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In this episode of SKILL for the Skilled I'll introduce a feature of the let primitive that Scheme programmers will find familiar, but other readers may have never seen before. The feature is called named let, and I'll show you how to use it to sum the numbers in a given list.
(defun sumlist_7a (numbers)
(REPEAT (plus sum_so_far (car rest))
Testing the function
(sumlist_7a '(1 2 3 4 5))
|sumlist_7a((1 2 3 4 5))
||(REPEAT 0 (1 2 3 4 5))
|||(REPEAT 1 (2 3 4 5))
||||(REPEAT 3 (3 4 5))
|||||(REPEAT 6 (4 5))
||||||(REPEAT 10 (5))
|||||||(REPEAT 15 nil)
|||||||REPEAT --> 15
||||||REPEAT --> 15
|||||REPEAT --> 15
||||REPEAT --> 15
|||REPEAT --> 15
||REPEAT --> 15
|sumlist_7a --> 15
Equivalent to labels
The named let is more or less equivalent to a declaration and call of a local function as if by using labels. If you recall, this is exactly what was shown in sumlist_3b in SKILL for the Skilled: Many Ways to Sum a List (Part 3).
(defun sumlist_3b (numbers) (labels ((sum (sum_so_far rest) (if rest (sum (plus (car rest) sum_so_far) (cdr rest)) sum_so_far))) (sum 0 numbers)))
If you trace sumlist_3b and sum you'll see that it executes pretty much the same thing as sumlist_7a.
(sumlist_3b '(1 2 3 4 5))
|sumlist_3b((1 2 3 4 5))
||(sum 0 (1 2 3 4 5))
|||(sum 1 (2 3 4 5))
||||(sum 3 (3 4 5))
|||||(sum 6 (4 5))
||||||(sum 10 (5))
|||||||(sum 15 nil)
|||||||sum --> 15
||||||sum --> 15
|||||sum --> 15
||||sum --> 15
|||sum --> 15
||sum --> 15
|sumlist_3b --> 15
Illusion of jumping to the top
The illusion (or abstraction) presented by the named let is that of being able to jump back to the top of the let form, and evaluate it again with different initialization values.
Consider this simple let example.
(println (list a b))
(when (plusp a)
(loop (sub1 a)
Doesn't work in traditional SKILL
If you try to evaluate a named let in traditional SKILL (e.g., with a .il file extension), you'll get an error something like the following, which basically means that SKILL let expects a list as its first operand and you have given the symbol loop instead.
*Error* let: local bindings must be a proper list - loop
let is syntactic sugar for lambda
In SKILL++ the normal let has the same semantics as calling an unnamed function with particular parameter values. For example:
(let ((a X)
(expr1 a b)
(expr2 b c))
(lambda (a b c) ...)
(funcall (lambda (a b c)
(expr1 a b)
(expr2 b c))
Named let and tail-call optimization
The illusion of jumping back to the top in sort of a goto fashion is indeed what happens if tail-call-elimination is enabled via the optimizeTailCall status flag explained in SKILL for the Skilled: Many Ways to Sum a List (Part 4).
In this post I've shown some examples of how to used the named let construct of SKILL++. This construct converts the conventional let into a loop --- a loop which can be repeated by calling the label as a function, providing the next iteration's variable values.
More to come
In upcoming posts we'll continue to survey the SKILL++ language using the example of summing a list.
SKILL for the Skilled: Many Ways to Sum a List (Part 3) SKILL for the Skilled: Many Ways to Sum a List (Part 4) Scheme In particular see the discussion of named let