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3 points by akkartik 2129 days ago | link | parent

In my example above, making an empty list truthy would cause this change:

    (def map1 (f xs)
    "Returns a list containing the result of function 'f' applied to every element of 'xs'."
   -  (if xs
   +  (if (~empty? xs)
        (cons (f car.xs)
              (map1 f cdr.xs))))
We can argue how important this is, but disambiguation does definitely make Arc programs less terse.


3 points by shawn 2129 days ago | link

Two points.

- the assumption baked into this argument is that cdr of an empty list returns an empty list. Switching nil to #f and letting empty list be truthy avoids this problem.

- Good names are important. ~empty? isn't really a fair characterization. Lumen uses (some? xs). There is also another way: Update `no` to be (or (is x nil) (empty x)), and then use (~no xs).

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2 points by akkartik 2129 days ago | link

First option makes sense.

Second option, part a: I actually find `~empty?` clearer than `some` in this case. Also `some` means something different in Arc.

Second option, part b: wait, then `(if xs ...)` would sometimes not do the opposite of `(if (no xs) ...)`!

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2 points by rocketnia 2128 days ago | link

For what it's worth, my approach here is pattern-matching. In Lathe Comforts for Racket I implement a macro `expect` which expands to Racket's `match` like so:

  (expect subject pattern else
    then)
  ->
  (match subject
    [pattern then]
    [_ else])
If Arc came with a similar pattern-matching DSL and `expect`, we could write this:

  (def map1 (f xs)
    (expect xs (cons x xs) ()
      (cons (f x) (map1 f xs))))
The line "expect xs (cons x xs) ()" conveys "If xs isn't a cons cell, finish with an empty list. Otherwise, proceed with x and xs bound to its car and cdr."

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