Better semantics for MemberActivity (#1)

Co-authored-by: Smaug123 <patrick+github@patrickstevens.co.uk>
Reviewed-on: #1

PureGym.App/Authenticate.fs

@@ -5,24 +5,45 @@ open System
 open PureGym
 
 type GetTokenArg =
-    | [<ExactlyOnce>] User_Email of string
-    | [<ExactlyOnce>] Pin of string
+    | [<Unique ; EqualsAssignmentOrSpaced>] User_Email of string
+    | [<Unique ; EqualsAssignmentOrSpaced>] Pin of string
+    | [<Unique>] StdIn
 
     interface IArgParserTemplate with
         member s.Usage =
             match s with
             | GetTokenArg.Pin _ -> "Eight-digit PureGym user's PIN"
             | GetTokenArg.User_Email _ -> "PureGym user's email address"
+            | GetTokenArg.StdIn _ -> "Read anything not specified on the command line from stdin"
 
     static member Parse (args : ParseResults<GetTokenArg>) : Result<UsernamePin, ArguParseException> =
-        try
-            {
-                Username = args.GetResult GetTokenArg.User_Email
-                Pin = args.GetResult GetTokenArg.Pin
-            }
-            |> Ok
-        with :? ArguParseException as e ->
-            Error e
+        let canUseStdin = args.TryGetResult(GetTokenArg.StdIn).IsSome
+
+        let username =
+            match args.TryGetResult GetTokenArg.User_Email, canUseStdin with
+            | None, true ->
+                Console.Error.Write "Enter username: "
+                Console.ReadLine ()
+            | None, false ->
+                // TODO: proper exception handling, this should be surfaced through the Error
+                failwith "You must supply --user-email or --stdin"
+            | Some s, _ -> s
+
+        let pin =
+            match args.TryGetResult GetTokenArg.Pin, canUseStdin with
+            | None, true ->
+                Console.Error.Write "Enter eight-digit PIN: "
+                Console.ReadLine ()
+            | None, false ->
+                // TODO: proper exception handling, this should be surfaced through the Error
+                failwith "You must supply --pin or --stdin"
+            | Some s, _ -> s
+
+        {
+            Username = username
+            Pin = pin
+        }
+        |> Ok
 
 [<RequireQualifiedAccess>]
 module Authenticate =