F# Analyzer to Restrict language

RestrictionAnalyzer.fs

module RestrictionAnalyzer

open FSharp.Analyzers.SDK
open FSharp.Compiler.Text
open FSharp.Compiler.Syntax

let hasTypeInNamespace (target:SynModuleOrNamespace): Message option =
  match target with
  | SynModuleOrNamespace(kind = SynModuleOrNamespaceKind.DeclaredNamespace; decls = decls)  ->
    let hasTypes =
      decls
      |> List.exists (function
        | SynModuleDecl.Types _ -> true
        | _ -> false)
    if hasTypes then
      {
        Type = "Restriction analyzer"
        Message = "Types not allowed"
        Severity = Severity.Error
        Code = "TR001"
        Range = Range.Zero
        Fixes = []
      } |> Some
    else
      None
  | _ -> None

[<Analyzer "RestrictionAnalyzer">]
let badCodeAnalyzer : Analyzer =
  fun (context: Context) ->
    match context.ParseTree with
    | ParsedInput.ImplFile (ParsedImplFileInput(contents=contents)) ->
      contents
      |> List.choose hasTypeInNamespace
    | _ -> [ ]

TestCase.fs

namespace TestCases


type ShouldFail = ShouldFail