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The Magnificent Sevenby Michael Foguscreating a Lisp variant in seven forms
Who am I?Michael FogusSoftware Programmer
12 years experienceLisp, C, CLIPS, Prolog, C++, Java, Jess, Python, Scala, ClojureCo-author of The Joy of Clojure@fogus on the Intertweets
Lisp
HistoryJohn McCarthy
carconsatomquote
cdrcondeq
1958Massachusetts Institute of Technology (MIT)IBM 704 (origin of car and cdr)Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I[1]
[1] http://www-formal.stanford.edu/jmc/recursive.html
Lisp InnovationsDynamic typesGarbage collectionif-then-else (via cond)Tree data structuresHomoiconicity...
McCarthy'sMagnificent Seven
McCarthy's Seven[2]
Had
label and lambda, dynamic scoping [3], lists (kinda)
Didn't Have
closures, macros, numbers
[2] paulgraham.com/rootsoflisp.html[3] github.com/fogus/lithp
Building from parts(label and (lambda (and_x and_y) (cond (and_x (cond (and_y t) (t nil))) (t nil))))
(and t nil);=> nil
(and t t);=> t
Building from parts (continued)(label list (lambda (x y) (cons x (cons y (quote ())))))
(def append (lambda (append_x append_y) (cond ((null append_x) append_y) (t (cons (car append_x) (append (cdr append_x) append_y))))))
(append (list 1 2) (list 3 4));=> (1 2 3 4)
You can see where this is going...
Meta-circular Evaluator FTW(def eval (lambda (expr binds) (cond ((atom expr) (assoc expr binds)) ((atom (car expr)) (cond ((eq (car expr) (quote quote)) (cadr expr)) ((eq (car expr) (quote atom)) (atom (eval (cadr expr) binds))) ((eq (car expr) (quote eq)) (eq (eval (cadr expr) binds) (eval (caddr expr) binds))) ((eq (car expr) (quote car)) (car (eval (cadr expr) binds))) ((eq (car expr) (quote cdr)) (cdr (eval (cadr expr) binds))) ((eq (car expr) (quote cons)) (cons (eval (cadr expr) binds) (eval (caddr expr) binds))) ((eq (car expr) (quote cond)) (eval-cond (cdr expr) binds)) (t (eval (cons (assoc (car expr) binds) (cdr expr)) binds)))) ((eq (caar expr) (quote def)) (eval (cons (caddar expr) (cdr expr)) (cons (list (cadar expr) (car expr)) binds))) ((eq (caar expr) (quote lambda)) (eval (caddar expr) (append (pair (cadar expr) (eval-args (cdr expr) binds)) binds))) (t (assoc expr binds)))))
note: not all code shown
Breathtaking!
Fojure
Feajures7 core funcjions and 2 spejial fjormsSymboljLajySingle immutable data strucjureFuncjionalLexical ScopjureClosures
The Magnificent Sevenfndef
No Need For car and cdr(def CAR (fn [[h & _]] h))(def CDR (fn [[_ & t]] t))
(CAR [1 2 3]);=> 1
(CDR [1 2 3]);=> (2 3)
Wait! What?!?I never mentioned anything about vectors
No Need For cons(def CONS (fn [h t] (fn ([] h) ([_] t))))
(CONS 1 (CONS 2 (CONS 3 nil)));=> #<user$CONS$fn__85 user$CONS$fn__85@445e228>
A closure over the head and tail
A good start...
Closure:A Poor Man's Object
Closure Dissection(def CONS (fn [h t] (fn ([] h) ([_] t))))
A closureheadtailA closure is an Object with a single method .apply(...)
The New first and rest(def FIRST (fn [s] (s)))(def REST (fn [s] (s nil)))
(def a (CONS 1 (CONS 2 (CONS 3 nil))))
(FIRST a);=> 1
(REST a);=> #<user$CONS$fn__85 user$CONS$fn__85@375e293a>
(FIRST (REST a));=> 2
Saplings1. 1 =2. 2 if3. 3 '4. 4 :keywords
Yet Another CONS(def CONS
(fn [h t] (fn [d] (if (= d :type) 'CONS (if (= d :head) h t)))))
(def $ (CONS 'a (CONS 'b nil)));=> #<user$CONS$fn__4 user$CONS$fn__4@61578aab>
($ :type);=> CONS
($ :head);=> a
(($ :tail) :head);=> b
Now what does this look like?
Cons Cell
Object:A Poor Man's Closure
A Protocol for seqsCall with :type to inspect the seq type
Return CONS when type is a cons cellCall with :head to get the headCall with antyhing else to get the tail
first and rest(def FIRST (fn [x] (if x (if (= (x :type) 'CONS) (x :head)
(if (x) ((x) :head))))))
(def REST (fn [x] (if x (if (= (x :type) 'CONS) (x :tail) (if (x) ((x) :tail))))))
(FIRST $);=> a
(REST $);=> #<user$CONS$fn__17 user$CONS$fn__17@2eb0a3f5>
(FIRST (REST $));=> b
We can do a ton with only CONS, FIRST and REST!
seq(def SEQ (fn [x] (if x (if (= (x :type) 'CONS) x (if (x) (SEQ (x)))))))
(SEQ $);=> #<user$CONS$fn__97 user$CONS$fn__97@293b9fae>
(FIRST (SEQ $));=> a
(SEQ (REST (REST $)));=> nil
prn(def PRN (fn [s] (if (SEQ s) (do (print (FIRST (SEQ s))) (print " ") (recur (REST s))) (println))))
(PRN $)
; a b
(PRN (CONS 'a nil)); a
This doesn't count
append(def APPEND (fn app [l r] (if (FIRST l) (CONS (FIRST l) (app (REST l) r)) r)))
(PRN (APPEND (CONS 'x nil) (CONS 'y (CONS 'z nil)))); x y z
But this is not a convenient way to deal with lists
Lists1. 5 apply
list(def LIST (fn ls ([h] (CONS h nil)) ([h t] (CONS h (CONS t nil))) ([h m & [f & r]] (if (CAR r) (if (CAR (CDR r)) (APPEND (LIST h m) (apply ls f (CAR r) (CDR r))) (APPEND (LIST h m) (LIST f (CAR r)))) (CONS h (LIST m f))))))
(PRN (LIST 'a 'b 'c 'd 'e 'f)); a b c d e f
(SEQ (REST (LIST 'a)));=> nil
(PRN (APPEND (LIST 'a 'b) (LIST 'x 'y))); a b x y
Using CAR, CDR, and destructuring as the primordial first and rest
Being Lazy
Being LazyTODO
Lazy seqs
Lazy seq(def LAZY-SEQ (fn [f] (fn ([x] (if (= x :type) 'LAZY-SEQ)) ([] (f)))))
(FIRST ((LAZY-SEQ (fn [] (LIST 'a 'b 'c)))));=> a
(PRN ((LAZY-SEQ (fn [] (LIST 'a 'b 'c))))); a b c
Now we have a protocol for lazy seqs
A Protocol for lazy seqsWrap the part that you want to be lazy in a fnPass that fn to LAZY-SEQConform to the semantics of :typeDeal with the extra level of indirection when dealing with lazy seqs
map(def MAP (fn [f s] (LAZY-SEQ (fn [] (if (SEQ s) (CONS (f (FIRST s)) (MAP f (REST s))))))))
(PRN (MAP keyword (LIST 'a 'b 'c))); :a :b :c
(PRN (MAP LIST (LIST 'a 'b))); #<user$CONS$fn__356 user$CONS$fn__356@54cb2185> ...
(PRN (FIRST (MAP LIST (LIST 'a 'b)))); a
Control Structures6 defmacro7 `
let(let [a 1] (let [b 2] (println [a b])) (println [a b]))
; java.lang.Exception: Unable to resolve symbol: b in this context
Defines a scope for named values
LET(defmacro LET [[bind val] & body] `((fn [~bind] ~@body) ~val))
(LET (a 1) (LET (b 2) (println [a b])))
produces...
((fn [a] ((fn [b] (println [a b])) 2)) 1)
more or less
More LET
(FIRST (LET (x 'a) (CONS x nil)))
;=> a
(PRN (LET (x 'x) (LET (y 'y) (CONS x (CONS y $)))))
; x y a b
And the rest is mechanical
but...
We didn't need keywords...
Symbols would have worked just as well(def CONS (fn [a b] (fn ([x] (if (= x 'lazy) 'CONS (if (= x 'head) a b))))))
(def $$ (CONS 'a (CONS 'b nil)))
($$ 'head);=> a
($$ 'tail);=> #<user$CONS$fn__91 user$CONS$fn__91@58e22f2b>
The Magnificent 6= if ' :keywords apply defmacro `
and...
We didn't need apply...
defmacro gives us that for free(defmacro APPLY [f args] `(~f ~@args))
(APPLY + [1 2 3 4]);=> 10
(PRN (APPLY LIST '[a b c d e])); a b c d e
The Magnificent 5= if ' :keywords apply defmacro `
and...
We didn't need defmacro and `...why not?
Meta-circular Evaluator FTW(def EVAL (fn (expr binds) (COND ((ATOM expr) (ASSOC expr binds)) ((ATOM (FIRST expr)) (COND ((= (FIRST expr) 'quote) (SECOND expr)) ((= (FIRST expr) 'ATOM) (ATOM (EVAL (SECOND expr) binds))) ((= (FIRST expr) '=) (= (EVAL (SECOND expr) binds) (EVAL (THIRD expr) binds))) ((= (FIRST expr) 'FIRST) (FIRST (EVAL (SECOND expr) binds))) ((= (FIRST expr) 'REST) (REST (EVAL (SECOND expr) binds))) ((= (FIRST expr) 'CONS) (CONS (EVAL (SECOND expr) binds) (EVAL (THIRD expr) binds))) ((= (FIRST expr) 'COND) (EVAL-COND (REST expr) binds)) ('true (EVAL (CONS (ASSOC (FIRST expr) binds) (REST expr)) binds)))) ((= (CAAR expr) 'def) (EVAL (CONS (CADDAR expr) (REST expr)) (CONS (LIST (CADAR expr) (FIRST expr)) binds))) ((= (CAAR expr) 'fn) (EVAL (CADDAR expr)
(APPEND (PAIR (CADAR expr) (EVAL-ARGS (REST expr) binds)) binds))) ('true (ASSOC expr binds)))))
note: not all code shown
The Magnificent 3= if ' :keywords apply defmacro `
The Magnificent 3!?!
Our Optionsdeftype defprotocol reify intern . defmulti defmethod defrecord first rest [] ^ {} delay force new defclass proxy list* fn* fn? seq clojure.lang.RT
and so on...
The Garden of Forking Paths
