Re: on call by reference

Scheme is call-by-value.  Python is call-by-value.  Java is
call-by-value.  C is call-by-value.  In none of those languages can you
write a procedure that assigns a variable used as an argument to that
procedure.  Read that carefully.  "Assigns" means to change what the
variable references.  "Assigns" does _not_ mean to make modifications to
the thing that the variable references.  In Python terms "x = 4" is an
assignment to "x", but "x[1] = 4" is not an assignment to "x" because
afterwards "x" still references the same object.

English is a bit slippery here, so people often find this distinction
confusing.  To make it clearer ask yourself what it means to "change
your socks".  Does it mean you took your socks off and put on a
different pair?  Or does it mean you dyed the socks you are wearing a
different color?

A call by value language doesn't let you put on a different pair of
socks, it only lets you dye them a different color.

- Alan