RFC: outplace argument _

[timotheecour]

proposal

Support the outplace argument _, for the common case of sugar.dup where the outplace argument is assumed to be initialized to its default value for its (inferred) type.

Example

proc merge[T](result: var seq[T], x, y: openArray[T]) = ...
let x = [1,3]
let y = [2,5]
let a = merge(_, x, y)
let a2 = _.merge(x,y) # also works

# same as:
var a: seq[int]
merge(a, x, y)
a

implementation

• requires compiler support.
• in sigmatch, the arguments are implicitly reordered so that _ is matched last, at which point all generic parameters must be bound, or a sigmatch error occurs.

links

• https://nim-lang.github.io/Nim/sugar.html#dup.m%2CT%2Cvarargs%5Buntyped%5D
• outplace argument _ nim-lang/RFCs#292
• [TODO] [WIP] new outplace operator without intermediate copies/temporaries by timotheecour · Pull Request #13337 · nim-lang/Nim
• new outplace operator: doAssert @[2,1,3].>sort() == @[1,2,3] by timotheecour · Pull Request #13309 · nim-lang/Nim
• outparam: allows to call in-place a proc with var result ; fixes #RFCs/issues/62 by timotheecour · Pull Request #11916 · nim-lang/Nim

[timotheecour]

=> nim-lang/RFCs#292