default should be public instead of redefined everywhere; + proposal for a better implementation

[timotheecour]

Currently template default[T](t: typedesc[T]): T = var v: T; v is defined in multiple places:

lib/core/seqs.nim:91:10:template default[T](t: typedesc[T]): T =
lib/pure/collections/queues.nim:157:10:template default[T](t: typedesc[T]): T =
lib/pure/collections/sets.nim:44:10:template default[T](t: typedesc[T]): T =
lib/pure/collections/sets.nim:284:10:template default[T](t: typedesc[T]): T =
lib/pure/collections/tableimpl.nim:119:10:template default[T](t: typedesc[T]): T =
tests/destructor/tcustomseqs.nim:101:10:template default[T](t: typedesc[T]): T =
tests/vm/tvmmisc.nim:16:8:  proc default[T](t: typedesc[T]): T {.inline.} = discard

instead of being defined once and exported via * (it's also useful in library and user code).

how about defining it (once) in system.nim, but using the following better definition which allows usage at compile time:

# NOTE: `defaultImpl` needed because of https://github.com/nim-lang/Nim/issues/8486
proc defaultImpl[T]():T=
  var a: T
  return a
template default*[T](t: typedesc[T]): T = defaultImpl[T]()

when defined(case_default_old_temp):
  template default*[T](t: typedesc[T]): T =
    # Error: cannot evaluate at compile time: v116407 var v: T
    var v: T
    v
when defined(case_default_workaround):
  proc defaultImpl[T]():T=
    var a: T
    return a
  # Works!
  template default*[T](t: typedesc[T]): T = defaultImpl[T]()
else:
  proc default*[T]():T=
    var a: T
    return a

proc len(T: typedesc[tuple]):auto=
  when defined(case_default_old) or defined(case_default_workaround):
    const t=T.default
  else:
    const t=default[T]()

  var n=0
  for _ in t.fields:
    inc n
  return n

proc test()=
  var a=(1,2,3)
  const length = a.type.len
  echo length

test()

EDIT /cc @LemonBoy see my edited proposal above.

NOTE: if nim-lang/Nim#8486 is fixed in the future, we can use the friendlier syntax default(T) / T.default

EDIT: added template default*[T](t: typedesc[T]): T = defaultImpl[T]()

[Araq]

I agree, we can do that once the compiler bugs are fixed.

[timotheecour]

@Araq I found a way to use the friendlier T.default using intermediate defaultImpl, see above.
PR accepted for that? (with that friendlier version, no need to wait until nim-lang/Nim#8486 is fixed since user code using default won't be affected by a change of implementation of default)

Which module, system.nim?

[Araq]

system.default has been added. Closing.