Support [<CallerArgumentExpression>]

[Happypig375]

Champion "CallerArgumentExpression" is currently in the C# team's working set.

I propose we support this attribute, which has been present since .NET Core 3.0. We can match this attribute by namespace so that it can be defined even when outside .NET Core.

The motivation, pros, and cons can be seen in the C# proposal. To summarize, this allows developers to capture the expressions passed to a method, to enable better error messages in diagnostic/testing APIs and reduce keystrokes.

Extra information

Estimated cost (XS, S, M, L, XL, XXL): M

Related suggestions: #463 - F# compiler should support CallerLineNumber, CallerFilePath etc (Completed for F# 4.1)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

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[dsyme]

Note F# does let you use ReflectedDefinition attribute on method parameters, e.g.

type B() =
    static member M([<ReflectedDefinition>] x: Quotations.Expr<int>) = printfn "x = %A" x

B.M(1+1)

gives

x = Call (None, op_Addition, [Value (1), Value (1)])

You can also add "WithValue" which both evaluates the expression and gives its quotation:

type B() =
    static member M([<ReflectedDefinition(includeValue=true)>] x: Quotations.Expr<int>) = printfn "x = %A" x

B.M(1+1)

gives

x = WithValue (2, Call (None, op_Addition, [Value (1), Value (1)]))

For example you can get both a closure and the quotation of the closure.

[Happypig375]

But that is not searchable in the codebase and is too verbose for debugging purposes.

[dsyme]

But that is not searchable in the codebase and is too verbose for debugging purposes.

Yes, capturing the string has different properties

[Happypig375]

This will be in C# vNext: dotnet/roslyn#51952

[abelbraaksma]

It has since gotten into C# 10. Here's some docs and use cases.

PS: Championing issue is still open only because ECMA Spec isn't written yet.

[dsyme]

Marked as approved-in-principle

[ijklam]

How can we implement the feature? At present, the compiler does not store the source code during compilation (or maybe I didn't find out), so it cannot obtain the original source text of the argument...

So there might be two way to make it.

One is to read the source text through the FileSystem, by this way we can gain the original texts, but may be unusable in the fsi (since the filename is default to "stdin").

The other is to convert the expressions of the argument into code text.