Sorting out Go's 'for ... = range ..' and when it copies things

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Sorting out Go's 'for ... = range ..' and when it copies things go utcc.utoronto.ca
via calvin 3 years ago | archive
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olegkovalov 3 years ago | link

BTW, for the such and similar situations go-critic has checks rangeValCopy and rangeExprCopy

See docs: https://go-critic.github.io/overview

Repo: https://github.com/go-critic/go-critic
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  mperham edited 3 years ago | link
  
  Thanks, I integrated it into my codebase. The default sizeLimit of 512 seems rather large, I would expect to see something like 64.
  
  https://github.com/contribsys/faktory/blob/master/.golangci.yml
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    olegkovalov 3 years ago | link
    
    go-critic has a support for config per checker, look for @rangeExprCopy.sizeThreshold on the overview page :)
    
    And from what I see there is no mention of go-critic config param (oops), we’ll update docs soon.
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      mperham 3 years ago | link
      
      Yeah, I dug thru it and config’d golangci-lint. See link above.
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    quasilyte 3 years ago | link
    
    Just in case if you didn’t know: you can change the default size with -@rangeExprCopy.sizeThreshold parameter.
    
    To get a list of checkers, run: gocritic doc.
    
    To get info about a checker, run: gocritic doc <checkerName>.
    
    So, in case of the rangeExprCopy you do “gocritic doc rangeExprCopy” and it tells what parameters are available.
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p5v 3 years ago | link

I think this is somewhat standard behavior. In other programming languages, the compiler, or latest, the runtime might try to prevent modifications to a looped-over collection. Go would allow that when using a pointer. Makes sense from a semantic point of view. Treat the ranged-over slice as immutable, unless you use a pointer to it.