Lua Learning Notes

As a long time nvim user, I am learning Lua and slowly transition my nvim config to lua. In this script, I will share some tips and lessons I have learned the hard way.

The colon operator

In Lua code, we may see the colon operator followed by some object method. For example, if we have a string x and want to find the length of this string. In Python, we do x.len(), but this will be an error in Lua:

stdin:1: bad argument #1 to 'len' (string expected, got no value)
stack traceback:
    [C]: in function 'string.len'
    stdin:1: in main chunk
    [C]: in ?

In Lua, we need to use x:len() (or you can use string.len(x) like len(x) in Python):

x = 'foo bar'
x.len()  --> this is an error
x:len()  --> works without errors

x is of type string, when we use x.len(), if it does not exist, lua will look at the metatable of x and find the __index key. The metatable of x is like this:

{
  __index = {
    byte = <function 1>,
    char = <function 2>,
    dump = <function 3>,
    find = <function 4>,
    format = <function 5>,
    gmatch = <function 6>,
    gsub = <function 7>,
    len = <function 8>,
    lower = <function 9>,
    match = <function 10>,
    rep = <function 11>,
    reverse = <function 12>,
    sub = <function 13>,
    upper = <function 14>
  }
}

It will then execute the function corresponding to key len. However, since function len() requires an argument, so we get the above error.

If we instead use x:len(), we are implicitly pass x itself as the first argument:

A call v:name(args) is syntactic sugar for v.name(v,args), except that v is evaluated only once.

So we are actually using x.len(x). In fact, if we use x.len(x), it works perfectly fine.

The difference between defining function with colon and dot

As discussed in the above section, when we use colon operator in Lua, we are implicitly passing the object itself (called self) to the function.

Suppose we want to implement a string function to concatenate two strings, we can use both the dot and colon operators. But the function signature will be a little different:

local x = "hello world"

function string.add1(str1, str2)
  print(string.format("str1: %s, str2: %s", str1, str2))

  return string.format("%s%s", str1, str2)
end

function string:add2(s)
  print(string.format("str1: %s, str2: %s", self, s))

  return string.format("%s%s", self, s)
end


print(x:add1("abc"))
print(x:add2("abc"))

In this example, if we use dot operator, we must explicitly provide the two parameters. For the colon operator, the first string is implicitly provided as self, which you can access in the function body. So this function has only one parameter.

If you run the code, their result will be exactly the same.

• Lua: colon notation, ‘self’ and function definition vs. call: https://stackoverflow.com/q/32080972/6064933

The weirdness of lua REPL

If we have the following code in test.lua:

local x = 1
print(x)

it will print out 1, not nil. Expected result, nothing special.

In the lua REPL, if we type the following command:

> local x = 1
> print(x)

the output is surprisingly nil, not 1! Crazy, isn’t it?

The reason is that in lua REPL, each line is treated as a block. Basically, a block is an area where a variable is visible. Local variables only work in that block. For example, the following works:

> local x = 1; print(x)

This is because when lua execute a file, it treat the file as a block, so local variable x does not expire in the next line. Actually we can manually create a block in lua file with do ... end (see doc here).

It is a little weird, but that is how lua REPL works.

In lua, both 0 and empty string is true

Unlike Python, both 0 and empty string is considered true in lua, which is weird.

if '' then
  print('str is not empty')
else
  print('str is empty')
end

if 0 then
  print('zero is true')
else
  print('zero is false')
end

If you run the above code, you will get the following output:

str is not empty
zero is true

Lua table index starts from 1, not 0

This feature trips me really hard and I ended up debugging my code for nearly half of an hour. So if we have a table local a = {1, 2, 3}, the first element will be a[1] not a[0].

I have repeated forgotten this and made mistakes.

Why use parentheses around literal strings when call its method?

If you use a literal string and want to call string method, you need to wrap the string with parentheses. This is how the Lua syntax works.

• Right: print(("hello"):len())
• Wrong: print("hello":len())

• https://stackoverflow.com/q/48143667/6064933