vim/README_VIM9.md

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# What is Vim9?

This is a new syntax for Vim script that was introduced with Vim 9.0.
It intends making Vim script faster and better.


# Why Vim9?

## 1. FASTER VIM SCRIPT

The third item on the poll results of 2018, after popup windows and text
properties, both of which have been implemented, is faster Vim script.
So how do we do that?

I have been throwing some ideas around, and soon came to the conclusion
that the current way functions are called and executed, with
dictionaries for the arguments and local variables, is never going to be
very fast.  We're lucky if we can make it twice as fast.  The overhead
of a function call and executing every line is just too high.

So what then?  We can only make something fast by having a new way of
defining a function, with similar but different properties of the old
way:
* Arguments are only available by name, not through the a: dictionary or
  the a:000 list.
* Local variables are not available in an l: dictionary.
* A few more things that slow us down, such as exception handling details.

I Implemented a "proof of concept" and measured the time to run a simple
for loop with an addition (Justin used this example in his presentation,
full code is below):

``` vim
  let sum = 0
  for i in range(1, 2999999)
    let sum += i
  endfor
```

| how     | time in sec |
| --------| -------- |
| Vim old | 5.018541 |
| Python  | 0.369598 |
| Lua     | 0.078817 |
| LuaJit  | 0.004245 |
| Vim new | 0.073595 |

That looks very promising!  It's just one example, but it shows how much
we can gain, and also that Vim script can be faster than builtin
interfaces.

LuaJit is much faster at Lua-only instructions.  In practice the script would
not do something useless counting, but change the text.  For example,
reindent all the lines:

``` vim
  let totallen = 0
  for i in range(1, 100000)
    call setline(i, '    ' .. getline(i))
    let totallen += len(getline(i))
  endfor
```

| how     | time in sec |
| --------| -------- |
| Vim old | 0.578598 |
| Python  | 0.152040 |
| Lua     | 0.164917 |
| LuaJit  | 0.128400 |
| Vim new | 0.079692 |

[These times were measured on a different system by Dominique Pelle]

The differences are smaller, but Vim 9 script is clearly the fastest.
Using LuaJIT is only a little bit faster than plain Lua here, clearly the call
back to the Vim code is costly.

How does Vim9 script work?  The function is first compiled into a sequence of
instructions.  Each instruction has one or two parameters and a stack is
used to store intermediate results.  Local variables are also on the
stack, space is reserved during compilation.  This is a fairly normal
way of compilation into an intermediate format, specialized for Vim,
e.g. each stack item is a typeval_T.  And one of the instructions is
"execute Ex command", for commands that are not compiled.


## 2. DEPRIORITIZE INTERFACES

Attempts have been made to implement functionality with built-in script
languages such as Python, Perl, Lua, Tcl and Ruby.  This never gained much
foothold, for various reasons.

Instead of using script language support in Vim:
* Encourage implementing external tools in any language and communicate
  with them.  The job and channel support already makes this possible.
  Really any language can be used, also Java and Go, which are not
  available built-in.
* No priority for the built-in language interfaces.  They will have to be kept
  for backwards compatibility, but many users won't need a Vim build with these
  interfaces.
* Improve the Vim script language, it is used to communicate with the external
  tool and implements the Vim side of the interface.  Also, it can be used when
  an external tool is undesired.

Altogether this creates a clear situation: Vim with the +eval feature
will be sufficient for most plugins, while some plugins require
installing a tool that can be written in any language.  No confusion
about having Vim but the plugin not working because some specific
language is missing.  This is a good long term goal.

Rationale: Why is it better to run a tool separately from Vim than using a
built-in interface and interpreter?  Take for example something that is
written in Python:
* The built-in interface uses the embedded python interpreter.  This is less
  well maintained than the python command.  Building Vim with it requires
  installing developer packages.  If loaded dynamically there can be a version
  mismatch.
* When running the tool externally the standard python command can be used,
  which is quite often available by default or can be easily installed.
* The built-in interface has an API that is unique for Vim with Python. This is
  an extra API to learn.
* A .py file can be compiled into a .pyc file and execute much faster.
* Inside Vim multi-threading can cause problems, since the Vim core is single
  threaded.  In an external tool there are no such problems.
* The Vim part is written in .vim files, the Python part is in .py files, this
  is nicely separated.
* Disadvantage: An interface needs to be made between Vim and Python.
  JSON is available for this, and it's fairly easy to use.  But it still
  requires implementing asynchronous communication.


## 3. BETTER VIM SCRIPT

To make Vim faster a new way of defining a function needs to be added.
While we are doing that, since the lines in this function won't be fully
backwards compatible anyway, we can also make Vim script easier to use.
In other words: "less weird".  Making it work more like modern
programming languages will help.  No surprises.

A good example is how in a function the arguments are prefixed with
"a:". No other language I know does that, so let's drop it.

Taking this one step further is also dropping "s:" for script-local variables;
everything at the script level is script-local by default.  Since this is not
backwards compatible it requires a new script style: Vim9 script!

To avoid having more variations, the syntax inside a compiled function is the
same as in Vim9 script.  Thus you have legacy syntax and Vim9 syntax.

It should be possible to convert code from other languages to Vim
script.  We can add functionality to make this easier.  This still needs
to be discussed, but we can consider adding type checking and a simple
form of classes.  If you look at JavaScript for example, it has gone
through these stages over time, adding real class support and now
TypeScript adds type checking.  But we'll have to see how much of that
we actually want to include in Vim script.  Ideally a conversion tool
can take Python, JavaScript or TypeScript code and convert it to Vim
script, with only some things that cannot be converted.

Vim script won't work the same as any specific language, but we can use
mechanisms that are commonly known, ideally with the same syntax.  One
thing I have been thinking of is assignments without ":let".  I often
make that mistake (after writing JavaScript especially).  I think it is
possible, if we make local variables shadow commands.  That should be OK,
if you shadow a command you want to use, just rename the variable.
Using "var" and "const" to declare a variable, like in JavaScript and
TypeScript, can work:


``` vim
def MyFunction(arg: number): number
   var local = 1
   var todo = arg
   const ADD = 88
   while todo > 0
      local += ADD
      todo -= 1
   endwhile
   return local
enddef
```

The similarity with JavaScript/TypeScript can also be used for dependencies
between files.  Vim currently uses the `:source` command, which has several
disadvantages:
*   In the sourced script, is not clear what it provides.  By default all
    functions are global and can be used elsewhere.
*   In a script that sources other scripts, it is not clear what function comes
    from what sourced script.  Finding the implementation is a hassle.
*   Prevention of loading the whole script twice must be manually implemented.

We can use the `:import` and `:export` commands from the JavaScript standard to
make this much better.  For example, in script "myfunction.vim" define a
function and export it:

``` vim
vim9script  " Vim9 script syntax used here

var local = 'local variable is not exported, script-local'

export def MyFunction()  " exported function
...

def LocalFunction() " not exported, script-local
...
```

And in another script import the function:

``` vim
vim9script  " Vim9 script syntax used here

import MyFunction from 'myfunction.vim'
```

This looks like JavaScript/TypeScript, thus many users will understand the
syntax.

These are ideas, this will take time to design, discuss and implement.
Eventually this will lead to Vim 9!


## Code for sum time measurements

Vim was build with -O2.

``` vim
func VimOld()
  let sum = 0
  for i in range(1, 2999999)
    let sum += i
  endfor
  return sum
endfunc

func Python()
  py3 << END
sum = 0
for i in range(1, 3000000):
  sum += i
END
  return py3eval('sum')
endfunc

func Lua()
  lua << END
    sum = 0
    for i = 1, 2999999 do
      sum = sum + i
    end
END
  return luaeval('sum')
endfunc

def VimNew(): number
  var sum = 0
  for i in range(1, 2999999)
    sum += i
  endfor
  return sum
enddef

let start = reltime()
echo VimOld()
echo 'Vim old: ' .. reltimestr(reltime(start))

let start = reltime()
echo Python()
echo 'Python: ' .. reltimestr(reltime(start))

let start = reltime()
echo Lua()
echo 'Lua: ' .. reltimestr(reltime(start))

let start = reltime()
echo VimNew()
echo 'Vim new: ' .. reltimestr(reltime(start))
```

## Code for indent time measurements

``` vim
def VimNew(): number
  var totallen = 0
  for i in range(1, 100000)
    setline(i, '    ' .. getline(i))
    totallen += len(getline(i))
  endfor
  return totallen
enddef

func VimOld()
  let totallen = 0
  for i in range(1, 100000)
    call setline(i, '    ' .. getline(i))
    let totallen += len(getline(i))
  endfor
  return totallen
endfunc

func Lua()
  lua << END
    b = vim.buffer()
    totallen = 0
    for i = 1, 100000 do
      b[i] = "    " .. b[i]
      totallen = totallen + string.len(b[i])
    end
END
  return luaeval('totallen')
endfunc

func Python()
  py3 << END
cb = vim.current.buffer
totallen = 0
for i in range(0, 100000):
  cb[i] = '    ' + cb[i]
  totallen += len(cb[i])
END
  return py3eval('totallen')
endfunc

new
call setline(1, range(100000))
let start = reltime()
echo VimOld()
echo 'Vim old: ' .. reltimestr(reltime(start))
bwipe!

new
call setline(1, range(100000))
let start = reltime()
echo Python()
echo 'Python: ' .. reltimestr(reltime(start))
bwipe!

new
call setline(1, range(100000))
let start = reltime()
echo Lua()
echo 'Lua: ' .. reltimestr(reltime(start))
bwipe!

new
call setline(1, range(100000))
let start = reltime()
echo VimNew()
echo 'Vim new: ' .. reltimestr(reltime(start))
bwipe!
```