[lazedo]

This EEP proposes that a map key in a pattern can use variables bound by other parts of the same pattern, in any order, as long as the keys do not depend on each other in a cycle:

local_number(Number, #{numbers := #{Number := Metadata}}) ->
    {Number, Metadata};
local_number(_, _) ->
    not_local.

EEP 43 described this when maps were introduced, but it was never implemented. The EEP also covers the problems that shelved the 2020 prototype: repeated variables such as foo(#{K := K}, K) work, and programs that stay invalid get the same error messages as today.

• I have tentatively used the next free number, 84, as EEP 1 allows. Please change it if needed.
• Reference implementation: Allow map keys bound in the same pattern otp#12188 (draft), on master (30.0-rc0), covering erl_lint, v3_core, erl_eval and the debugger, with tests and updates to the Reference Manual.
• Forum discussion: https://erlangforums.com/t/pre-eep-map-keys-bound-in-the-same-pattern-as-in-f-k-k-v/6133

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