Update: The OTP team implemented it! Should be coming to an erlang version near you soon. And I hope/think that elixir should automatically pick it up (@jose ?) https://github.com/erlang/otp/pull/8600
On Mon, Jun 10, 2024 at 2:35 PM Tobias Pfeiffer <prag...@gmail.com> wrote: > 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/ >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "elixir-lang-core" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to elixir-lang-core+unsubscr...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/elixir-lang-core/CAG3Z5YTVO1Wrvx76fg%2BBEmqC3PgZsRG0rEpLwVt7fdOWFZ8n2w%40mail.gmail.com >>> <https://groups.google.com/d/msgid/elixir-lang-core/CAG3Z5YTVO1Wrvx76fg%2BBEmqC3PgZsRG0rEpLwVt7fdOWFZ8n2w%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "elixir-lang-core" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to elixir-lang-core+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/elixir-lang-core/CAHMz6kwcOeu3aVrvbGQPN3t1DznfXwguMWwwEH8O7hgnU06O%3DA%40mail.gmail.com >> <https://groups.google.com/d/msgid/elixir-lang-core/CAHMz6kwcOeu3aVrvbGQPN3t1DznfXwguMWwwEH8O7hgnU06O%3DA%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. 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