Copy and update inside a folder function with an anonymous record prints a misleading error message

[giorgiovilardo]

When using an anonymous record as the initial state of a fold, I noticed that I can't use copy and update expressions in the folder function. See this screenshot:

After a bit of troubleshooting on the FSSF Slack server, we noticed that the anonymous record has to be constructed "fully" once, and then the copy and update syntax is available.

Code snippet:

type Command =
    | Add of int
    | Sub of int

let anonymousRecordFoldMatchBug x =
    let commandList = [Add 3; Sub 1; Add 10]
    commandList
    |> List.fold (fun state value ->
        match value with
        | Add x -> {| Result = state.Result + x |}
        | Sub x -> {| state with Result = state.Result + x |})
        {| Result=0 |}

Expected behavior

Being able to use the copy and update syntax from the start, or at least a more explicative error message that points out the steps needed to fix the problem.

[T-Gro]

I think the type checker should not look in advance. So in this code construct, it is not yet knowing about the anonymous record supplied as the last argument.

If you rewrote the same thing using ||>like this, it works:

type Command =
    | Add of int
    | Sub of int

let anonymousRecordFoldMatchBug x =
    let commandList = [Add 3; Sub 1; Add 10]
    ({| Result=0 |},commandList)
    ||> List.fold (fun state value ->
        match value with
        | Add x -> {| state with Result = state.Result + x |}
        | Sub x -> {| state with Result = state.Result + x |})

[T-Gro]

What can be improved is the error message in this situation - to refocus it on the fact that "state" is not bound to any particular type (yet), and a type annotation might be needed.

[T-Gro]

The type annotated version would then look like this, alternative to the ||> syntax mentioned above: