Evaluate expressions within Vim

I put together my own solution using VimScript and the Mathematica script interface. This works for Linux; for other OSs modify as necessary.

First we create a file, say ~/bin/mathpipe.m which takes a Mathematica expression from standard input and prints the result to standard output. As far as I know, there's no built-in way to do this, please correct me if I'm wrong. I'm using Mathematica 10; consult the documentation to check the right hashbang line for you (it may have changed from MathematicaScript to wolfram).

#!/usr/local/bin/MathematicaScript -script

(* read standard input, one line at a time, evaluating each one.
 print the InputForm of the evaluated last line. *)

val = Input[];
ret = val;
While[val = Input[]; val =!= EndOfFile, ret = val];
Print[ret];

Do chmod +x ~/bin/mathpipe.m. We can run

$ echo "2+2" | ~/bin/mathpipe.m
4

Now, in our .vimrc we add one helper function:

function! s:get_visual_selection()
  " from http://stackoverflow.com/a/6271254/371334
  let [lnum1, col1] = getpos("'<")[1:2]
  let [lnum2, col2] = getpos("'>")[1:2]
  let lines = getline(lnum1, lnum2)
  let lines[-1] = lines[-1][: col2 - (&selection == 'inclusive' ? 1 : 2)]
  let lines[0] = lines[0][col1 - 1:]
  "return join(lines, "\n")
  return lines
endfunction

This gets the current (or most recent) visual selection and returns it as a list of string values. With this we can define a command for echoing the evaluation of a selected expression (also goes in .vimrc):

function! Mathpipe1()
  let mathpipe = s:get_visual_selection()
  call writefile(mathpipe, '/tmp/mathpipein')
  silent !cat /tmp/mathpipein | ~/bin/mathpipe.m
endfunction

Add this line to map the function to the <leader>m key sequence:

xnoremap <leader>m :<c-h><c-h><c-h><c-h><c-h>call Mathpipe1()<CR>

So now if we visually select some text in Vim that is a Mathematica expression and hit <leader>m (usually \m), the result is shown in the output area on the ex command line. Multi-line selections work too.

If we want the result printed in the current Vim buffer right below what we selected, we can do that too:

function! Mathpipe2()
  let mathpipe = s:get_visual_selection()
  call writefile(mathpipe, '/tmp/mathpipein')
  silent !cat /tmp/mathpipein | ~/bin/mathpipe.m > /tmp/mathpipeout
  normal `>
  r /tmp/mathpipeout
endfunction
xnoremap <leader>M :<c-h><c-h><c-h><c-h><c-h>call Mathpipe2()<CR>

With this, we can select text, hit <leader>M and the evaluation is printed on the next line.