Error with self invoking anonymous functions in Lua

user_501266 0
Viewed 552

I'm trying to use self invoking anonymous functions in Lua, and am seeing some strange behavior.

This:

(function ()
  print("self-invoking approach")
end)()

print("standard approach")

works ok and prints the following output:

self-invoking approach
standard approach

but reversing the two:

print("standard approach")

(function ()
  print("self-invoking approach")
end)()

results in this error:

➜  hammerspoon   lua temp.lua
standard approach
lua: temp.lua:1: attempt to call a nil value
stack traceback:
    temp.lua:1: in main chunk
    [C]: in ?

Strangely, when code is run in the Lua REPL, the failure only occurs when the function form is second, and both calls are wrapped in an outer function that is called:

function foo()
    print("standard approach")

    (function ()
      print("self-invoking approach")
    end)()
end

foo()

What's happening here?

interpreter lua function lambda
1 Answers

It's a parsing ambiguity. The non-working case is being parsed as:

print("standard approach")(function ()
  print("self-invoking approach")
end)()

In other words, it's printing standard approach, then taking the return value of that print (which is nil), and trying to call that with your self-invoking function as the argument (after which it would have tried to call the result of that too, had it not already crashed). To fix it, add a semicolon at the end of the first print function call.

user_75090650
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