Why does my iex return a '-C' or a '-A' when I run this function

Question

I've been learning Elixir for awhile now but I came across something today that totally confused me.

I made this filtering function:

thingy = for a <- ["may", "lay", "45", "67", "bay", "34"], do: Integer.parse(a)
for {n, _} <- thingy, do: n

output: '-C"'

Completely unexpected output, yet the version below 'works'

parseds = for i <- [ "10", "hot dogs", "20" ], do: Integer.parse(i)
for {n, _} <- parseds, do: n

output: [10, 20]

However if I change the numbers to something like 45 and 65 I get '-A' as the result.

Is this just the underlying binary functions permitting me from using which numbers I like?

Answer 1

This is because Elixir, like Erlang, doesn't internally have a String type. Single-quoted strings are represented as character lists, and these are commonly used when dealing with Erlang libraries. When you give Elixir the list [45, 67, 34], it displays it as a list of ASCII characters 46, 67, and 34; which are -, C, and ".

If at least one number in your list doesn't represent a printable character, you see the list of numbers. Because 10 doesn't map to a printable character, in the second example, you see 10 and 20.

It's important to note that the list you've created is still internally represented as [45, 67, 34] so any list operations you do will work exactly as you'd expect with your numbers.

Answer 2

This is because Elixir, like Erlang, doesn't internally have a String type

Whatever that means. Strings, Smings. It's as simple as:

iex(4)> [45, 67, 34]
'-C"'

In iex, a list of numbers is interpreted as a sequence of characters, where each number is a numerical code for some character. If you look at an ascii chart, you will see that:

45 -> -
67 -> C
34 -> "

Look at this:

iex(5)> 'hi' == [104, 105]
true

In Elixir, [104, 105]and [45, 67, 34] are called charlists. A shortcut for creating the charlist [104, 105] is 'hi'. This is the result of a terrible feature of Erlang, but because Elixir is able to interface with Erlang, there is a need for charlists. An Elixir charlist is the equivalent of an Erlang string, and an Erlang string is equivalent to a list of numbers.

Suppose your Elixir program does a bunch of critical mathematical calculations with the result being:

result = [76, 64, 78, 79]

and you want to display that info to the user so that they can tune a defibrillator's settings in order to save a patient's life, so you do this:

IO.puts "Set the defibrillator dials to these numbers: #{result}"

Here's what the user will see:

Set the defibrillator dials to these numbers: L@NO

Patient dies here.