Manipulate chars with their ascii code

Question

Let say I have

let l:letter = 'A'

is there a function that takes l:letter ascii code and transforms it to 'B'? I mean something like

let l:next_letter = l:letter + 1

I know there's <Ctrl-A> with set nf=alpha, but that requires to have the cursor over the character in the document and I was wondering if it's possible to do it with a variable without modifying the document.

Here's what I try so far, but as I said, it modifies the document and the cursor position:

fu! s:get_next_char(letter)
    set nf=octal,hex,alpha
    call setline(line('$') + 1, a:letter)
    exe ':normal! G^'."\<C-A>"
    set nf=octal,hex

    let l:next_letter = getline('$')
    normal! Gdd
    return l:next_letter
endfu

Answer 1

It's easy to do with :help nr2char() and :help char2nr().

Example usage:

:let b_char = nr2char(char2nr('A') + 1)

Then, :echo b_char will return 'B'

Answer 2

The question is (emphasis mine):

is there a function that takes l:letter ascii code and transforms it to 'B'?

Since you want a function, the first thing to try is :help function<C-d> which lists every documentation tag containing function:

:function                    list-functions               folding-functions
function()                   time-functions               history-functions
functions                    mark-functions               mapping-functions
function-key                 text-functions               various-functions
function_key                 compl-function               numbered-function
function-list                local-function               autoload-functions
:function-verbose            spell-functions              quickfix-functions
function-argument            float-functions              complete-functions
nb-functions                 style-functions              anonymous-function
function-search-undo         ft-ada-functions             Dictionary-function
gui-functions                string-functions             completion-functions
arg-functions                server-functions             window-size-functions
var-functions                cursor-functions             interactive-functions
function-range-example       window-functions             command-line-functions
expr-function                buffer-functions             highlighting-functions
new-functions-5.2            system-functions             curly-braces-function-names
date-functions               syntax-functions             python-Function
dict-functions               xterm-function-keys          :endfunction
file-functions               vt100-function-keys          :delfunction
user-functions               bitwise-function

The most obvious candidate is function-list, let's try it:

There are many functions.  We will mention them here, grouped by what they are
used for.  You can find an alphabetical list here: |functions|.  Use CTRL-] on
the function name to jump to detailed help on it.

String manipulation:                    *string-functions*
    nr2char()       get a character by its ASCII value
    char2nr()       get ASCII value of a character
    [...]

You may have noticed a pattern, here: functions are grouped by theme. This means that, if you need a function to filter a list you can go directly to :help list-functions.

The Vim help pages are very useful; often times :help <keyword><C-d> will get you a long way ;-)