Python-like context managers and

[wizzardx]

Hi there.

I like the Python RAII pattern of "with" statements, alongside context managers.

eg, this is nice:
    
    
    with open("test.txt") as f:
        print f.read()
    

Closest thing to that I could find was the "withFile" example over in the 
second Nim tut.

So I've tried to adapt that example a bit, and ended up with this:
    
    
    template withAs(x: typed, y: untyped,
                    body: untyped): typed =
      var x2 = x
      var y = x2.enter()
      try:
        body
      finally:
        exit(x2)
    
    proc enter(f: File): File =
        echo "Entering context manager"
        return f
    
    proc exit(f: File) =
        echo "Leaving context manager"
        f.close()
    
    open("test.txt").withAs(txt):
      echo txt.readLine()
    

So when I run that, I get this output:
    
    
    Entering context manager
    HOW ARE YOU GENTLEMEN
    Leaving context manager
    

So it looks like things are working.

Is "withAs" (and context managers) an existing well-known Nim pattern?

Would be nice to be able to eg, do "nim install contextlib", and then have 
"withAs" as well as some other useful things.

Comments?