 # How do I use an operation on every element of a list?

Hello there, prolog newbie here!

Simple question, probably with a simple answer:
How do I use an operation on every element of a list?

For example:
Lets say I have a predicate adding 4 to the given variable:

`plusFour(X,Y) :- Y is X+4.`

And now I want to use that on the list [1,2,3], so that it outputs [4,5,6].
How exactly would I do that?

Thanks in advance! ``````1 ?- [user].
|: plusFour(X,Y) :- Y is X+4.
|:
% user://1 compiled 0.00 sec, 1 clauses
true.

2 ?- maplist(plusFour, [1,2,3,4], Z).
Z = [5, 6, 7, 8].
``````

Is there a way to do it without a predefined predicate (i.e. maplist)?

`maplist/3` is written in Prolog. You can see the code here: https://github.com/SWI-Prolog/swipl-devel/blob/deb87d4b884c7e68349a8c2ecbe49615c995cc5e/library/apply.pl#L177

You can even generalize it to `plusN`:

``````?- assert(plusN(N, X, Y) :- Y is X + N).
true.

?- maplist(plusN(4), [1,2,3,4], Z).
Z = [5, 6, 7, 8].
``````

I think you’re looking for library(yall). It’s autoloaded, so you don’t need to explicitly load it (by use_module or :- [library(yall)]). That is:

``````?- Xs=[1,2,3],N=4,maplist({N}/[X,Y]>>(Y is X+N),Xs,Ys).
...
Ys = [5, 6, 7].

``````
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