Type inference issue with applicative2 about fsharpplus HOT 1 CLOSED

@anpin Sorry for the late reply and thanks for taking the time for reporting this.

It seems to me you have an issue understanding the Validation mechanics: the 'Error type has to be something like a semigroup that is a type which supports a kind of "self folding" operation, in F#+ that operator should be static member (+).

So, to fix the issue you can define your Errors type as Error and in turn make Errors a list of Error:

#r "nuget: FSharpPlus"

open FSharpPlus
open FSharpPlus.Data
open System.Threading.Tasks

[<Struct>]
type Error =
  | UnexpectedError of string

type Errors = Error list

[<Struct>]
type dto = {
  A : string
  B : string
}
[<Struct>]
type TypedTuple<'a,'b> = TypedTuple of struct('a option *  'b option )

let validateA (TypedTuple (did, dto): TypedTuple<string,dto>)   =
  match did with
  | Some _ -> Validation<Errors, _>.Success dto
  | None -> Validation<Errors, _>.Failure [Error.UnexpectedError "ooops"]
  |> Task.FromResult

Another option could be defining internally your original Error as a list and add the (+) static member.

When you use a string it just works because string is a semigroup and it supports the + operation, although the error accumulation will be a bit awkward as it will concatenate all error messages.

from fsharpplus.