Default value of function parameter in Vim script

How to specify default value (e开发者_StackOverflow.g. 0, None) for a parameter in Vim script?

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From the docs, it seems that arguments can't have default values in Vim script. However, you can emulate this by defining a function with variable number of arguments, and using a:0 to determine the number of extra arguments and a:1 through a:n to access them:

function Foo(bar, ...)
  if a:0 > 0
    let xyzzy = a:1
  else
    let xyzzy = 0
  end
endfunction

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You can use get to select an argument in the specific position or a default value if it's not present.

function! Foo(bar, ...)
    let baz = get(a:, 1, 0)
endfunction

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Since Vim 8.1.1310 Vim also supports real optional function arguments.

However, that means that most vim installation don't support this yet. Neovim has that feature since version 0.7.0.

Example from :help optional-function-argument:

  function Something(key, value = 10)
     echo a:key .. ": " .. a:value
  endfunction
  call Something('empty')   "empfty: 10"
  call Something('key', 20) "key: 20"