Auto-generated static field in F# class/module prevents creation of SQL CLR function - sql-server

Question

I'm trying to write a CLR user-defined function in F#, but CREATE ASSEMBLY gives the error:
CREATE ASSEMBLY failed because type 'StringMetrics' in safe assembly 'MyNamespace.SqlServer.Text' has a static field 'field1776#'. Attributes of static fields in safe assemblies must be marked readonly in Visual C#, ReadOnly in Visual Basic, or initonly in Visual C++ and intermediate language.
Here's how it looks in Reflector. This is not a field I've explicitly created.
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
internal static <PrivateImplementationDetails$MyNamespace-SqlServer-Text>.T1775_18Bytes# field1776#; // data size: 18 bytes
I've tried using a module and a class. Both generate the field, just in different places. What is this field for? Is there a way to avoid its creation? Is there another approach I should be using to create a CLR function in F#? Is it even possible?
Complete Code
namespace MyNamespace.SqlServer.Text
module StringMetrics =
open System
open System.Collections.Generic
open System.Data
open System.Data.SqlTypes
[<CompiledName("FuzzyMatch")>]
let fuzzyMatch (strA:SqlString) (strB:SqlString) =
if strA.IsNull || strB.IsNull then SqlInt32.Zero
else
let comparer = StringComparer.OrdinalIgnoreCase
let wordBoundaries = [|' '; '\t'; '\n'; '\r'; ','; ':'; ';'; '('; ')'|]
let stringEquals a b = comparer.Equals(a, b)
let isSubstring (search:string) (find:string) = find.Length >= search.Length / 2 && search.IndexOf(find, StringComparison.OrdinalIgnoreCase) >= 0
let split (str:string) = str.Split(wordBoundaries)
let score (strA:string) (strB:string) =
if stringEquals strA strB then strA.Length * 3
else
let lenA, lenB = strA.Length, strB.Length
if strA |> isSubstring strB then lenA * 2
elif strB |> isSubstring strA then lenB * 2
else 0
let arrA, arrB = split strA.Value, split strB.Value
let dictA, dictB = Dictionary(), Dictionary()
arrA |> Seq.iteri (fun i a ->
arrB |> Seq.iteri (fun j b ->
match score a b with
| 0 -> ()
| s ->
match dictB.TryGetValue(j) with
| true, (s', i') -> //'
if s > s' then //'
dictA.Add(i, j)
dictB.[j] <- (s, i)
| _ ->
dictA.Add(i, j)
dictB.Add(j, (s, i))))
let matchScore = dictB |> Seq.sumBy (function (KeyValue(_, (s, _))) -> s)
let nonMatchA =
arrA
|> Seq.mapi (fun i a -> i, a)
|> Seq.fold (fun s (i, a) ->
if dictA.ContainsKey(i) then s
else s + a.Length) 0
let wordsB = HashSet(seq { for (KeyValue(i, _)) in dictB -> arrB.[i] }, comparer)
let nonMatchB =
arrB |> Seq.fold (fun s b ->
if wordsB.Add(b) then s + b.Length
else s) 0
SqlInt32(matchScore - nonMatchA - nonMatchB)

Answer 1

It seems it's generated by the wordBoundaries array. If you express it as a list instead, and convert it to an array at runtime, this internal static field is not generated:
let wordBoundaries = Array.ofList [' '; '\t'; '\n'; '\r'; ','; ':'; ';'; '('; ')']
However, it seems that also the function itself is represented as a static, non-readonly
field:
public static SqlInt32 fuzzyMatch(SqlString strA, SqlString strB);
Maybe using a class instead of a module cures this.