How to create key-mapping for vimgrep with dynamic values for {file} argument

Question

Problem

I am trying to create a more dynamic mapping for vimgrep. Many project directories I work in don't have consistent names for directories in which source/test code lives (ie. test vs. tst). I would like to run a command when I first begin a vim session at the root of the project to change the mapping for vimgrep to limit my searching to only user-provided directories.

The problem is that I am having difficulty creating a mapped expression to provide this functionality.

Desired workflow:

$ (.../proj_root) vim .

[in vim session]
:<user-command> src test

:<user-command> src test would execute the following and create the mapping:

:nn <c-\> :vimgrep /<c-r><c-w>/j src/** test/** <bar>:cw<cr>

Attempted solution:

{.vimrc}

command! -nargs=* ExeVpaths call ExeVpaths(<f-args>)
function! ExeVpaths(...)
    if len(a:000) == 0
        nn <expr> <c-\> printf(":vimgrep /<c-r><c-w>/j %s <bar>:cw<cr>", "*/**")
    else
        let l:vpaths = join(a:000, "/** ") . "/** "
        nn <expr> <c-\> printf(":vimgrep /<c-r><c-w>/j %s <bar>:cw<cr>", l:vpaths)
    endif
endfunction

Solution error

The above function executes with no errors for the following two examples:

(1) :ExeVpaths

(2) :ExeVpaths src test

However, using <c-\> after having run (2) fails with the two error messages:

E121: Undefined variable: l:vpaths
E116: Invalid arguments for function printf(":vimgrep /^R^W/j %s |:cw^M", l:vpaths)

This seems to be a scoping issue, seeing l:vpaths appears to be undefined. But that does not make sense to me because the variable is defined right above it.

Comments and critiques are appreciated.

Answer 1

The problem you're having is that l:vpaths is no longer in context when the mapping is invoked, and the printf() will only be evaluated at invocation time.

Using a global variable (of buffer variable) would work to fix the issue here, since it would be available after the function has returned.

But I'm guessing what you want to do here is incorporate l:vpaths into the mapping that you define in the function.

You can easily do so by having your printf() produce the whole nn command and then use execute on it.

function! ExeVpaths(...)
    if len(a:000) == 0
        let l:vpaths = "*/**"
    else
        let l:vpaths = join(a:000, "/** ") . "/** "
    endif
    execute printf('nn <c-\> :vimgrep /<c-r><c-w>/j %s <bar>:cw<cr>', l:vpaths)
endfunction

Note that you don't need the <expr> anymore, since you're evaluating the printf() expression at the time when you're creating the mapping.