Note how the ``let fn = filename`` statement ensures that ``filename`` is evaluated only once

[greenfork]

The title is the last sentence from the 
[https://nim-lang.org/docs/tut2.html#templates](https://nim-lang.org/docs/tut2.html#templates)
 and it refers to the following snippet of code:
    
    
    template withFile(f: untyped, filename: string, mode: FileMode,
                      body: untyped): typed =
      let fn = filename
      var f: File
      if open(f, fn, mode):
        try:
          body
        finally:
          close(f)
      else:
        quit("cannot open: " & fn)
    
    withFile(txt, "ttempl3.txt", fmWrite):
      txt.writeLine("line 1")
      txt.writeLine("line 2")
    
    
    Run

I can see `fn` is used in `if open(f, fn, mode):` and in `quit("cannot open: " 
& fn)`. But why would it be evaluated twice if we did not use `let`? Aren't 
arguments for template immutable?