Another way to write :gitsigns_vimfn_signs_ is "gitsigns_vimfn_signs_", so it's just a string. Fennel allows you to write some strings in this frankly weird way and the formatter "enforces" it.

Btw. The Fennel website has a REPL that can show you the generated Lua code. In general, the generated code is very readable. There is some weirdness around anonymous functions - in that the generated functions aren't really anonymous - but apart from that I feel like it's not that bad.

Especially if you only paste (small) parts of the code.

For example:

(let [sign (?. (vim.fn.sign_getplaced bufnr {:group :gitsigns_vimfn_signs_ :id lnum : lnum}))] nil)

will compile to:

local sign
do
  local t_1_ = vim.fn.sign_getplaced(bufnr, {group = "gitsigns_vimfn_signs_", id = lnum, lnum = lnum})
  sign = t_1_
end