The below obviously doesn't work and hence this question.
How do I correct the below code so that the value of somelist becomes '(("abc" . 123)) ?
(setq x "abc")
(setq y 123)
(setq somelist nil)
(add-to-list 'somelist '(x . y))
asked Jan 17, 2015 at 21:03
2 Answers
The general issue is that you need x and y to be evaluated before they get inserted in somelist. The issue with the quoted list (with ' as reader syntax) is that quote is a special form that does not evaluate its argument. According to the docstring:
(quote ARG)Return the argument, without evaluating it.
(quote x)yieldsx. Warning:quotedoes not construct its return value, but just returns the value that was pre-constructed by the Lisp reader...
Hence, you either need to backquote or use a function that evaluates the arguments.
Backquoting allows you to evaluate elements of a backquoted list selectively with the , syntax:
(setq x "x-val" y "y-val" z "z-val" somelist nil)
'(x y z) ; => (x y z)
`(x ,y z) ; => (x "y-val" z)
(add-to-list 'somelist `(x y ,z)) ; => ((x y "z-val"))
Alternately, you can use cons (as @tarsius suggests in his answer) or, for an arbitrary number of elements, list:
(add-to-list 'somelist (cons x y)) ; => (("x-val" . "y-val"))
(setq somelist nil) ; reset
(add-to-list 'somelist (list x y z)) ; => (("x-val" "y-val" "z-val"))
Which to use depends on what you need to do with the elements.
-
Sorry, I have read this and other answers over and over, but am still clueless how to do that. That's because everywhere lists are used, but what if I don't have lists? What if I have just the
'xfrom your answer. No lists,'xis an argument passed to a function (like(foo 'x)), and inside the function I need both the'x(to mutate it), and the value ofxwhich is"x-val"in your code. So, no lists. Then, how do I get the value of'xout of it (i.e. evaluate it)? I tried a bunch of combinations with backticks, commas, and@s, to no avail.– Hi-AngelAug 9, 2021 at 20:38 -
I found out: one needs
eval. So, calling(eval 'x)will return"x-val". Worth mentioning in the answer I think, because similar questions are marked as duplicates of this page.– Hi-AngelAug 10, 2021 at 6:29
Do not quote the cons cell, because quoted expressions are not evaluated. That's exactly why one quotes - to prevent evaluation. But that's not what you want, so don't.
Instead use the form that creates a cons cell from two evaluated values, its arguments.
(cons x y)
Of course you can also quasiquote but that doesn't really make sense here, and looks worse. Only use ` and , when that improves readability, i.e. when doing something more complex than constructing a cons cell or adding an atom or list at the beginning of some existing list.
Using quasiquoting it would look like this:
`(,x . ,y)
Which is worse because it uses additional syntax which isn't required at all in this case and obfuscates that cons is being used.
-
3Good point on
consing. Quasiquoting strikes me as being more about fine-grained control of the list contents rather than readability, but I agree that the use-case makes sense forcons.– Dan ♦Jan 17, 2015 at 23:02 -
Thank you for your answer. That was a great TIL moment for me. I was blindly putting quotes before lists and cons. Jan 17, 2015 at 23:02
-
1@Dan, well yes - and no. Quasiquoting cannot do anything that you couldn't do with just
cons,list, andnconc. Except being prettier. It's syntactic sugar which is useful when you need "fine-grained control of the list contents" (as in "doing something more complex than adding an atom or list at the beginning"). And the added benefit of using that syntactic sugar is: readability. Quasiquoting doesn't give you additional more fine-grained control - it just allows you to do the same thing with fewer bugs in the initial attempt. :-)– tarsiusJun 22, 2015 at 18:23
`(,x . ,y).