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Preserve left element types in list subtraction #15808

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Preserve left element types in list subtraction
lukaszsamson Aug 29, 2026
3a7af7a
Add :erlang.--/2 parametric TODO and cover static non-empty subtraction
lukaszsamson Aug 30, 2026
002cfc2
Merge branch 'main' into ls-list-minus-types
josevalim Aug 31, 2026
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13 changes: 13 additions & 0 deletions lib/elixir/lib/module/types/apply.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -1169,6 +1169,19 @@ defmodule Module.Types.Apply do
end
end

defp remote_apply(:erlang, :--, _info, [left, right], stack) do
# TODO: remove once we add parametric types, this will just be:
# list(a), list(term()) -> list(a)
case {list_of(left), list_of(right)} do
{{_, list_of}, {_, _}} ->
result = if list_of, do: list(list_of), else: empty_list()

@josevalim josevalim Aug 29, 2026

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Isn't this the same as left?

@lukaszsamson lukaszsamson Aug 30, 2026

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No. Trivial counterexample [:foo] -- [:foo] === []. If left is nonempty the result may be empty list. But even if we returned union(left, empty_list()) it would be unsound with negations. Given left as non_empty_list(atom()) and not list(atom([:a])), in [:b, :a] -- [:b] the result [:a] would fall outside the union type (credit Fable for this one counterexample).

@josevalim josevalim Aug 30, 2026

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Fascinating example where list(a), list(term()) -> list(a) when a: term() gives a different result than a, list(term()) -> a when a: list(term()), which I would expect them to be equivalent. Does this make sense to you @gldubc?

@lukaszsamson lukaszsamson Aug 31, 2026 •
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Fascinating sure but currently purely synthetic. I don't think elixir can produce such type negation via guards/patterns. Guards do not allow expressing whole list properties and patterns only match on empty or list head. It may not be synthetic when type definition syntax lands.
Are they equivalent - it boils down to what -- actually preserves. Contains some type or is empty: yes, Is the list proper: yes, is the list not empty: no, contains at least one sth: no, contains at least one not sth: no

{:ok, return(result, [left, right], stack)}

_ ->
{:error, badremote(:erlang, :--, [left, right])}
end
end

defp remote_apply(Kernel, :elem, info, [tuple, _index] = args_types, stack) do
remote_apply_tuple_element(info, args_types, tuple, stack)
end
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9 changes: 9 additions & 0 deletions lib/elixir/test/elixir/module/types/expr_test.exs
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Expand Up @@ -173,6 +173,15 @@ defmodule Module.Types.ExprTest do
non_empty_list(term()), term()
"""
end

test "--" do
assert typecheck!([x], [1, 2, 3] -- x) == list(integer())
assert typecheck!([1] -- [1]) == list(integer())
assert typecheck!([x], [] -- x) == empty_list()

assert typeerror!([x], [1, 2, 3] -- String.to_integer(x)) |> strip_ansi() =~
"incompatible types given to Kernel.--/2"
end
end

describe "funs" do
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