[erlang-questions] What is allowed in a pattern in terms of string concatenation?

Jachym Holecek freza@REDACTED
Mon Jun 27 19:23:36 CEST 2011

# Daniel Dormont 2011-06-27:
> Suppose I want to test that a certain string String1 ends with a certain
> other string String2. The best I could come up with is:

There are no strings in Erlang! :-)

> case string:substr(lists:reverse(String1), 1, length(String2)) of
> 	String2 -> ok;
> 	_ -> not_ok
> end

Kevin already pointed you to lists:suffix/2 which is good if suffix
isn't statically known, and very easy on the eyes too. If suffix is
known you could do:

  has_suffix(L) ->

  has_suffix2("xiffus" ++ _) ->
  has_suffix2(_) ->

Which I think answers your other question. More precisely, AFAIU this

  has_suffix2("xiffus" ++ _)

is just syntactic sugar for

  has_suffix2([$x, $i, $f, $f, $u, $s | _])

So in a way you could perhaps say "++" isn't really supported in pattern
matching (but other will know this with more certainty). One doesn't see
"++" used very often overall, in my experience.

Also, this would have been so much cheaper (but harder to read) if you
used binaries to represent strings:

  has_suffix(S, B) ->
      has_suffix2(byte_size(B) - byte_size(S), S, B).

  has_suffix2(N, _, _) when N < 0 ->
  has_suffix2(N, S, <<_:N/binary, D/binary>>) ->
      S == D.

Or somesuch, you get the idea.

	-- Jachym

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