To make it full circle, I've filed a feature request with erlang:
https://github.com/erlang/otp/issues/8558
@Jason: Good point - thanks, I had forgotten that these are a thing. If
elixir was to consider this, I'd still vote for a warning though. Like, yes
technically it can match. Practically, I don't think these ill-formed
structs are something that is commonly used or the default that's expected.
And so, possibly degrading DX for them isn't great imo. It's also just a
warning, and I think using them in this context is warning worthy :) And if
this was the behavior one wanted, one could just match against the
:__struct__ key. I also think that with the full complexity of this,
including it in something like credo is too much to ask - this is very much
a compiler/type checker concern as you need knowledge of the struct keys
etc.
On Mon, Jun 3, 2024 at 6:40 PM Jason Axelson <jason.axel...@gmail.com>
wrote:
> I quite like the idea of more compilation-warnings for impossible pattern
> matches! However, it does seem like a lot of work will be needed to get
> them reliable and performant enough to get there and I don't have any
> insight to add there.
>
> But I do want to point out that the "Map matching on struct specific keys"
> isn't an impossible match since not all structs are well-formed, e.g. the
> "range" function head can be hit like so:
> ```
> iex(1)> defmodule MapVsStruct do
> ...(1)> def key_match(%{first: _first}), do: "map"
> ...(1)> def key_match(%Range{}), do: "range"
> ...(1)> end
> {:module, MapVsStruct,
> <<70, 79, 82, 49, 0, 0, 5, 188, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0,
> 209,
> 0, 0, 0, 20, 18, 69, 108, 105, 120, 105, 114, 46, 77, 97, 112, 86, 115,
> 83,
> 116, 114, 117, 99, 116, 8, 95, 95, 105, ...>>, {:key_match, 1}}
> iex(2)> MapVsStruct.key_match(%{__struct__: Range})
> "range"
> ```
>
> So given that I'm not sure we'd want a compilation warning for that
> pattern match, so perhaps that warning would make more sense in a linter
> like Credo.
>
> -Jason
>
> On Thu, May 30, 2024 at 3:25 AM Tobias Pfeiffer <prag...@gmail.com> wrote:
>
>> Hello everyone,
>>
>> as usual thanks for all your outstanding work on Elixir & everything else.
>>
>> Courtesy of a reddit discussion [1] I'd like to propose implementing some
>> more compiler warnings for matches that are impossible.
>>
>> ## Background
>>
>> Elixir is very programmer friendly and issues compiler warnings in many
>> cases if something is impossible. So if we match just against `variable` in
>> a function head and further down match against a more specific value we get
>> a warning.
>>
>> Similarly if we match against `__struct__` and the actual struct we get a
>> warning:
>>
>> def struct_match(%{__struct__: Range}), do: "__struct__"
>> def struct_match(%Range{}), do: "struct"
>>
>> This one warns as:
>>
>> warning: this clause cannot match because a previous clause at line
>> 17 always matches
>> │
>> 18 │ def struct_match(%Range{}), do: "struct"
>> │ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> │
>> └─ lib/compiler_warnings_impossible_matches.ex:18
>>
>> This is great! The idea here is to take it further.
>>
>> ## Proposal
>>
>> The code examples used here can be found here:
>> https://github.com/PragTob/elixir_playground/blob/main/lib/compiler_warnings_impossible_matches.ex
>>
>> I tried the examples on 1.18.0-dev (ed67d6b) with OTP 27.0 to make sure
>> none of them emit a warning right now.
>>
>> In short, it's more warnings where for some of which I thought they'd
>> already warn you about impossible matches but found out they don't. I think
>> these warnings would be helpful to avoid bugs & help newcomers.
>>
>> ### Simple map match vs. Struct
>>
>> def map_match(%{}), do: "map"
>> def map_match(%Range{}), do: "range"
>>
>> Ideally this should warn as the match is impossible. Possibly also when
>> using the `is_map` guard.
>>
>> ### Map matching on struct specific keys
>>
>> def key_match(%{first: _first}), do: "map"
>> def key_match(%Range{}), do: "range"
>>
>> If we had a map where all keys overlap with a struct matched further down
>> in the function headers, ideally this should also warn. Of course, if we
>> add a second key to the match that the struct doesn't have it should not
>> warn.
>>
>> Of this proposal, I think this is the one that I see causing bugs most
>> easily.
>>
>> ### atoms, nil and booleans
>>
>> def atom_match(value) when is_atom(value), do: "atom"
>> def atom_match(nil), do: "nil"
>> def atom_match(value) when is_boolean(value), do: "bool"
>>
>> I'm not sure how much guards should be taken into account with these
>> kinds of warnings, but these are also "unmatchable".
>>
>> ## Implementation
>> I'm aware there is a chance you're aware of this and it wasn't
>> implemented for compilation performance considerations. If so, sorry and
>> happy to learn how I could look something like this up.
>>
>> If you agree that this could be worthwhile to implement, I'd be happy to
>> give the implementation a shot myself but I'd at least need basic guidance
>> as I don't know the internals all that well :)
>>
>> Thanks y'all and have a splendid day!
>>
>> [1] https://www.reddit.com/r/elixir/comments/1cibtia/comment/l2ckqjr/
>>
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