marick
Why is the syntax tree for a 2-tuple different than for other tuples?
Because Elixir code is parsed into 3-tuples, there needs to be a different notation for an embedded 3-tuple. It is what I think of as “constructor” notation:
iex(12)> quote do: {1, 2, 3}
{:{}, [], [1, 2, 3]}
That’s true for other tuple lengths, which makes sense: why not be consistent?
Except for 2-tuples, which are represented unchanged:
iex(7)> quote do: {1, 2}
{1, 2}
My hunch is that’s because lists are unchanged:
iex(8)> quote do: [1, 2, 3]
[1, 2, 3]
… and making 2-tuples unchanged means that keyword lists are also unchanged:
iex(11)> quote do: [a: 1, b: 2]
[a: 1, b: 2]
I can’t think of a case where treating 2-tuples like other tuples would cause ambiguity, though. Is this just to make literal keyword lists more readable?
Maps also have a constructor representation:
iex(13)> quote do: %{a: 1}
{:%{}, [], [a: 1]}
Why is that needed?
Marked As Solved
dorgan
It’s because the Elixir AST is optimized for developer ergonomics when writing macros, it’s not a goal to be regular(in which case it would be more verbose).
Having them as 2-tuples, in combination with lists being literals in the AST, is what makes it easier to write macros for keyword blocks, like
defmacro schema(table, do: fields) do ...
If the AST were regular, you’d need to do something like this
defmacro schema(table, [{:{}, _, [:do, fields]}]) do ...
And you can imagine how it would be more complicated for other macros. Processing the keyword syntax for Ecto queries would be much more verbose, for example.
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mudasobwa
FWIW, Elixir properly handles 2-element tuples in uniform AST notation, for the sake of convenience.
defmodule Tuple2 do
defmacro t(list), do: {:{}, [], list}
end
require Tuple2
Tuple2.t([:a, :b])
#⇒ {:a, :b}
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