Fl4m3Ph03n1x
Dialyzer does not catch errors on returned functions
Background
While playing around with dialyzer, typespecs and currying, I was able to create an example of a false positive in dialyzer. After talking to some people, I was recommended to post the issue here.
For the purposes of this MWE, I am using diallyxir (versions included) because it makes my life easier. The author of dialyxir confirmed this was not a problem on their side, so that possibility is excluded for now.
Environment
$ elixir -v
Erlang/OTP 24 [erts-12.2.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]
Elixir 1.13.2 (compiled with Erlang/OTP 24)
- Which version of Dialyxir are you using? (cat mix.lock | grep dialyxir):
"dialyxir": {:hex, :dialyxir, "1.1.0", "c5aab0d6e71e5522e77beff7ba9e08f8e02bad90dfbeffae60eaf0cb47e29488", [:mix], [{:erlex, ">= 0.2.6", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "07ea8e49c45f15264ebe6d5b93799d4dd56a44036cf42d0ad9c960bc266c0b9a"},
"erlex": {:hex, :erlex, "0.2.6", "c7987d15e899c7a2f34f5420d2a2ea0d659682c06ac607572df55a43753aa12e", [:mix], [], "hexpm", "2ed2e25711feb44d52b17d2780eabf998452f6efda104877a3881c2f8c0c0c75"},
Current behavior
Given the following code sample:
defmodule PracticingCurrying do
@spec greater_than(integer()) :: (integer() -> String.t())
def greater_than(min) do
fn number -> number > min end
end
end
Which clearly has a wrong typing, I get a success message:
$ mix dialyzer
Compiling 1 file (.ex)
Generated grokking_fp app
Finding suitable PLTs
Checking PLT...
[:compiler, :currying, :elixir, :gradient, :gradualizer, :kernel, :logger, :stdlib, :syntax_tools]
Looking up modules in dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt
Finding applications for dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt
Finding modules for dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt
Checking 518 modules in dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt
Adding 44 modules to dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt
done in 0m24.18s
No :ignore_warnings opt specified in mix.exs and default does not exist.
Starting Dialyzer
[
check_plt: false,
init_plt: '/home/user/Workplace/fl4m3/grokking_fp/_build/dev/dialyxir_erlang-24.2.1_elixir-1.13.2_deps-dev.plt',
files: ['/home/user/Workplace/fl4m3/grokking_fp/_build/dev/lib/grokking_fp/ebin/Elixir.ImmutableValues.beam',
'/home/user/Workplace/fl4m3/grokking_fp/_build/dev/lib/grokking_fp/ebin/Elixir.PracticingCurrying.beam',
'/home/user/Workplace/fl4m3/grokking_fp/_build/dev/lib/grokking_fp/ebin/Elixir.TipCalculator.beam'],
warnings: [:unknown]
]
Total errors: 0, Skipped: 0, Unnecessary Skips: 0
done in 0m1.02s
done (passed successfully)
Expected behavior
I expected dialyzer to tell me the correct spec is @spec greater_than(integer()) :: (integer() -> bool()).
As a side note (and comparison, if you will) gradient does pick up the error.
I know that comparing these tools is like comparing oranges and apples, but I think it is still worth mentioning.
Questions
- Is dialyzer not intended to catch this type of error?
- If it should catch the error, what can possibly be failing? (is it my example that is incorrect, or something inside dialyzer?)
I personally find it hard to believe this could be a bug in Dialyzer, the tool has been used rather extensively by a lot of people for me to be the first to discover this error. However, I cannot explain what is happening.
Help is appreciated.
Marked As Solved
Fl4m3Ph03n1x
Answer
As far as I can understand, Dialyzer wont perform checks on input and output types of returned anonymous functions. The following example, much like mine, is incorrect, but the tool won’t complain:
# no error
@spec add(integer()) :: (String.t() -> String.t())
def add(x) do
fn y -> x + y end
end
It will however point out a mismatch in arity, e.g.
# invalid_contract
# The @spec for the function does not match the success typing of the function.
@spec add2(integer()) :: (integer(), integer() -> integer())
def add2(x) do
fn y -> x + y end
end
If however, we try to call the function, Dialzyer might detect that the input and output types are incorrect, but if it does so there is a good chance it will default to the "Function has no local return" error message.
There is an excellent reply in StackOverflow (which I have quickly summarized here) that explains this behavior in more detail and even though we (I and the person replying) were not able to find a specific reason to the “why” of this happening, the empirical tests and samples are still of considerable value.
Upon skimming through the Dialyzer paper I found I was still unable to understand why the algorithm behaves this way, but I will leave it here in case someone else has a different take on it and is able to better understand this specific case:
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