ollran

ollran

How can I pattern match against an arbitrary sized tuple? This works:

{status, _, _} = {:ok, 1, 2}

But what if I don’t know the structure of the tuple on the right side and I only care about the first element? This doesn’t work:

{status, _} = {:ok, 1, 2, 3, 4, 5, 6}
{status, _} = {:ok, {:random, :things}, [1, 2, 3]}

Is it possible to get the first value from the tuple with pattern matching?

Showing Posts 1 to 6

NobbZ

NobbZ

No. A tuple is of fixed size.

You can either use status = elem({:ok, 1, 2, 3}, 0) or [status | _] = Tuple.to_list({:ok, 1, 2, 3}.

lpil

lpil

Creator of Gleam

Tuple.to_list/1 will allocate a new list, Kernel.elem/2 can be used to retrieve the first element without creating this intermediate list :slight_smile:

dkuku

dkuku

would be handy to have some macro that generates the missing underscores:

 {:ok, info} <- :file.read_file_info(pid),
 {:file_info, file_size, _, _, _, _, _, _, _, _, _, _, _, _} <- info,
{:file_info, file_size, _escape_next(12)} <- info,
ityonemo

ityonemo

for that particular tuple, I think you can use the Record module.

dkuku

dkuku

Thanks, this makes the code much more readable, not much examples for this module, if someone needs one for this particular case:

defmodule FileInfo do
  require Record
  Record.defrecord :file_info, Record.extract(:file_info, from_lib: "kernel/include/file.hrl")
end

and then instead my previous line I have a keyword list that can be accessed with

iex(3)> import FileInfo
FileInfo
iex(4)> info
{:file_info, 2333891, :regular, :read, {{2020, 12, 29}, {19, 18, 12}},
 {{2020, 12, 29}, {19, 27, 58}}, {{2020, 12, 29}, {19, 27, 58}}, 33188, 1,
 66310, 0, 302070, 33, 19}
iex(5)> file_info(info)
[
  size: 2333891,
  type: :regular,
  access: :read,
  atime: {{2020, 12, 29}, {19, 18, 12}},
  mtime: {{2020, 12, 29}, {19, 27, 58}},
  ctime: {{2020, 12, 29}, {19, 27, 58}},
  mode: 33188,
  links: 1,
  major_device: 66310,
  minor_device: 0,
  inode: 302070,
  uid: 33,
  gid: 19
]
iex(6)> file_info(info, :size)
2333891

glmeocci

glmeocci

I’d love it!

{:file_info, file_sieze | _the_rest_as_tuple}  <- info
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