Why does my simple string function fail to grab the only element in a List?

Question

The idea is to normalize a string before processing it to extract information from it. Strings may come with embedded outer brackets which are easily stripped or as elements of a true list. This is early in the development of parse but I just cannot figure out why it works fine with pure strings but fails when given in a list. It's not like I haven't worked much with Lists before. I'm now toward end of year 2 in experience with MMA. But other than labels and similar constructs I really have not done anything with strings considered intermediate or advanced.

Below is my nearly nude parse. In comments above it are things i tried but failed.

(* LabeledEcho 0.5.0 utilities: parse *)

ClearAll[LabeledEcho,parse];
ClearSystemCache[]

(* other things attempted on True *) 
(* condition to lift out the string *)
(* Take[str,{1,-1}] *)
(* First@str *)
(* Identity@@str *)
(* Replace[str,List->Identity] *)
(* Replace[str,List->Sequence] *)

parse[str : _String | List] := With[{
    expr = If[ListQ@str, Sequence @@ str, str]
    },
   cln = If[StringFreeQ[expr, {"{", ",", "}", Whitespace}], 
     Return[expr], StringDelete[expr, {"{", "}"}]];
   Echo[ cln, "cln \[Rule] "]
   ];

string = "{ABC,Its,easy,as,123},{as simple as Do re mi}, ABC, \
123,{Baby you and me girl,   }";
Head@string
str = parse[string]


list = {"{ABC,Its,easy,as,123},{as simple as Do re mi}, ABC, \
123,{Baby you and me girl,   }"};
Head@list
lst = parse[list]

Answer 1

I recommend handling the String vs. List arguments with alternative definitions of your function. That would simplify your work, since each function definition would then "know what it has to deal with".

For instance, the following seems to reproduce the behavior of your parse function:

Clear[parse2]
parse2[str_String] := Echo[StringDelete[str, {"{", "}"}], "cln->"]
parse2[{str_String}] := parse2[str]

parse2[string]
parse2[list]

A couple of notes:

• There is no need to check whether you have the characters you want to delete before deleting them. If they are not present, then StringDelete will simply return its argument unchanged. I noted that you were also checking for the presence of white space but then did not replace it out, so consider that as well.
• In the handling of lists, your example has a list containing a single element. If you need to handle lists of strings with more than one element, then you could change the second definition to something like parse2[lst_?(VectorQ[#, StringQ]&)] := ... and then perhaps map the desired action over lst. However, it is not clear to me what your desired output should be if you have lists of more than one string.