Incorrect type narrowing of `for` comprehension initial accumulator
维护者通常 1 天内回复
还没有人认领这个 Issue。
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调研方向
Start by running both scripts from the issue as reduce_repro.exs and reduce_repro_1.exs to reproduce the false-positive type warnings. Trace the type handling for reduce: comprehensions in the compiler/type checker, using the gradual typing reference only to assess the documented alternative. Done means the initializer retains its input type and both examples execute without warnings while preserving their outputs.
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描述
Existing issue
- I have searched existing issues and could not find a duplicate.
Elixir and Erlang/OTP versions
Erlang/OTP 28 [erts-16.4.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]
Elixir 1.21.0-dev (7a40579)
Operating system
any
Current behavior
The current implementation types the initializer with the expected type of the comprehension result. An initializer is always evaluated but its value may never reach the final reduction result.
Repro:
save this as reduce_repro.exs and execute
defmodule ReduceRepro do
def calculate(initial) do
(for _ <- [1], reduce: initial do
_ -> 1
end) + 1
end
def run, do: calculate(:old)
end
IO.inspect(ReduceRepro.run())
Result:
the code works fine and prints 2 but emits a false positive warning:
warning: incompatible types given to calculate/1:
calculate(:old)
given types:
:old
but expected one of:
float() or integer()
type warning found at:
│
8 │ def run, do: calculate(:old)
│ ~
│
└─ reduce_repro.exs:8:20: ReduceRepro.run/0
another example:
defmodule ReduceRepro1 do
def original(initial) do
:ok =
for _ <- [1], reduce: initial do
_ -> :ok
end
case initial do
:old -> :kept
_ -> :other
end
end
end
IO.inspect(ReduceRepro1.original(:old))
result: code prints kept and warns
warning: the following clause will never match:
:old ->
because it attempts to match on the result of:
initial
which has type:
dynamic(:ok)
type warning found at:
│
17 │ :old -> :kept
│ ~~~~~~~~~~~~~
│
└─ reduce_repro_1.exs:17: ReduceRepro1.original/1
initial is incorrectly narrowed to dynamic(:ok)
I believe this is a separate problem from https://github.com/elixir-lang/elixir/issues/15451 - covers refinements inside conditional code branches
Expected behavior
Initializer type should not be narrowed. Precise typing is probably not possible as the reducer callback is not guaranteed to execute (as in #15451).
Alternatively, if the behaviour is intended it should be documented in https://github.com/elixir-lang/elixir/blob/main/lib/elixir/pages/references/gradual-set-theoretic-types.md#false-positives
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