Old Lisp programs

I found an old chess program that Mark Watson (somehow I can’t find the original link anymore, my Gist mirroring it is here). I was wondering what it would take to get it to run again, and I was shocked to find out that when I copy-pasted it in, loaded the file, it … it just worked !!

This is not supposed to happen, really. Code from 28 years ago isn’t supposed to just run. I don’t need to talk about how even Python code from 10 years ago would require some work to run.

Also notable is the lack of required scaffolding in terms of makefiles, build tools or anything of the sort. Yes, you could imagine breaking the code up, but it’s just 585 lines. Drop it in and load the code, and you’re good to go. No extensions, shims, nothing.

As proof, here is a short transcript of me playing terribly with it.

 

Advertisements

Common Lisp nits/tips/libraries: Iteration

Background

Every once in a while, I see a mention of how Common Lisp really does support “non-functional” styles, and then cl:loop is trotted in, with attendant examples that make your eyes glaze over.

As you might expect, this isn’t the only way to do it; there’s a “Lispier” way, if you will (none of this is new, the original manual/memo dates from 1989!).

Installing

CL-USER> (ql:quickload "iterate")
To load "iterate":
Load 1 ASDF system:
asdf
Install 1 Quicklisp release:
iterate
; Loading "iterate"
[package iterate]...........................
("iterate")

(while I’m here, a plug for sly instead of slime; you know you’re successfully connected when you see the [sly] Connected. Take this REPL, brother, and may it serve you well. message 😀)

Simple uses

For a really simple example, iter is not too different from loop, but still:

Basic for loop

CL-USER> (loop for i from 1 to 10 collect i)
(1 2 3 4 5 6 7 8 9 10)
CL-USER> (iter:iter
(for i from 1 to 10)
(collect i))
(1 2 3 4 5 6 7 8 9 10)

Collecting tuples

CL-USER> (loop
for x in '(a b c d)
for y in '(d e f g)
collect (list x y))
((A D) (B E) (C F) (D G))
CL-USER> (iter:iter
(for x in '(a b c d))
(for y in '(d e f g))
(collect (list x y)))
((A D) (B E) (C F) (D G))

Intermediate example

Here is an example (from the CL cookbook) of looping, with an auxiliary variable on which we have a terminating condition, with a combination of “doing something” and collecting something else, at the same time:

CL-USER> (loop for x from 1
for y = (* x 10)
while (< y 100) do (print (* x 5)) collect y) 5 10 15 20 25 30 35 40 45 (10 20 30 40 50 60 70 80 90)
CL-USER> (iter:iter
(for x upfrom 1)
(for y = (* x 10))
(while (< y 100)) (print (* x 5)) (collect y)) 5 10 15 20 25 30 35 40 45 (10 20 30 40 50 60 70 80 90)

Another example, though a bit contrived (there’s a one-liner to do this without using either of these two, but …)

CL-USER> (let ((s "alpha45"))
(loop for i from 0 below (length s)
for ch = (char s i)
when (find ch "0123456789" :test #'eql)
return ch) )
#\4
CL-USER> (let ((s "alpha45"))
(iter:iter
(for ch in-string s)
(finding ch such-that
(find ch "0123456789" :test #'eql))))
#\4

Misc cool stuff

Making modifications

I find it easier to “splice in” new changes to iter. This is another contrived example, but sort of shows what I mean:

CL-USER> (iter:iter
(for i from 1 to 10)
(collect i into nums)
(finally (return nums)))
(1 2 3 4 5 6 7 8 9 10)
CL-USER> (iter:iter
(for i from 1 to 10)
(collect i into nums)
(collect (* i i) into nums)
(finally (return nums)))
(1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100)

Natural iteration for different types

The (for ... in ...) gathering clause applies quite naturally to a great many types of structures.

CL-USER> (iter:iter
(for x in '(1 5 6))
(when (oddp x)
(collect x)))
(1 5)
CL-USER> (iter:iter
(for x in-vector #(1 5 6))
(when (oddp x)
(collect x)))
(1 5)
CL-USER> (iter:iter
(for (k v) in-hashtable (alexandria:alist-hash-table '((foo bar) (baz quux))))
(collect v))
((BAR) (QUUX))

Accessing previous values

CL-USER> (iter:iter
(for x from 1 to 10)
(for p-x previous x initially 0)
(collect (+ x p-x)))
(1 3 5 7 9 11 13 15 17 19)

Collecting all vs. unique values

CL-USER> (iter:iter
(for x in '(7 1 4 3 2 1 7 1 0))
(collect x))
(7 1 4 3 2 1 7 1 0)
CL-USER> (iter:iter
(for x in '(7 1 4 3 2 1 7 1 0))
(adjoining x))
(7 1 4 3 2 0)

Reductions

You can splice in a reduction step (counting, summing, maximizing, minimizing, etc.) in ad-hoc way.

This extremely contrived example is essentially equivalent to (reduce #'* '(1 2 3 4 5 6 7 8 9 10)), but hopefully you get the point:

CL-USER> (iter:iter
(for x from 1 to 10)
(reducing x by #'* ))
3628800 (22 bits, #x375F00)

Conclusion

YMMV, but iter seems to have (for me) a more uniform syntax, a few extra features, better comparability of clauses, and I personally prefer it to loop. If you’ve never used either, I’d recommend just sticking with the former.