[erlang-questions] lists

[Richard O'Keefe]

"sam1" is a list of length 4 whose first element is the integer $s.
["sam1","sam2"] is a list 2 whose first element is the list "sam1".

I'm guessing that you're heading towards a problem that I discussed
in my Prolog book: "How do I design a data structure in this language?"
The basic rule is
  - what are the different situations I need to represent?
    - for each of these different situations, what additional
      information is there?
    - decide how to represent that context-dependent information
  - in Prolog, give each situation its own function symbol;
    in Erlang, consider the {Case_Label,Case_Dependent,Info...}
    approach.  Whatever you do, make each case OBVIOUSLY different
    by a trivial pattern match (or a similarly trivial guard test
    in Erlang).
  - if some cases have one chunk of info and that is a sequence,
    you might use a bare list but ONLY for one.

With a well-designed Erlang data structure, there won't be any
point in asking the question 'how do I tell the difference
between "sam1" and ["sam1","sam2"]' because you will have made
sure it is never the case that both are live possibilities.
For example, if you want to model JSON, you might choose
true | false | null    -- the corresponding Erlang atom
Number                 -- the same number
String                 -- {string,String}
[T1,....,Tn]           -- {array,[T'1,....,T'n]}
{K1:V1,...,Kn:Vn}      -- [{K1,V1},...,{Kn,Vn}]

Of course in current Erlang you might use a 'map' for the last.
The point is that {string,"..."} and {array,[...]} are trivially
easy to distinguish.

You might want to give some consideration to using
<<"sam1">> and
[<<"sam1">>,<<"sam2">>]
instead.

Perhaps you could go into more detail about what you want to achieve.


On Tue, 17 Sep 2019 at 15:44, Sam Overdorf <soverdor@REDACTED> wrote:

> How do I tell the difference between:
> "sam1"
> ["sam1","sam2"]
> They are both lists but behave differently when I apply
>   [H|T] = List.
>   H = "s"
>   H = "sam1".
>   The second one is what I want and then stop breaking them down.
>
> Weird problem...
>
> Thanks,
> Sam
> _______________________________________________
> erlang-questions mailing list
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> http://erlang.org/mailman/listinfo/erlang-questions
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