map(key, key, ...) on the left side of a pattern and in expressions/right side

[kvakvs]

Having:
#{key := A, x := X, y := Y, key := B} = ZZZ,
on the left of a pattern is not an error but can be very confusing. It could be a warning.

Having it on the right side of a pattern, or in a free-standing expression — is not an error but most likely is a sign of a typo, or a copy-paste error.
ZZZ = #{key => A, x => xxx, y => yyy, key => B}

[paulo-ferraz-oliveira]

I don't see how #{key := A, x := X, y := Y, key := B} = ZZZ can "be very confusing". It's pattern-matching at its best.

If I understand your message well, there's some confusion here.

The first clause is pattern-matching. The second clause is single variable assignment.

[elbrujohalcon]

@paulo-ferraz-oliveira it's related to this. Notice the repetition of key in the first pattern.

[paulo-ferraz-oliveira]

@paulo-ferraz-oliveira it's related to this. Notice the repetition of key in the first pattern.

Oh, I got it. Them details...

In any case, what's so wrong about assigning to two different variables? You'll eventually either use both or only one (if using only one the compiler will already warn you). Is the confusion part the fact that they hold the same value for different names?

[elbrujohalcon]

Assigning the same value to 2 variables can be done like this…

#{key := A = B, x := X, y := Y} = ZZZ,

…which is, at least, more explicit.

[elbrujohalcon]

This is already taken care of by the Erlang compiler. It emits warnings such as…

your_module.erl:9:60: Warning: key 'a' will be overridden in expression
%    9|     Other = NewMap#{even := using, different => accessors, a => "arrow", a := "not arrow"},
%     |                                                            ^