Has anyone written a very simple introduction to Clojure for LISP
hackers? I've spend an evening playing with it pretty intensively, and
thus far I haven't got a thing to work. I've read
http://en.wikibooks.org/wiki/Clojure_Programming#Clojure_for_Common_Lisp_Programmers,
but it hasn't helped me.
In LISP:
* (defun fact (n) (cond ((= 1 n) 1)(t (* n (fact (- n 1))))))
FACT
* (fact 10)
3628800
that is Common LISP, but what the hell; in Portable Standard LISP it
would have been identical except 'de' instead of 'defun'; in Scheme
it's a little different:
> (define (fact n) (cond ((= n 1) 1)(#t (* n (fact (- n 1))))))
> (fact 10)
3628800
But the family resemblance is there... So, let's try it in Clojure:
user=> (defun fact (n) (cond ((= n 1) 1) (t (* n (fact (- n 1))))))
java.lang.Exception: Unable to resolve symbol: defun in this context
user=> (de fact (n) ( cond ((= n 1) 1) (t (* n (fact (- n 1))))))
java.lang.Exception: Unable to resolve symbol: de in this context
user=> (def fact (n) ( cond ((= n 1) 1) (t (* n (fact (- n 1))))))
java.lang.Exception: Too many arguments to def
user=> (defn fact [n] (cond ((= n 1) 1)(t (* n (fact (- n 1))))))
java.lang.Exception: Unable to resolve symbol: t in this context
user=> (defn fact [n](cond ((= n 1) 1)(#t (* n (fact (- n 1))))))
java.lang.Exception: No dispatch macro for: t
user=> (defn fact [n](cond ((= n 1) 1)(#true (* n (fact (- n 1))))))
java.lang.Exception: No dispatch macro for: t
user=> (defn fact [n] (cond ((= n 1) 1)(true (* n (fact (- n 1))))))
#'user/fact
OK, what's with the funny square brackets? A sequence of forms
enclosed in square brackets is a vector, not a list. Why is the
arglist for a function a vector? For now let's accept that it is, and
pass on.
user=> (fact 10)
java.lang.ClassCastException: java.lang.Boolean cannot be cast to
clojure.lang.IFn
OK, so that doesn't work. As Zaphod memorably put it, 'hey, what is
truth, man?' Good question:
user=> (true? 'true)
true
user=> (true? true)
true
user=> (true? t)
java.lang.Exception: Unable to resolve symbol: t in this context
user=> (true? 't)
false
user=> (def t 'true)
#'user/t
user=> (true? t)
true
OK, now we know the truth, surely we can write a valid fact?
user=> (defn fact [n] (cond ((= n 1) 1)('true (* n (fact (- n 1))))))
#'user/fact
user=> (fact 10)
java.lang.ClassCastException: java.lang.Boolean cannot be cast to
clojure.lang.IFn
user=> (defn fact [n] (cond ((= n 1) 1)((true? 'true) (* n (fact (- n
1))))))
#'user/fact
user=> (fact 10)
java.lang.ClassCastException: java.lang.Boolean cannot be cast to
clojure.lang.IFn
OK, it looks like whatever's causing the break isn't the guard on the
second cond branch. Let's for a moment try something that's purely
boolean:
user=> (defn band [l] (cond ((nil? l) true)((true? (car l))(band (cdr
l)))))
java.lang.Exception: Unable to resolve symbol: cdr in this context
No CDR? It's LISP, but it can't fetch the contents of the decrement
register? fifty years of LISP history tossed into the dirt. So if the
CDR isn't called the CDR, what is it called (and what's the CDADR
called)?
user=> (defn band [l] (cond ((nil? l) true)((true? (first l))(band
(rest l)))))
#'user/band
user=> (band '(true true true))
java.lang.ClassCastException: java.lang.Boolean cannot be cast to
clojure.lang.IFn
OK, can we write any function at all that works?
user=> (defn square [n] (* n n))
#'user/square
user=> (square 4)
16
Fine, so recurse up from that:
user=> (defn power [n m] (cond ((= m 0) 1)(true (* n (power n (- m
1))))))
#'user/power
user=> (power 2 2)
java.lang.ClassCastException: java.lang.Boolean cannot be cast to
clojure.lang.IFn
Arrrrggghhh...
Getting LISP to work seamlessly inside a Java environment with easy
intercalling between LISP and Java is a very big win, and potentially
knocks things like JScheme and Armed Bear Common LISP into a cocked
hat...
But it would be nice to be able to get started!
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