Ever wanted a simple calculator within Vim? Since vimscript is already a fully featured programming language, you've already got it! For instance you can type :echo 2 + 5 in normal mode and get the result 7. You can also enter Ex-mode by typing Q in normal mode. In Ex-mode you can type commands without first typing :, however you still have to type echo to print the result of expressions. When in insert mode you can also press <c-r>= (an equals sign should appear in the bottom left corner) followed by a vimscript expression. This inserts the result of the expression at the current cursor position.

To get a proper read–eval–print loop (REPL) for vimscript expressions, I've added the following function to my .vimrc:

function! Repl()
  while 1
    let expr = input('> ', '', 'expression')
    if expr == 'q' | break | endif
    if expr != ''
      echo "\n"
      if expr =~ '='
        execute 'let ' . expr
      else
        let ans = eval(expr)
        echo string(ans)
      endif
    endif
  endwhile
endfunction
nnoremap <leader>c :call Repl()<cr>

You start it by pressing <leader>c in normal mode (or :call Repl()). I use space as my leader (let mapleader = " "), so by pressing space followed by c a prompt displays at the bottom of the window. To exit the REPL, you type q and then press enter. Any expression you type while in the REPL is evaluated. The result of the expression is printed and saved in the ans-variable, so that it can be reused in the next expression.

> 25 - 5
20
> ans * 5 / 2
50
> ans / (2 + 3)
10

If the input contains an equals sign it is interpreted as a let-command. This can be used to easily define variables:

> a = 2
> b = 4
> c = pow(a, b)
> c
16.0

When Vim is compiled with floating point support (:echo has('float') returns 1), you can also do floating point arithmetic.

> 3 / 2
1
> 3 / 2.0
1.5

The following mathematical functions are built into vimscript (when compiled with floating point support):

abs() trunc() floor() ceil() round() float2nr()
fmod() pow() sqrt() exp() log() log10()
sin() cos() tan() sinh() cosh() tanh()
asin() acos() atan() atan2()

That's about it. It's pretty simple, but it can be quite useful when you need to do a couple of calculations.