While building the demo application from last weeks On .NET Live stream I was needing to do some writing of data to CosmosDB and figured I’d use the output bindings. With the docs only containing C# examples (at least, at the time of writing), I thought I’d use this post to show how to do it in F#.
out
arguments
One way which we can use an output binding is to have an out
argument, essentially somewhere that you’re passing a reference to a variable that the host will send to CosmosDB. Since out
is a C# keyword we need to use the F# equivalent, which is outref<T>
:
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In this example, we have the outref<ToDo>
as the second argument of our Function and we use the <-
operator to do assignment to the mutable value (outref
is a mutable reference, similar to let mutable
makes a mutable binding).
Dealing with async
Here’s a slightly more challenging problem, if you’re doing something that’s requiring an asynchronous process to happen, like reading the request body, and then writing to the output binding, we can’t use outref<T>
, as the way async operations work (whether it’s Task
or Async
based) means that you can’t capture an outref
parameter (nor in C# can you use an out
parameter in an async
function).
This is what the IAsyncCollector<T>
is for, it gives us an interface which we can push output to from within an async operation.
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In this example, we’re reading the req.Body
stream and then creating a new record that is passed to the IAsyncCollector.AddAsync
, and since it returns Task
, not Task<T>
, we need to ignore the result.
Lastly, we convert the async
block to Task<T>
using Async.AwaitTask
, since the Functions host requires Task<T>
to be returned. You could optimise this code using Ply or TaskBuilder.fs, but I kept it simple for this example.
Conclusion
This post shows how we can use the CosmosDB output bindings for Azure Functions from F# in the two most common scenarios, outputting a single item directly or outputting an item as part of an async operation.
You’ll find a much more complete example in the demo app I built.