Is there a way in F# to type-test against a generic type without specifying the instance type?

Question

I'm trying to pattern match against a few types that I care about for SQL generation. Ideally I'd like to do this:

let rec getSafeValue record (prop: PropertyInfo) = 
    match prop.GetValue(record, null) with
    | :? string as str -> "'" + str + "'"
    | :? Option<_> as opt -> 
        match opt with
        | Some v -> getSafeValue v prop
        | None -> "null"
    | _ as v -> v.ToString()

The problem is that here, the type parameter to Option<_> gets constrained to match that of record, which ends up being just obj.

I know I can do some pain-in-the-behind reflection-based check (check that it's a generic type and that it's an option type based on the name), but I'd rather avoid that if at all possible.

Answer 1

No, there's no good way to do this using F#'s built-in constructs. However, you could build your own reusable active pattern for this sort of thing:

open Microsoft.FSharp.Reflection
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.DerivedPatterns
open Microsoft.FSharp.Quotations.Patterns

let (|UC|_|) e o =
  match e with
  | Lambdas(_,NewUnionCase(uc,_)) | NewUnionCase(uc,[]) ->
      if (box o = null) then
        // Need special case logic in case null is a valid value (e.g. Option.None)
        let attrs = uc.DeclaringType.GetCustomAttributes(typeof<CompilationRepresentationAttribute>, false)
        if attrs.Length = 1
           && (attrs.[0] :?> CompilationRepresentationAttribute).Flags &&& CompilationRepresentationFlags.UseNullAsTrueValue <> enum 0
           && uc.GetFields().Length = 0
        then Some []
        else None
      else 
        let t = o.GetType()
        if FSharpType.IsUnion t then
          let uc2, fields = FSharpValue.GetUnionFields(o,t)
          let getGenType (t:System.Type) = if t.IsGenericType then t.GetGenericTypeDefinition() else t
          if uc2.Tag = uc.Tag && getGenType (uc2.DeclaringType) = getGenType (uc.DeclaringType) then
            Some(fields |> List.ofArray)
          else None
        else None
  | _ -> failwith "The UC pattern can only be used against simple union cases"

Now your function might look something like this:

let rec getSafeValue (item:obj) = 
    match item with
    | :? string as str -> "'" + str + "'"
    | UC <@ Some @> [v] -> getSafeValue v
    | UC <@ None @> [] -> "null"
    | _ as v -> v.ToString()