wow, I’d never thought about this one!

“the docs imply that if the last argument isn’t a list that the result would be an improper list” It is producing an improper list.

I’m not clear what an improper list is

I thought (1 2 . 3)

Does ( . 3) evaluate to 3 ?

An improper list is one whose final cons cell doesn’t have null in the cdr.

(. 3) is a read error

And finding all the exciting identity functions in Racket is a fun game.

Yes it is a read error but I don’t know how to express (append '() 1 )

well, turns out it’s 1 :smile:

Right, there’s no read syntax that just disappears like that

Should a do a pr for ‘append’in the racket reference? Something like;

> Note: (append '() n) is an identity function.

I think that would be more confusing than helpful.

How about > Note: Appending to an empty list e.g. (append '() n) is an identity function and will return the value of n, not a list.

Did this bite you?

Yes. I was wanting to use a list to record a series of actions in a little pict-draw thing I am making as a learning exercise. (Each action is drawing a circle or rectangle with parameters) I came across this by accident

In any case I think I should be using an array…

Probably the issue is that the last element was supposed to be a list but was not, rather than (λ (x) (append '() x)) being the identity function?

(also note that (append '() a-list) does of course return a-list)

yes I made that mistake too

What I wanted (append commands-list new-command) '()=> (('circle 20 30 10)) What I got ('() 'circle 20 30 10) :rolling_on_the_floor_laughing:

Heh :slightly_smiling_face:

Looks like a fork of racket-paint? :slightly_smiling_face: