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1642 :: Template parameters

• mora (orginal) [2015-09-17T22:33:40+02:00] view original

  Hi,

  Is it possible to receive a template parameter just the same way as a proc parameter? For example if I have a template like

  template Foo(x: seq[int]): expr =
    echo x.len
    echo x.len
  Run

  Then the call Foo(f()) will call f() twice, which is not desied in my case. I could write

  template Foo2(x: seq[int]): expr =
    var y = x
    echo y.len
    echo y.len
  Run

  which solves this problem, and only calls f() once. However, the problem with Foo2 is that if I write Foo2(largeSeq), then it will copy the largeSeq to y, which is costly and not necessary.

  Do you have any ideas how to make both examples effective? (Please correct me if I misunderstand any of these behaviours.)

  If I used a proc instead of a template, then it would solve both of these examples (f() would be called only once, there is no need to copy the seq). However, I need to use a template because of other reasons.

  Thank you,

  Peter

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• Jehan (orginal) [2015-09-17T23:26:16+02:00] view original

  Use let instead of var; this should avoid the copying.

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• Araq (orginal) [2015-09-18T02:54:05+02:00] view original

  let indeed has been designed to model parameter passing for templates that need non-lazy parameters.

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• mora (orginal) [2015-09-18T08:38:40+02:00] view original

  Thank you, I'll use let. I've checked it in one example, and in the generated C it creates a slice and not a copy, so looks good.

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