Why does pattern matching on this string in erlang result in a "string" for the tail and an ascii value for the list? - erlang

Question

I was trying to write a pattern matched function in erlang like:
to_end("A") -> "Z".
The whole idea is to transform a string such as "ABC" into something different such as "ZYX" using pattern matched functions. It looks like a string is represented as a list under the hood...
I was depending on the fact that pattern matching on a "string" in erlang would result in individual string characters. But I find this:
21> F="ABC".
22> F.
"ABC"
23> [H | T]=F.
"ABC"
24> H.
65
25> T.
"BC"
Why does the head of this type of pattern matching on list always result in an ASCII value and the tail result in letters? Is there a better way to pattern match against a "list of string"?

Answer 1

In Erlang, strings are just a list of ascii values. It also displays lists of integers, where every integer is a printable ascii code, as strings. So [48, 49] would print out "01" since 48 corresponds to 0 and 49 to 1. Since you have the string "ABC", this is the same as [65 | [66 | [67]]], and [66, 67] will display as "BC".
If you want to write a function to pattern match on characters, you should use the character literal syntax, which is $ followed by the character. So you would write
to_end($A) -> $Z;
to_end($B) -> $Y;
to_end($C) -> $X;
...
to_end($Z) -> $A.
instead of to_end("A") -> "Z" which is the same as to_end([65]) -> [90].

Answer 2

Why does the head of this type of pattern matching on list always
result in an ASCII value and the tail result in letters?
In erlang, the string "ABC" is a shorthand notation for the list [65,66,67]. The head of that list is 65, and the tail of that list is the list [66,67], which the shell happens to display as "BC". Whaa??!
The shell pretty much sucks when displaying strings/lists: sometimes the shell displays a list and sometimes the shell displays a double quoted string:
2> [0, 65, 66, 67].
[0,65,66,67]
3> [65, 66, 67].
"ABC"
4>
...which is just plain dumb. Every beginning and intermediate erlang programmer gets confused by that at some point.
Just remember: when the shell displays a double quoted string, it should really be displaying a list whose elements are the character codes of each character in the double quoted string. The fact that the shell displays a double quoted string is a TERRIBLE ??feature?? of erlang, and it makes it hard to decipher what is going on in a lot of situations. You have to mentally say to yourself, "That string I'm seeing in the shell is really the list ..."
That fact that the shell displays double quoted strings for some lists really sucks when you want to display, say, a list of a person's test scores: [88, 97, 92, 70] and the shell outputs: "Xa\\F". You can use the io:format() method to get around that:
6> io:format("~w~n", [[88,97,92,70]]).
[88,97,92,70]
ok
But, if you just want to momentarily see the actual list of integers that the shell is displaying as a string, a quick and dirty method is to add the integer 0 to the head of the list:
7> Scores = [88,97,92,70].
"Xa\\F"
Huh?!!
8> [0|Scores].
[0,88,97,92,70]
Oh, okay.
The whole idea is to transform a string such as "ABC" into something
different such as "ZYX" using pattern matched functions.
Because a string is shorthand for a list of integers, you can change those integers by using addition:
-module(my).
-compile(export_all).
cipher([]) -> [];
cipher([H|T]) ->
[H+10|cipher(T)]. %% Add 10 to each character code.
In the shell:
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:cipher("ABC").
"KLM"
3>
By the way, all functions are "pattern matched", so saying "a pattern matched function" is redundant, you can just say, "a function".