Understanding Elixir's List.to_string

So I came across this (quite old) post on stackoverflow, someone wanted to print a list next to a string and he was struggling with it

Elixir: Printing list along with string

Aug 5 '17 Comments: 1 Answers: 2

16

I would like to print a list along with a string identifier like

list = [1, 2, 3]
IO.puts "list is ", list

This does not work. I have tried few variations like

# this prints only the list, not any strings
IO.inspect list
# using puts which also does

…

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and I was a bit baffled as of why this is an issue at all, just convert your list to a string and print it, this logic should work in any language in any print function, should be simple and straightforward

Elixir surely have a function that convert a lists to string, and it does List.to_string

But to my surprise List.to_string was doing weird things

iex(42)> [232, 137, 178] |> List.to_string
<<195, 168, 194, 137, 194, 178>>
iex(43)> [1, 2, 3] |> List.to_string
<<1, 2, 3>>

That was not what I was expecting, I was expecting a returned value that is similar to what IO.inspect produce

So to verify my expectation, I checked what clojure does, because clojure to me is the epitome of sanity

Clojure 1.12.4
user=> (str '(232 137 178))
"(232 137 178)"
user=> (str '(1 2 3))
"(1 2 3)"

it was more or less what IO.inspect does
it returns something that look like how lists "look like" as code

So next step was more introspection

iex(47)> [232, 137, 178] |> List.to_string |> i
Term
  <<195, 168, 194, 137, 194, 178>>
Data type
  BitString
Byte size
  6
Description
  This is a string: a UTF-8 encoded binary. It's printed with the `<<>>`
  syntax (as opposed to double quotes) because it contains non-printable
  UTF-8 encoded code points (the first non-printable code point being
  `<<194, 137>>`).
Reference modules
  String, :binary
Implemented protocols
  Collectable, IEx.Info, Inspect, JSON.Encoder, List.Chars, String.Chars

What the heck is BitString?

So I kept fiddling with the function, until I finally got it

iex(44)> ["o","m",["z","y"]] |> List.to_string
"omzy"
iex(45)> ["o","m",["z","y"]] |> IO.inspect
["o", "m", ["z", "y"]]
["o", "m", ["z", "y"]]

List.to_string , does not transform a list to a string (preserving its structure), List.to_string flattens a list, take each element and transform it to a UTF-8 code point, and if you try to print that, you will get whatever string those codes points produce

iex(49)> [232, 137, 178] |> List.to_string |> IO.puts
è²
:ok
iex(50)> [91, 50, 51, 50, 44, 32, 49, 51, 55, 44, 32, 49, 55, 56, 93] |> List.to_string |> IO.puts
[232, 137, 178]
:ok

In retrospect this is what the docs says

hexdocs.pm

but well, the docs wasn't telling what I wanted it to say 😀

And oh, before I forget, Elixir have the inspect function, which does exactly was I thought List.to_string would do

iex(53)> [232, 137, 178] |> inspect |> IO.puts
[232, 137, 178]
:ok
iex(54)> ["o","m",["z","y"]] |> inspect |> IO.puts
["o", "m", ["z", "y"]]
:ok