Use a (named) type

Author: gusty

src/FSharpPlus/Extensions/Dict.fs

@@ -1,5 +1,82 @@
 namespace FSharpPlus
 
+[<AutoOpen>]
+module Auto =
+    let icollection value =
+        {
+            new  ICollection<'t> with
+                member _.Contains (item: 't) = obj.ReferenceEquals (item, value)
+                member _.GetEnumerator () = (Seq.initInfinite (fun _ -> value)).GetEnumerator () :> System.Collections.IEnumerator
+                member _.GetEnumerator () = (Seq.initInfinite (fun _ -> value)).GetEnumerator () : IEnumerator<'t>
+                member _.IsReadOnly = true
+
+                member _.Add (_item: 't) : unit                          = raise (System.NotImplementedException())
+                member _.Clear () : unit                                 = raise (System.NotImplementedException())
+                member _.CopyTo (_array: 't [], _arrayIndex: int) : unit = raise (System.NotImplementedException())
+                member _.Count : int                                     = raise (System.NotImplementedException())
+                member _.Remove (_item: 't): bool                        = raise (System.NotImplementedException())
+        }
+
+type DefaultableDict<'TKey, 'TValue>(source: 'TValue) =
+    interface IDictionary<'TKey, 'TValue> with
+        member _.TryGetValue (_key: 'TKey, value: byref<'TValue>) = value <- source; true
+        member _.Count =
+            if typeof<'TKey>.IsValueType then                
+                if typeof<'TKey> = typeof<bool> then 2
+                else
+                    let s = sizeof<'TKey>
+                    if s < 4 then pown 2 (sizeof<'TKey> * 8)
+                    else -1
+            elif typeof<'TKey> = typeof<unit> then 1
+            else -1
+        member _.ContainsKey (_key: 'TKey) = true
+        member _.Contains (item: KeyValuePair<'TKey,'TValue>) = obj.ReferenceEquals (item.Value, source)
+        member _.GetEnumerator () = invalidOp "Key set is potentially infinite." : System.Collections.IEnumerator
+        member _.GetEnumerator () = invalidOp "Key set is potentially infinite." : IEnumerator<KeyValuePair<'TKey,'TValue>>
+        member _.IsReadOnly = true
+        member _.Values = icollection source
+        member _.Item
+            with get (_key: 'TKey)   : 'TValue = source
+            and set  (_key: 'TKey) (_: 'TValue) : unit = raise (System.NotImplementedException())
+
+        member _.Add (_key: 'TKey, _value: 'TValue) : unit                              = raise (System.NotImplementedException())
+        member _.Add (_item: KeyValuePair<'TKey,'TValue>) : unit                        = raise (System.NotImplementedException())
+        member _.Clear () : unit                                                        = raise (System.NotImplementedException())
+        member _.CopyTo (_arr: KeyValuePair<'TKey,'TValue> [], _arrayIndex: int) : unit = raise (System.NotImplementedException())
+        member _.Keys : ICollection<'TKey>                                              = raise (System.NotImplementedException())
+        member _.Remove (_key: 'TKey) : bool                                            = raise (System.NotImplementedException())
+        member _.Remove (_item: KeyValuePair<'TKey,'TValue>) : bool                     = raise (System.NotImplementedException())
+(*
+    interface IReadOnlyDictionary<'TKey, 'TValue> with
+        member _.TryGetValue (_key: 'TKey, value: byref<'TValue>) = value <- source; true
+        member _.Count =
+            if typeof<'TKey>.IsValueType then                
+                if typeof<'TKey> = typeof<bool> then 2
+                else
+                    let s = sizeof<'TKey>
+                    if s < 4 then pown 2 (sizeof<'TKey> * 8)
+                    else -1
+            elif typeof<'TKey> = typeof<unit> then 1
+            else -1
+        member _.ContainsKey (_key: 'TKey) = true
+        // member _.Contains (item: KeyValuePair<'TKey,'TValue>) = obj.ReferenceEquals (item.Value, source)
+        member _.GetEnumerator () = invalidOp "Key set is potentially infinite." : System.Collections.IEnumerator
+        member _.GetEnumerator () = invalidOp "Key set is potentially infinite." : IEnumerator<KeyValuePair<'TKey,'TValue>>
+        // member _.IsReadOnly = true
+        member _.Values = icollection source
+        member _.Item
+            with get (_key: 'TKey)   : 'TValue = source
+            // and set  (_key: 'TKey) (_: 'TValue) : unit = raise (System.NotImplementedException())
+
+        // member _.Add (_key: 'TKey, _value: 'TValue) : unit                              = raise (System.NotImplementedException())
+        // member _.Add (_item: KeyValuePair<'TKey,'TValue>) : unit                        = raise (System.NotImplementedException())
+        // member _.Clear () : unit                                                        = raise (System.NotImplementedException())
+        // member _.CopyTo (_arr: KeyValuePair<'TKey,'TValue> [], _arrayIndex: int) : unit = raise (System.NotImplementedException())
+        member _.Keys : IEnumerable<'TKey>                                              = raise (System.NotImplementedException())
+        member _.Remove (_key: 'TKey) : bool                                            = raise (System.NotImplementedException())
+        member _.Remove (_item: KeyValuePair<'TKey,'TValue>) : bool                     = raise (System.NotImplementedException())
+        *)
+
 /// Additional operations on IDictionary<'Key, 'Value>
 [<RequireQualifiedAccess>]
 module Dict =