I think you have a synthax error on the way you’re trying to define the map. It is not about the for or do.

One way you could achieve the map definition would be :

%{“#{key}”: val * val}

or

%{“#{key}” => val * val}

Exactly, the first one works because what he is defining is a tuple, not a map.

Yes, it’s a tuple not a map, and not even a tuple, because tuple doesn’t have %{.

I did it accidentally and it took time to catch the mistake. I wanted to see if someone else can catch the mistake, because the error message doesn’t specifically point it out.

%{"#{ isn’t even needed, using key directly works.

for {key, val} <- %{"a" => 1, "b" => 2}, do: %{key => val * val} works.

Your map syntax itself is invalid

iex(1)> {"a", 1} # valid tuple syntax
{"a", 1}

iex(2)> %{"a" => 1} # valid map syntax
%{"a" => 1}

iex(3)> %{"a", 1} # invalid
** (SyntaxError) iex:3:6: syntax error before: ','
    |
 30 | %{"a", 1}
    |      ^

Ah you’re right ! ^^
No string interpolation is necessary indeed.

I tought it won’t work for map with atom keys, but it also works as well:

iex(1)> key = :a
:a
iex(2)> %{key => 1}               
%{a: 1}
iex(3)>