How to provide default value for fun arg

[q0w]

Without default values I explicitly pass platform_ as null

let install_gh_release = fun name_ repo_ tag_ platform_ =>
    let action = {
      name = "Install %{name_}",
      uses = "jaxxstorm/[email protected]",
      with = {
        repo = repo_,
        tag = tag_,
        cache = "enable",
      }
    }
    in
    if platform_ != null then
      action & { with = { platform = platform_ } }
    else
      action

[ErinvanderVeen]

Hej @q0w, I think this question was directed at the Nickel team (this is the Topiary repository). I've pinged them for you and transferred the discussion to their repository!

[q0w]

Oh, yes, sorry

[vkleen]

There aren't default values for functions per se in Nickel. Two options come to mind. First, you can do the Nix thing and take a record as the single argument. In this record, you can have default values:

let install_gh_release = fun { name, repo, tag, platform ? "<default>" } => ...

The second, and the one that I would recommend, is not using a function to generate this data at all, but instead encode it as a recursively defined record with some fields left to be merged in. Fields that should end up in the final configuration can be marked as not_exported to hide them from serialization in simple cases. Sometimes it's more comfortable to have a dedicated output attribute. For example,

let gh_release = {
  name,
  repo,
  tag,
  platform | default = null,
  output =
    let name' = name in
    {
      name = "Install %{name'}",
      uses = "jaxxstorm/[email protected]",
      with.repo = repo,
      with.tag = tag,
      with.cache = "enable",
    } & (if platform == null then {} else {
      with.platform = platform,
    }),
} in
gh_release & {
  name = "some name",
  repo = "some repo",
  tag = "some tag",
}

Ideally one would accumulate these kinds of release until the very top of the configuration and only then select the output field in them. That way, you can keep as much overridability as possible.

[q0w]

Thanks!