How do you convert between non-literal strings and literal strings?

Question

I was writing a small plugin, when I decided that I should give the option of customizing which key triggers the plugin's function. Here is an example of this:

let g:foo_key = "\<Tab>"

While this works for part of the plugin's functionality, I also need that variable as a literal string, which would be '\<Tab>' in this case. Is it possible to convert from the resulting key code to a literal string? Is there a standard way of handling this kind of option?

Answer 1

The hex value of g:foo_key will be 0x09 (a tab). You can use the strtrans() function to get a readable representation of this:

:echo strtrans("\<Tab>")
<09>

Note that this prints <09>, and not <Tab>. They are the same, and there is no way to know which one the user has entered. It may also show up as ^I (depends on value of display).
Getting the key name is not something I can figure out how to do with a simple function call, but in the key_name_entry struct in misc2.c you can find a list of keynames; making a function for this is easy:

let g:chars = {
    \ '<09>': '<Tab>',
    \ '<1b>': '<Esc>',
\ }

fun! StringTrans(char)
    if get(g:chars, a:char, -1) != -1
        return g:chars[a:char]
    else
        return strtrans(a:char)
    endif
endfun

You probably want to expand g:chars; note that you have to type the literal hex value with <C-v><Tab>, <C-v><Esc>, etc.

Note that this still won't return exactly what the user has typed, since there are multiple names for some characters (for example <NL>, <LF>, <Newline>, and <Linefeed> are all the same).

---

Doing the reverse with eval() is perhaps easier:

:let foo = '\<Tab>'
:echo foo
\<Tab>

:let foo_expanded =  eval('"' . foo . '"')
:echo strtrans(foo_expanded)
<09>