Calling functions passed as arguments in Lua

Question

I have this code

Option = { }


function Option.nothing( )
  local self = { isNone = true, isSome = false }

  function self:orElse( alt )
    return alt
  end

  function self:map( f )
    return Option.nothing( )
  end

  function self:exec( f )
  end

  function self:maybe( alt, f )
    return alt
  end

  return self
end



function Option.just( val )
  local self = { isNone = false, isSome = true }
  local value = val

  function self:orElse( alt )
    return value
  end

  function self:map( f )
    return Option.just( f(value) )
  end

  function self:exec( f )
    f( value )
  end

  function self:maybe( alt, f )
    return f(value)
  end

  return self
end



function printOpt( opt )
  local str = opt.maybe( "Nothing", function(s) return "Just " .. s end )
  print( str )
end


x = Option.nothing( )
y = Option.just( 4 )

printOpt(x)
printOpt(y)

But I keep getting 'attempt to call local 'f' (a nil value)' here:

  function self:maybe( alt, f )
    return f(value)
  end

It seems I'm having trouble calling a function passed as a argument.

Answer 1

You declared the function as self:maybe(), but you're calling it as opt.maybe(). You should call it as opt:maybe().

Declaring it as self:maybe(alt, f) is equivalent to declaring it as self.maybe(self, alt, f). So if you call it with a . you need 3 args. You're passing 2, so self ends up as "Nothing", and alt ends up as the function object.

However, by calling it as opt:maybe("Nothing", f) this is equivalent to saying opt.maybe(opt, "Nothing", f) which provides the required 3 args.