculations easy, which is crucial to achieve acceptable response times in our scenario.

Deciding to use F# was easy, but it turns out I faced some problems integrating with our cloud hosting platform of choice AppHarbor. This post will explain what needs to be done to get F# code to compile on AppHarbor and also how to run unit tests there.

Compiling F# 3.0 code on AppHarbor

Visual Studio 2012 installs the F# “SDK” (there is no official one for F#3.0) into C:\Program Files\Microsoft SDKs\F#, and that’s where the default F# project templates point to.

<Import Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets" Condition=" Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets')" />

We will fix this (and another issue) by copying the whole “SDK” folder into our source repository at tools/F# (yes, everything). Next up, we will create a Custom.FSharp.Targets file, that we will reference instead. Replace the project line above with:

  <Import Project="$(SolutionDir)\build\RowingInMotion.FSharp.Targets" />

We will also have to delete the FSharp.Core reference from the fsproj file. Since the AppHarbor build machines don’t have FSharp.Core 4.3.0 in the GAC (or in a ReferenceAssemblies location), we have to include this into the project too. I copied mine from C:\Program Files (x86)\Reference Assemblies\Microsoft\FSharp to lib\FSharp

The Custom.FSharp.Targets we created earlier will take care of including the correct reference, as well as pointing the Microsoft.FSharp.Targets to the correct F# compiler directory in our source tree.

<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  
  <!--Include a default reference to the correct FSharp.Core assembly-->
  <ItemGroup>
	  <Reference Include="FSharp.Core, Version=4.3.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
		  <HintPath>$(SolutionDir)\lib\FSharp\3.0\Runtime\v4.0\FSharp.Core.dll</HintPath>
	  </Reference>
  </ItemGroup>
  
  <!--Override the Path to the FSharp Compiler to point to our tool dir-->
  <PropertyGroup>
	<FscToolPath>$(SolutionDir)\tools\F#\3.0\Framework\v4.0</FscToolPath>
  </PropertyGroup>
  <Import Project="$(SolutionDir)\tools\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets" />
  
</Project>

One last thing that needs to be fixed is that the F# compiler (itself written in F#) also needs a copy of FSharp.Core, so I simply dropped one right next to it. That’s it, now you should be able to compile F# 3.0 projects on AppHarbor. It’s nice that F# is “standalone” enough from the rest of the .NET Framework that it can be pulled apart this easily, but it would be even better if Microsoft offered a F# SDK that the guys at AppHarbor could install on their build servers.

Running F# xUnit tests on AppHarbor

AppHarbor uses Gallio to run unit tests. Unfortunately, Gallio is not able to detect static test methods. That means you cannot write tests as modules. Instead you have to resort to declaring normal types with members, which is a bit heavier on the syntax and feels considerably less idiomatic (and its more typing…). I have filed a bug with the Gallio Team, which can be tracked here: http://code.google.com/p/mb-unit/issues/detail?id=902. It should be noted that the xUnit Visual Studio runner can run F# Xunit tests just fine. We’ll see if I see the need to switch to a more specific F# testing framework in the future.

Categories: .NET, F#, Testing

SubSpec available on NuGet

May 27, 2011 Leave a comment

SubSpec is finally available as a NuGet package. See http://nuget.org/ on how to get started with NuGet. Once you have NuGet installed, it’s a simple matter of running Install-Package SubSpec or Install-Package SubSpec.Silverlight from the Package Manager console to get SubSpec integrated into your project.

Integrated into your project you said? You mean “get the dll and reference it”? No, in fact, deployment as a separate dll is a thing of the past for SubSpec. SubSpec is an extremely streamlined extension of xUnit and as such it fits into less than 500 lines of C# (excluding xmlDocs). This approach has several advantages:

  1. Faster builds, 500 lines of C# are faster to compile than resolving and linking against a library
  2. It fosters the creation of extensions (which is extremely common, at least in my usage of it)
  3. No need to get the source separately, you already have it!
  4. Experimental extensions can be easily shared as single files too, such as Thesis, AutoFixture integration…

I hope you like the new packages, please feel free to upvote SubSpec and SubSpec.Silverlight on the NuGet gallery and feel encouraged to write a review.

Multiple Test Runner Scenarios in MSBuild

April 15, 2011 Leave a comment

Scenario:

SubSpec is built for .NET as well as for Silverlight. For the .NET test suite, we use the xUnit MSBuild task to execute the tests, for Silverlight we use a combination of Statlight and xunitContrib. Whenever you run a suite of tests, it’s usually desirable to have a failing test break the build, however under all circumstances the complete suite of tests should be run to give you an accurate feedback.

Our build script looks something like this:
SubSpec.msbuild:

    <Target Name="Test" DependsOnTargets="Build">
		<MSBuild
            Projects="SubSpec.tests.msbuild"
            Properties="Configuration=$(Configuration)" />
    </Target>

SubSpec.tests.msbuild:

  SilverlightTests"/>

  <Target Name="xUnitTests">
    <xunit
      Assemblies="@(TestAssemblies)"/>
  </Target>

  <Target Name="SilverlightTests">
    <Exec
      Command=""tools\StatLight\StatLight.exe" @(SilverlightTestXaps -> '-x="%(Identity)"', ' ') --teamcity" />
  </Target>

Problem:

When using each of the build runners (xUnit MSBuildTask, Statlight) in isolation with multiple assemblies, they do the right thing: Run all tests, fail if at least one test failed, succeed otherwise. Now imagine we have a test succeeding under .NET but failing under Silverlight. When we run xUnit first, we get the desired result. But if Statlight was to run before xUnit, we would never know if the .NET suite would actually succeed, because Statlight stops the build.

(Non-)Solutions:

The first (and most intuitive) idea was to move the test targets into a separate MSBuild project and call MSBuild on that project with ContinueOnError=”false”:

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <Target Name="Build">
    <MSBuild
      Projects="test.msbuild"
      Targets="Test"
      ContinueOnError="true"/>
  </Target>

  <Target Name="Test" DependsOnTargets="Foo;Bar">
  </Target>

  <Target Name="Foo">
    <Error Text="Foo"/>
  </Target>

  <Target Name="Bar">
    <Error Text="Bar"/>
  </Target>
</Project>

But this yields only Foo as the error (I wanted to see error: Foo and error: Bar).

MSDN says about ContinueOnError:

Optional attribute. A Boolean attribute that defaults to false if not specified. If ContinueOnError is false and a task fails, the remaining tasks in the Target element are not executed and the entire Target element is considered to have failed.

This is probably why it doesn’t make sense on the MSBuild task, it would only allow another task after the MSBuild task in “Build” to execute. We confirm this by:

  <Target Name="Build">
    <MSBuild
      Projects="test.msbuild"
      Targets="Test"
      ContinueOnError="true"/>
    <Message Text="Some Message"/>
  </Target>
  

And we see Foo as well as Some Message. At this point, it was clear me to me that I want a target that fails if any of its tasks failed, but executes all of them.

In MSDN, we discover StopOnFirstFailure:

true if the task should stop building the remaining projects as soon as any one of them may not work; otherwise, false.

If we specified separate projects, it would work, but we’re in the same project, so unfortunately this won’t help

The next idea was to use CallTarget with ContinueOnError=”true”, like:

  <Target Name="Build">
    <MSBuild
      Projects="test.msbuild"
      Targets="Test"
      ContinueOnError="false"/>
        <Message Text="I should not be executed"/>
  </Target>

  <ItemGroup>
    <TestTargets
        Include="Foo;Bar" />
  </ItemGroup>

  <Target Name="Test">
    <CallTarget Targets="%(TestTargets.Identity)" ContinueOnError="true"/>
  </Target>

  <Target Name="Foo">
    <Error Text="Foo"/>
  </Target>

  <Target Name="Bar">
    <Error Text="Bar"/>
  </Target>
  

However, “I should not be executed” appears in the output log, what happened? Build called MSBuild with ContinueOnError=false (the default). Because all tasks in Test were ContinueOnError=true, no error bubbled up to MSBuild and it executed without error. This is dangerous, because it makes our build appear succeeded when it’s not.

The next option I tried was using RunEachTargetSeparately:

Gets or sets a Boolean value that specifies whether the MSBuild task invokes each target in the list passed to MSBuild one at a time, instead of at the same time. Setting this property to true guarantees that subsequent targets are invoked even if previously invoked targets failed. Otherwise, a build error would stop invocation of all subsequent targets. The default value is false.

  <Target Name="Build">
    <MSBuild
      Projects="test.msbuild"
      Targets="Foo;Bar"
      RunEachTargetSeparately="true"/>
  </Target>

  <Target Name="Test" DependsOnTargets="Foo;Bar">
    <Error Text="Foo"/>
  </Target>

  <Target Name="Foo">
    <Error Text="Foo"/>
  </Target>
  <Target Name="Bar">
    <Error Text="Bar"/>
  </Target>
  

This gives us exactly what we want, but it doesn’t allow test runs to be parallelized. To achieve that, we need to put each test target in a separate project file. It turns out, that using this strategy, we don’t need to worry about controlling our failure strategy: Both projects get build and the MSBuild task reports an error when any of the projects have failed:

  <Target Name="Build">

  </Target>

  <Target Name="Test">
    <MSBuild
      Projects="SubSpec.test.msbuild;SubSpec.Silverlight.test.msbuild"/>
  </Target>
  

Whats the alternative? The Alternative is capturing the ExitCodes of the runners, as described in http://stackoverflow.com/questions/1059230/trapping-error-status-in-msbuild/1059672#1059672, however I don’t like that approach since it’s a bit messy. The only thing we give up by using multiple projects is that it’s harder to get an overview of what happens where, but I think in this case the separation might also aid a proper separation of concerns.

Categories: .NET, MSBuild, Testing, Tools

Running MSTest 9 on a CI Server without installing Visual Studio

April 2, 2011 3 comments

Disclaimer: I would have loved to migrate to a different framework (and I would strongly advice you do so if you’re not a full stack TeamSystem shop), however I have a couple of consultants on that project who are not very test experienced and having built-in MSTest has compelling advantages. Having said that, I know that Gallio has nice VS integration that you can use to run any frameworks’ tests inside Visual Studios Test windows, however that would require each developer to install gallio on their machine (which is bad too).

Without reiterating the tirades of hate Microsoft has earned for making it impossible to run MSTest on a build server without installing Visual Studio, I want to present what I have compiled from several sources to get it working for me:

  1. See this post on Stackoverflow for an overview of the issue and possible solutions
  2. Mark Kharitonov has compiled a basic set of instructions that allow installing MSTest on a Build Server

My setup consists of a Teamcity Build Agent running on Windows Server 2008R2 x64, so I needed to change all registry keys in the reg file to point at HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ instead of HKEY_LOCAL_MACHINE\SOFTWARE\.

Next, I am using Gallio to run the tests instead of executing them directly using MSTest. Even though Gallio is considerably slower than native MSTest, which you can also use with a built-in Teamcity buildstep, there are a couple of advantages:

  1. Pretty Reports
  2. No need to deal with test run configurations and test metadata (I’ve got no idea what they are and why I would need them)
  3. Teamcity picks up the test resulty properly
  4. I can use a MSBuild script to pick up my Test dlls via wildcards, no need to have extra MSTest build tasks.

As a reference, here’s my MSBuild script for running the tests using Gallio:

<Project DefaultTargets="Test" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    
	<!-- This is needed by MSBuild to locate the Gallio task -->
    <UsingTask AssemblyFile="tools\Gallio\Gallio.MSBuildTasks.dll" TaskName="Gallio" />
    
	<!-- Specify the tests assemblies -->
    <ItemGroup>
        <TestAssemblies Include="src\test\**\bin\$(Configuration)\*Tests.dll" />
	</ItemGroup>
    
	<Target Name="Test">
        <Gallio 
            Files="@(TestAssemblies)"
            IgnoreFailures="true" 
            ReportDirectory="build\"
            ReportTypes="html"> 
            <!-- This tells MSBuild to store the output value of the task's ExitCode property
                 into the project's ExitCode property -->
            <Output TaskParameter="ExitCode" PropertyName="ExitCode"/>
        </Gallio>
		<Error Text="Tests execution failed" Condition="'$(ExitCode)' != 0" />
	</Target>
	
</Project>
Categories: .NET, Testing, Tools

Contrasting .NET and Java – Finalizers

February 11, 2011 Leave a comment

One of the interesting differences between .NET and Java I ran into while reading Joshua Bloch’s excellent “Effective Java” book is the handling of finalizers.

In both, .NET and Java, the Garbage Collector will implicitly place objects implementing a finalizer and are eligible for collection on a finalizer queue. This queue is processed by a separate finalizer thread, and no guarantees are made as to when an object may be finalized or if it will be finalized at all. The reason finalization cannot be guaranteed is that the process may crash, finalization throws an exception or similar mishappenings.

Due to the nature of the finalization, it comes with significant overhead. To avoid this overhead, .NET provides the IDisposable pattern. The Dispose method takes care of releasing all allocated resources and then instructs the Garbage Collector with a call to GC.SupressFinalizer(this) that it need not be finalized anymore.
Clients of objects implementing IDisposable are responsible (though not forced to) call IDisposable.Dispose whenever they are done with an instance. To support this scenarios, many .NET languages feature a using keyword, which is syntactic sugar for providing a scope for disposable objects. Whenever this scope is left, the appropriate Dispose method is called.

Java on the other does not provide a general purpose equivalent to IDisposable, for IO related the Closeable interface exists. As of Java 6 there is no equivalent to the using keyword, although “Automatic Resource Block Management” using the try keyword  has been announced for Java 7. Until then, you’re left with manually implementing scoping with a try/finally block. Java also doesn’t provide an equivalent of GC.SupressFinalize which means that finalizable objects will always have significant performance impacts.

A further difference is in the way base class finalization is handled. Although the .NET CLI Spec does not enforce base class finalizers are called from a derived class’ finalizer, C# and C++/CLI enforce this with their destructor syntax. In Java, it is up to the implementor not to forget calling the base class finalizer.

All these aspects, combined with the significantly better support for code issue warnings around undisposed Disposables in the .NET stack, make .NET’s handling look superior to Java’s.

Categories: .NET, .NET vs. Java, Java Tags:

Check if a PDB matches a Dll

December 27, 2010 1 comment

A common issue when modifying .NET assemblies by using IL Round-Trip compiling or a library like Mono.Cecil is preserving Debug information across modifications. You will need to take big care not to lose your PDBs along the way.

Hence it would be handy to have a tool to check if your assembly and its PDB still line up. This is exactly where ChkMatch comes in, a very handy utility I found via this SO question.

Categories: .NET, Tools

Building Mono.Cecil

September 7, 2010 Leave a comment

Building for .NET 3.5

If you want to use Cecil in a 3.5 project, you need to define the NET_3_5 symbol and change the target framework to 3.5 in Mono.Cecil.csproj.

Running the test suite:

Cecil will build fine after checkout from [http://github.com/jbevain/cecil|jbevain’s github repo], the test suite will not however.

The following steps are necessary to sucessfully build and run the cecil test suite:

  1. Add the Framework SDK (PEVerify, ILDasm) and the Framework install directory (ILAsm)to your PATH variable (we need the 4.0 tool set because the tests run over a few 4.0 assemblies): C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools;C:\Windows\Microsoft.NET\Framework\v4.0.30319
  2. Install NUnit 2.4.8 (its a little outdated but Mono compatible): http://www.nunit.org/index.php?p=download
  3. The tests can’t be run using Ad-hoc TD.Net but must be run using the NUnit runner. There’s a NUnit GUI project in the projects root.
Categories: .NET, Open Source

SubSpec: Assert and Observation

August 24, 2010 Leave a comment

When writing a test, we should make sure only to have one Assertion per test. The reasoning behind this constraint is simple. If we used multiple assertions and our first one fails, we are not able to retrieve the results from the other ones.



In this example, if the assertion on stack.IsEmpty() fails, we are unable to retrieve the results of the next two Assertions. We can see that our test consists of three parts:

  1. Arrange the System Under Test (SUT)
  2. Act on SUT
  3. Assert the SUT’s state has changed accordingly.

If we want to have one Assertions per test, we need to write three tests, duplicating the Arrange and Act for each test. As always, repetition is suboptimal, so let’s see what we can do about it.

SubSpecs’ core idea is that each test (we call them Specification) that you write consists of the above mentioned primitives.  Each primitive can be represented by an action and a corresponding description. Using fluent syntax, a SubSpec Specification for the above mentioned Scenario looks like this:

Each of the primitive test actions is represented by a description and a lambda statement.  The big difference to a traditional test is that SubSpec knows about these primitive actions and can compose them to generate three Tests from the above Specification, one for each Assertion. What it does under the hood is pretty much what you’d expect it to do: SubSpec repeats the Context and Do action for each Assertion and wraps it inside a single test. That’s the power of declarative tests!

This is one of the features SubSpec has supported since it’s beginning. But there’s one thing we can improve about the above example. We have got three Assertions in our above test, but only one of them is destructive. You guessed correct, it is the second one. By popping an element from the stack, it modifies the system under test. This is a more general problem. Although we should try to avoid this situation, sensing something in our  SUT cannot always be made side-effect free. (Anyone feels reminded of quantum physics? 😀 )

The first and third Assertion on the other hand are side effect free. If the Context and Do Action were possibly expensive (such as when involving an external resource), repeating them for each of our Isolated Assertions would be a waste of time. But tests need to be as fast as possible. What can we do about it?

Given the distinction between a destructive Assertion and a side effect-free Observation we can check against our SUT, we should split our Assert primitive accordingly. An Assertion is a destructive operation on our SUT, which therefore needs to be recreated for each Assertion we check. For an Observation on the other hand, the SUT can be shared. Let’s get back to our exmaple:

The Context and Do action are executed once for each Assertion (once in this case) and once for all Observations. Given the declarative nature of SubSpec, we can easily mix and match Observations and Assertions in one Specification and still get a single test for each. Pretty cool, isn’t it?

The distinction between Assert (verb) and Observation (noun) is intentional to highlight the difference between those two concepts.

Categories: .NET, SubSpec, Testing

SubSpec: A declarative test framework for Developers

August 23, 2010 Leave a comment

In my last post I described Acceptance Testing and why it is an important addition to the developer-centric way of integration and unit testing.

I also described that Acceptance Tests should  be as expressive as possible and therefore benefit from being written in a declarative style. From learning F# at the moment, I came to the conclusion that writing declarative code is the key to avoid accidental complexity (complexity in your solution domain that is not warranted by complexity in your problem domain). But not only acceptance tests benefit from a declarative style, I do also think that it helps a long way to make unit and integration tests easier to understand.

SubSpec has originally been written by Brad Wilson and Phil Haack. It was their motivation to write a framework that enables xUnit based BDD-Style testing. Given my desire to support a declarative approach for writing tests at all layers, I decided to fork the project and see what can be accomplished. I’m actively working on it and the code can be found on my bitbucket site. I like the idea of having a vision statement, so here is mine:

SubSpec allows developers to write declarative tests operating at all layers of abstraction. SubSpec consists of a small set of primitive concepts that are highly composable. Based on the powerful xUnit testing framework, SubSpec is easy to integrate with existing testing environments.

Here’s a short teaser to show you how expressive a SubSpec test is:

Mapping SelectMany to Monads

August 21, 2010 1 comment

This is just a quick post to write down one of my findings while I’m currently doing a bit of F# hacking.

The bind operation  is the backbone of Monads. For the collection Monad (M<T> where M is IEnumerable) this is Enumerable.SelectMany for C# and Seq.concat or Seq.collect respectively. The last detail is important because in C# we have two overloads for these different operations (there are actually two more passing the index of the current element but I’ll ignore that for now).

In general, a bind operation looks like this (F# syntax):

('T -> M<'U>) -> M<'T> -> M<'U>

The bin operation takes a function that maps a value of type T into the monadic type M<U>. As a second argument it is passed a monadic type M<T> and it returns a monadic type M<U>. Given this signature, we can easily infer what the operation does: The value of type T is un-wrapped from its monadic type M<T>. The unwrapped value is passed as an argument to the function specified as a first argument of the bind operation. The result of calling the specified function with the value of type T is the monadic type M wrapping a value of type U, which is also the return value of the bind operation.

The SelectMany function looks accordingly (think M=IEnumerable):

IEnumerable<TResult> SelectMany<TSource, TResult>(IEnumerable<TSource> source , Func<TSource, IEnumerable<TResult>> selector)

Ignoring that the order of parameters is different, we can see that the SelectMany function corresponds to the bind operation. It is noteworthy however, that different semantics for this operation are possible given the same signature. Un-wrapping the value of type T from our IEnumerable monad corresponds to enumerating over the sequence. Therefore we invoke the selector on each element and it returns our value mapped to TResult wrapped inside an IEnumerable monad (M<U>). Now, the problem is that, in order to statisfy the method signature it would be possible for the SelectMany implementation to simply return the result of the first invocation of the selector. Nonetheless, this is not what we expect the bind operation for IEnumerables to do, we expect it to flatten the returned sequence. So we have a bunch of IEnumerbale<U> that we need to bring into a single IEnumerable<U>. To do this, we simply concat all the sequences together.

So there are two distinct steps happening here: Mapping each value T into a new M<U> and then perform a selection on all returned M<U>’s to return a single M<U>.

This is why there’s also a second overload of SelectMany:

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector,
 Func<TSource, TCollection, TResult> resultSelector)

This second overload allows you to customize the last step of the operation. From a theoretical point of view, this second overload is not necessary but it certainly makes things easier in C#.

Consider the following example, we have two lists of numbers from 1 to 3 and want to select pairs from it where the numbers match (using F# with LINQ):

open System.Linq

let numbers = [1..3];

numbers.SelectMany(fun n -> 
    numbers.SelectMany( fun m -> 
    Seq.ofList(
        if (n = m) then [(m, n)] // sequence with a single tuple (m,n)
        else []  // empty sequence
 )))

// Prints (F# Interactive):
// val it : seq<int * int> = seq [(1, 1); (2, 2); (3, 3)]

A similar implementation in C# would look like this:

var numbers = Enumerable.Range(1, 3);
			
var q =
	numbers.SelectMany(n =>
		numbers.SelectMany(m =>
		{
			if (n == m) return new { n, m }.Return();
			else return new { n, m }.Ignore();
		}));

In order to get the same expressiveness as in F# we needed to add to extension methods. Return is the second important operation on monads, it simply takes a value and wraps it inside the monad. In this case it returns an IEnumerable with a single element. Returning an empty sequence is not so easy because we do not have the same type inference capabilities as in F#. The Ignore operation returns an empty sequence (or empty monad) and is only invoked on an instance of the anonymous type so we know the type of sequence to return.

Doing it this way is a pretty cumbersome, so when the C# team decided to implement LINQ they used a different approach to translate a query as our example above. Section 7.15.2. of the C# Spec specifies this.

var numbers = Enumerable.Range(1, 3);
			
var q =
	from n in numbers
        from m in numbers 
        where n==m
        select new {n, m};

This translates to using the second overload of SelectMany to produce a cross join of both sequences and then filtering the result:

var numbers = Enumerable.Range(1, 3);
			
var q5 = numbers
        .SelectMany(n => numbers, (n, m) => new { n, m })
	.Where(x => x.n == x.m);

I might not be 100% correct on this, but I think this the only reason the second SelectMany overload exists is to compensate for the missing type inference and make query expression translation easier. From the monad point of view, it’s not necessary, I believe.

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