How do I find an `f` that makes `pcall(pcall, f)` return `false` in Lua? -

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i'm using lua 5.3.

reading 'programming in lua 3rd edition', came exercise asking me find f makes pcall(pcall, f) return false. think equivalent make pcall(f) raise error. seems use pcall on won't that. example, let f = nil , nothing bad happens.

however, found on internet letting f = (function() end)() trick. don't understand why. here f return value (which nil) of function function() end right?

this not work

> f = (function() end)() > pcall(pcall, f) true    false   attempt call nil value

but does:

> pcall(pcall,(function() end)()) false   bad argument #1 'pcall' (value expected)

the difference (function() end)() returns no value, different returning nil. functions written in lua cannot make distinction pcall written in c , can make distinction. (other examples print , type.)

note behaves expected (equivalent first attempt):

> pcall(pcall,(function() return nil end)()) true    false   attempt call nil value

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