eintr: Using set
1.9.1 Using ‘set’
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To set the value of the symbol ‘flowers’ to the list ‘'(rose violet
daisy buttercup)’, evaluate the following expression by positioning the
cursor after the expression and typing ‘C-x C-e’.
(set 'flowers '(rose violet daisy buttercup))
The list ‘(rose violet daisy buttercup)’ will appear in the echo area.
This is what is _returned_ by the ‘set’ function. As a side effect, the
symbol ‘flowers’ is bound to the list; that is, the symbol ‘flowers’,
which can be viewed as a variable, is given the list as its value.
(This process, by the way, illustrates how a side effect to the Lisp
interpreter, setting the value, can be the primary effect that we humans
are interested in. This is because every Lisp function must return a
value if it does not get an error, but it will only have a side effect
if it is designed to have one.)
After evaluating the ‘set’ expression, you can evaluate the symbol
‘flowers’ and it will return the value you just set. Here is the
symbol. Place your cursor after it and type ‘C-x C-e’.
flowers
When you evaluate ‘flowers’, the list ‘(rose violet daisy buttercup)’
appears in the echo area.
Incidentally, if you evaluate ‘'flowers’, the variable with a quote
in front of it, what you will see in the echo area is the symbol itself,
‘flowers’. Here is the quoted symbol, so you can try this:
'flowers
Note also, that when you use ‘set’, you need to quote both arguments
to ‘set’, unless you want them evaluated. Since we do not want either
argument evaluated, neither the variable ‘flowers’ nor the list ‘(rose
violet daisy buttercup)’, both are quoted. (When you use ‘set’ without
quoting its first argument, the first argument is evaluated before
anything else is done. If you did this and ‘flowers’ did not have a
value already, you would get an error message that the ‘Symbol's value
as variable is void’; on the other hand, if ‘flowers’ did return a value
after it was evaluated, the ‘set’ would attempt to set the value that
was returned. There are situations where this is the right thing for
the function to do; but such situations are rare.)
Info Catalog
eintr: set & setq
eintr: Using setq