I Would like to understand why this outputs what it does

In iex when I run IO.puts("#{Enum.map(1..2000, fn x -> x end)}")
It prints to stdout:

^A^B^C^D^E^F^G^H        
^N^O^P^Q^R^S^T^U^V^W^X^Y^Ze^\^]^^^_ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_\`abcdefghijklmnopqrstuvwxyz{|}~^€‚ƒ
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I would like to know why.

:wave:

It tries to print the “printable representation” (consult your ascii/unicode tables) of

iex(3)> "#{Enum.map(1..2000, fn x -> x end)}"
<<1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
  23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
  42, 43, 44, 45, 46, 47, 48, 49, 50, ...>>
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Enum.map/2 will return a list. In your case a list of numbers. If you look into the documentation for List.to_string/1 (which will be called implicitly by the string interpolation) you will see this:

Converts a list of integers representing codepoints, lists or strings into a string.
Notice that this function expects a list of integers representing UTF-8 codepoints.

To avoid this you need to explicitly turn your list into a string. You probably want to pipe it into Enum.join/2 or inspect/2.

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I had a similar question recently. See if this helps:

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