[<DefaultParameterValue>] is ignored in F#-authored code

[KevinRansom]

Originally opened at CodePlex latkin

Reported by Tomas P via fsbugs.

The F# compiler ignores the [<DefaultParameterValue>] attribute. For example, the following always prints null when called by C# code (calling from F# without specifying message is not supported):

open System
open System.Runtime.InteropServices
type C = 
  static member Foo([<Optional; DefaultParameterValue("Hello world")>] message) =
         printfn "%s" message

Analysis from Don:

This is correct, in F# 3.0 and F# 3.1 DefaultParameterValue is ignored for F#-authored code, both by F# consumers and .NET consumers of F# code.

The relevant code is infos.fs GetParamAttribs, where we look for OptionalArgumentAttribute and not DefaultParameterValueAttribute:

        | FSMeth(g,_,vref,_) -> 
            vref 
            |> ArgInfosOfMember g 
            |> List.mapSquared (fun (ty,argInfo) -> 
                let isParamArrayArg = HasFSharpAttribute g g.attrib_ParamArrayAttribute argInfo.Attribs
                let isOutArg = HasFSharpAttribute g g.attrib_OutAttribute argInfo.Attribs && isByrefTy g ty
                let isOptArg = HasFSharpAttribute g g.attrib_OptionalArgumentAttribute argInfo.Attribs
                // Note: can't specify caller-side default arguments in F#, by design (default is specified on the callee-side) 
                let optArgInfo = if isOptArg then CalleeSide else NotOptional
                (isParamArrayArg,isOutArg,optArgInfo))

Likewise in ilxgen.fs, we never generate a “Default” value in the Abstract IL metadata for an F# assembly. There is even a “TODO” marker there...

       let param = 
            { Name=nmOpt;
              Type= ilArgTy;  
              Default=None; (* REVIEW: support "default" attributes *)   
              Marshal=Marshal; 
              IsIn=inFlag;    
              IsOut=outFlag;  
              IsOptional=optionalFlag; 
              CustomAttrs= mkILCustomAttrs (GenAttrs cenv eenv attribs) }

We should fix this.

[latkin]

This definitely feels like a bug but changing the behavior would seem to be a pretty serious breaking change, no? Omitted params are uninitialized in one version, given default values in the next.

[dsyme]

Whatever we call it, we really should just fix this and make it do the right thing by default.

But it is possible we need a "compat" flag to allow the previous behaviour.

[dsyme]

Hi @latkin - I've been looking at this again - fixing this would only affect F# code that specifies the DefaultParameterValue("Hello world")>] attribute on parameters - it would cause the correct C#-compatible IL to be emitted for that case.

i don't think anyone is using that attribute on F# parameters today since it doesn't do anything (and it's quite hard to find it!)

So I think fixing this makes sense.

[latkin]

I agree it's a narrow slice of users would would be affected. In principle I'm against it since the behavior is dramatically different, but practically speaking we could probably get away with it.

In our changelist file maybe we should keep track of stuff like this that is technically breaking, but deemed to be acceptable.

[dsyme]

OK. I suppose my estimate is that there would be no F# code using DefaultParameterValue today. And any code where it was being used would expect the fixed behaviour. So yes, acceptable "breaking change in theory".