Watch window not working for F#

Question

I had noticed some time ago that the "Watch" window in VS2012 for Web doesn't work for default functions in FSharp. For example, cos someValue doesn't work, neither does the workaround where let _cos = cos or let _cos x = cos x is inserted in the beginning of the function and _cos(someValue) is used. The error is something like "cos doesn't exist in the current context" or "_cos isn't valid in the current scope", among others.

Should I change some settings or is this an unexpected bug? Of course I can declare all the results I need to watch, but that's a bit of overhead and it is quite impractical. What can I do to fix this?

Answer 1

As mentioned in the referneced answer , the watches and immediate windows only support C#, so they are not able to evaluate F# expressions and they are not aware of the F# context (such as opened namespaces).

In summary storing the result in a local variable (which is compiled to an ordinary local variable) is the best way to see the result.

More details: In some cases, you can write C# code that corresponds to what you want to do in F#. This is probably only worth for simple situations, when the corresponding C# is not too hard to write, but it can often be done. For example to call cos 3.14 , you need to write something like:

Microsoft.FSharp.Core.Operators.Cos(3.14)

If you find the cos function in the F# source code (it righ here, in prim-types.fsi ), then you can see that it comes with CompiledName attribute that tells the compiler to compile it as a method named Cos (to follow .NET naming guidelines). It is defined in module named Operators (see it here ), which is annotated with AutoOpen so you do not need to explicitly write open in the F# code, but it is actually the name of the class that the F# compiler generates when compiling the code.