[erlang-questions] Pattern match error in Maps

[Avinash Dhumane]

avinash@REDACTED:~/tws$ erl
Erlang/OTP 18 [erts-7.3] [source] [64-bit] [smp:2:2] [async-threads:10]
[hipe] [kernel-poll:false]

Eshell V7.3  (abort with ^G)
1> M = #{{a, 1} => 100, {a, 2} => 200, {a, 3} => 300}.
#{{a,1} => 100,{a,2} => 200,{a,3} => 300}
2>  lists:map(fun(N) -> #{{a, N} := V}= M, V div 10 end, lists:seq(1,3)).
* 1: illegal map key in pattern
3>

I can get it working with maps:get/2

But, I seem not to understand when and where may variables be used in the
pattern.

What am I  missing?

Thanks
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