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tree-walkers

Replacement of car, cdr and consorts by one operator, walk, of integer arguments. (walk -1) is car, (walk 1) cdr, (walk k) is (iterate cdr k) and (walk -k) (iterate car (- k)). These operaters can (and will) be composed, so, for example, (walk -1 2) is (o (walk -1) (walk 2)), in other words caddr.

The more arguments you give to walk, the deeper you can dig into a tree, i.e. a nested pseudolist.

If you like, you can interprete the arguments of walk as a path for traverseing the tree. For compatibility with the c*r routines, you interprete the arguments from walk from right to left. It might be more convenient, to do it left to right; walk* does this.

The routines

tree-walkers

[procedure] (tree-walkers [sym])

documentation-procedure; returns the list of exported symbols if called with no symbol, otherwise the signature of that exported symbol sym.

iterate

[procedure] (iterate fn k)
[procedure] (iterate fn k start)

The first version creates a new function, composing fn k times. The second applies this new function to a start value.

walk

[procedure] (walk . ks)

Creates a procedure to dig into a tree argument, by transforming the integer arguments, ks, from right to left into corresponding car's and cdr's to be successively applied to the tree.

Here, car is represented by -1, cdr by +1

walk*

[procedure] (walk* . ks)

The same as walk, but accessing the ks from left to right.

Requirements

none

Example

 

(import tree-walkers)

(iterate add1 5 1)
;-> 6

(define flat '(0 1 2 3))
  (iterate cdr 2 flat)
  ; -> '(2 3)
  ((walk -1 2) flat)
  ; -> 2
  (caddr flat)
  ; -> 2
  ((walk 2) flat)
  ; -> '(2 3)
  (cddr flat)
  ; -> '(2 3)

(define deep '(0 (1 (2) 3) . 4))
  ((walk* 1 -1 1 -2) deep)
  ; -> 2
  ((walk 2) deep)
  ; -> 4
  )

Last update

Aug 09, 2022

Author

Juergen Lorenz

License

 
Copyright (c) 2022, Juergen Lorenz
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Version History
1.0
initial version