extend .GetTag method to also accept expr constructors

Author: eiriktsarpalis

RELEASE_NOTES.md

@@ -1,4 +1,4 @@
-### 3.0.0-alpha002
+### 3.0.0-alpha003
 * Add subcommand support.
 * Add support for list and option parameters.
 * Add support for grouped switches.

src/Argu/ArgumentParser.fs

@@ -154,6 +154,13 @@ type ArgumentParser<'Template when 'Template :> IArgParserTemplate> private (arg
         let tag = argInfo.TagReader.Value (value :> obj)
         argInfo.Cases.[tag].UnionCaseInfo
 
+    /// <summary>
+    ///     Gets the F# union tag representation for given union case constructor
+    /// </summary>
+    /// <param name="ctorExpr">Quoted union case constructor.</param>
+    member __.GetTag(ctorExpr : Expr<'Fields -> 'Template>) : UnionCaseInfo =
+        expr2Uci ctorExpr
+
     /// <summary>
     ///     Prints command line syntax. Useful for generating documentation.
     /// </summary>