Why does this fail and then succeed if i simply pull the expression out into a variable?

[unphased]

This looks like a lua language issue but I cannot rule out that it is related to hammerspoon and/or metamethods or something.

I have this code:

local function findSessionFiles()
    print("Finding session files...")
    local sessionDir = home .. "/.local/share/nvim/sessions"
    local sessions = {}
    if hs.fs.attributes(sessionDir, "mode") == "directory" then
        for file in hs.fs.dir(sessionDir) do
            if file ~= "." and file ~= ".." then
                print("Found file: " .. file, type(file))
                local s = file:gsub("__", "/")
                print("Converted to: " .. s)
                table.insert(sessions, file:gsub("__", "/"))
            end
        end
    else
        print("Session directory not found: " .. sessionDir)
        hs.alert.show("Session directory not found: " .. sessionDir)
    end
    print("Found " .. #sessions .. " file(s)")
    return sessions
end

The error I get is:

2024-08-17 04:00:37: Finding session files...
2024-08-17 04:00:37: Found file: __Users__slu__util	string
2024-08-17 04:00:37: Converted to: /Users/slu/util
2024-08-17 04:00:37: 04:00:37 ERROR:   LuaSkin: hs.hotkey callback: /Users/slu/.vim/hammerspoon/main-init.lua:73: bad argument #2 to 'insert' (number expected, got string)
stack traceback:
	[C]: in function 'table.insert'
	/Users/slu/.vim/hammerspoon/main-init.lua:73: in upvalue 'findSessionFiles'

If I change this:

--- i/hammerspoon/main-init.lua
+++ w/hammerspoon/main-init.lua
@@ -70,7 +70,7 @@ local function findSessionFiles()
                 print("Found file: " .. file, type(file))
                 local s = file:gsub("__", "/")
                 print("Converted to: " .. s)
-                table.insert(sessions, file:gsub("__", "/"))
+                table.insert(sessions, s)
             end
         end
     else

The code runs fine.

[unphased]

A simple failing test in lua 5.4 repl

❯ lua  
Lua 5.4.6  Copyright (C) 1994-2023 Lua.org, PUC-Rio
> test = {}; str = "a__b"; function insert_and_print() table.insert(test, str:gsub("__", "/")); print(#test, test[1]) end
> insert_and_print()
stdin:1: bad argument #2 to 'insert' (number expected, got string)
stack traceback:
        [C]: in function 'table.insert'
        stdin:1: in function 'insert_and_print'
        (...tail calls...)
        [C]: in ?
>

indicates this is a standard lua behavior. Sorry, it's not a hammerspoon specific question then. But surely you Lua experts could school me (as well as the LLMs I've managed to stump with this).

[asmagill]

This is one of those subtle lua quirks that crops up and bites everyone at least once... it has to do with functions that return multiple values and their position in an argument list.

gsub returns two values -- the changed string and the number of changes made. If a function returns multiple values and is at the end of an argument list, all of the return values are appended to the list:

a = "1, 2, 3, 4"
print(a:gsub(", ", "-"))

1-2-3-4	3

But, if the same function is not the final argument in the argument list, then only the first return value is used:

print(a:gsub(", ", "-"), "is new")

1-2-3-4	is new

The fix is to either capture the result you want into a local variable and use that, or to enclose the function which returns multiple arguments in an extra set of parenthesis:

print((a:gsub(", ", "-")))

1-2-3-4

[unphased]

Thank you for patiently explaining this. I actually realized it all on my own last night. I read the doc for the other functions but not gsub but I was able to convince myself that it is gsub and built in language syntax sugar that played me here.