Arc Forum | You will need to use my fork (rather than base ar) for two reasons:1) I fixed se...

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2 points by Pauan 4925 days ago | link | parent

You will need to use my fork (rather than base ar) for two reasons:

1) I fixed setforms so it passes all the args to sref, rather than only the first.

2) My fork has an awkward and inefficient (but working) form of defcall.

I hope to get both those things into ar soon, but for now, just use my fork.

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After loading up "lib/slices.arc", you should be able to both get and set slices of a list:

  (= foo '(1 2 3 4 5))

  (foo 2 5)              -> (3 4 5)     ; this is the slice
  (= (foo 2 5) '(6 7 8)) -> (1 2 6 7 8) ; foo is now this

It also supports negative start and end indices, like Python. So -1 is the last element, -2 is the second-to-last, etc:

  (foo -3 -1)             -> (6 7)
  (= (foo -3 -1) '(9 10)) -> (1 2 9 10 8)

And you can use nil as either the start or end to mean 0 and (len foo) respectively:

  (foo 2 nil) -> (9 10 8)
  (foo nil 2) -> (1 2)

I also changed ordinary element notation so it understands negative indices:

  (foo -1) -> 8
  (foo -2) -> 10

In case it wasn't obvious from the examples above, slices use half-open notation, just like Arc's cut and Python slices. In fact, Arc slices should be exactly the same as Python slices, with two exceptions:

1) If the start index is higher than the end index, my implementation will throw an error:

  (foo 5 3) -> error: start index must be smaller than the end index

In Python, as near as I can tell, that will always return an empty list. I'm not entirely sure how to handle that situation, so for now it just throws an error. If you can think of a useful behavior for that, please share.

Lastly, you can now delete elements/slices by assigning a slice to nil:

  (= (foo 0 1) nil)   -> (2 9 10 8)
  (= (foo 2 nil) nil) -> (2 9)

2) However, you cannot set the entire list to nil:

  (= (foo nil nil) nil) -> error: cannot set the entire list to nil

So the idiomatic Python way of using `del foo[:]` won't work. Instead, you'll have to set the variable to nil:

  (= foo nil)

This is because of the difference between arrays (in Python) and lists (in Arc). I can, of course, special-case this with the `del` operator, but I haven't done so yet.