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13774 :: Help me understand expected syntax for Concept.

• sandwood (orginal) [2026-03-09T21:19:02+01:00] view original

  Hello.

  I am trying to express a concept for accepting 'any' regular expression Engine, providing that the user perform a small wrapping effort.

  Here is the best I could achieve (MRE).

  
  #A simple concept for a regexp engine, so that we can use ANY one.
  
  type
    
    ReEngine  = concept r, var v, type R
      loadEnginePattern(v, type string) is R
      reEngineMatch(r, type string) is bool
  
  proc buildAndMatch[T:ReEngine] (pattern: string, payload: string) : bool =
    var r = T()
    r = r.loadEnginePattern(pattern)
    return r.reEngineMatch(payload)
  
  
  import regex
  
  type ReWrap = object
    reg : Regex
  
  proc loadEnginePattern(r: var ReWrap, p: string) : Rewrap =
    return ReWrap(reg : re(p))
  
  proc reEngineMatch(r : ReWrap, s: string) : bool =
    return s.contains(r.reg)
  
  echo buildAndMatch[ReWrap](pattern = "bc" , payload = "aaabccc")
  echo buildAndMatch[ReWrap](pattern = "bc" , payload = "abaabaccc")
  Run

  compile and run ok with Nim 2.2.8.

  Now, the question : as you can notice, the loadEnginePattern signature is ugly (force to have first the var T=T() in the buildAndMatch proc.

  I would like to express that

  ` newEnginePattern(s: String) is R`

  ( in human text : there is a constructor for this type accepting a string as unique parameter).

  I've tried many various things, could not get a compiling version...

  Any advice welcome ! Thanks.

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• Araq (orginal) [2026-03-10T06:40:59+01:00] view original

  Use new-styled concepts:

  
  type
    ReEngine  = concept
      proc loadEnginePattern(v: var Self; s: string): Self
      proc reEngineMatch(r: Self; s: string): bool
  Run

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