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6849 :: Why can't I define custom `find` proc and override one from the system module?

• alexeypetrushin (orginal) [2020-09-23T13:35:52+02:00] view original

  It conflicts with the built-in find in system, how can I override it?

  import sugar, options
  
  proc find*[T](list: openarray[T], check: (T) -> bool): Option[T] =
    for v in list:
      if check(v): return v.some
    T.none
  
  echo @[1, 2, 3].find((v) => v == 2)
  Run

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• cblake (orginal) [2020-09-23T13:55:43+02:00] view original

  Try:

  import system except find
  Run

  before your other imports?

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• Stefan_Salewski (orginal) [2020-09-23T14:00:15+02:00] view original

  Some years ago I read that we should never import module system directly. Has that been fixed?

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• cblake (orginal) [2020-09-23T14:12:08+02:00] view original

  There is a test for it in tests/modules/texplicit_system_import.nim. I haven't looked into the VC/github history as to how reliable it is.

  If you really hate everything from system you can do this:

  from system import nil
  import options
  
  proc find*[T](list: system.openarray[T],
                check: (proc(v:T): system.bool)): Option[T] =
    for v in system.items(list):
      if check(v): return v.some
    T.none
  
  system.echo [1,2,3].find(proc(v: system.int): system.bool =
                             system.`==`(v,2))
  Run

  Therein lies the "guidance" to not import system explicitly (or with the except or from system import nil constructs). Many facilities probably just assume namespace qualification is not necessary. In this example, the => macro from sugar wanted auto..So, I just de-sugared manually. There may be other ways to workaround it, though. In any event, "should never" is probably slightly too strong a wording for the guidance. "Don't unless you really need to/know what your doing/what the costs are" is probably a better way to phrase it.

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• Stefan_Salewski (orginal) [2020-09-23T14:12:17+02:00] view original

  https://nim-lang.org/docs/system.html

  Each module implicitly imports the System module; it must not be listed explicitly.

  For me and google the translation seems to be "es darf nicht", that is it is forbidden.

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• cblake (orginal) [2020-09-23T14:24:03+02:00] view original

  Nim is "re-definable" to a very unusual degree. That can confuse some people, yet other people have such re-definability as a high priority. I'm unsure what the manual/doc comment should say..Probably move the "; it must not" to after the "don't call some other module system" and soften the explicit import language slightly as per my above suggestions.

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• cblake (orginal) [2020-09-23T14:46:32+02:00] view original

  It decidedly can break stdlib/other code instantiated in context (as per my auto mention) or create situations of extreme confusion. An example may be defining == to mean what != usually does. ;-) There's probably a way in German to create a word meaning "possible but tricky, with hard to see downstream consequences, and likely ill-advised". :)

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• cblake (orginal) [2020-09-23T15:38:43+02:00] view original

  Also, looks like it was added in Summer 2016 & noted in the 0.15 release.

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• alexeypetrushin (orginal) [2020-09-23T16:49:06+02:00] view original

  Can we consider maybe renaming find to find_index and moving to sequtils?

  And maybe limit its scope somehow, as currently it's impossible to override it and there are lots of methods that can use find name, like:

  
  sequence.find((v) => v == 2)
  db.collection("posts").find(id = 10)
  ...
  Run

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• Araq (orginal) [2020-09-23T17:06:21+02:00] view original

  No, that's why we're in version 1 land. We stopped moving stuff around pointlessly. To call your find use the mymodule.find syntax or use import system except find.

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• cblake (orginal) [2020-09-23T18:28:25+02:00] view original

  Also, the current system.find is close to strutils.find in its signature as well as what doc/apis.rst says. You could/maybe should also rename your find to findOption, optionFind or something.

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• pietroppeter (orginal) [2020-09-24T00:47:04+02:00] view original

  btw, your example will work if you qualify parameter check

  https://play.nim-lang.org/#ix=2yyj

  import sugar, options
  
  proc find*[T](list: openarray[T], check: (T) -> bool): Option[T] =
    for v in list:
      if check(v): return v.some
    T.none
  
  echo @[1, 2, 3].find(check=((v) => v == 2))
  Run

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