Linus Ericsson <mailto:oscarlinuserics...@gmail.com>
July 10, 2014 at 6:52 AM
You should try Clojure Programming (Halloway, Bedra). I felt
enlightened after reading the first edition, the second edition is
also very good!
http://pragprog.com/book/shcloj2/programming-clojure
/Linus
On Thursday, July 10, 2014, Stephen Feyrer <stephen.fey...@gmail.com
<mailto:stephen.fey...@gmail.com>> wrote:
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Stephen Feyrer <mailto:stephen.fey...@gmail.com>
July 10, 2014 at 2:37 AM
Hi Sam, Lee.
Thank you both.
It would appear that I am faced with the old adage, "A little
knowledge is a dangerous thing".
Again thank you, you ' ve been a great help. If I may impose upon you
a little further? Would either of you be able to recommend an
introductory book either for Clojure or FP that might fit well with
Clojure, for the mildly bewildered? I have a book Programming Clojure
but it is written with an assumption of prior knowledge/experience
which I don't have. By the way, Programming Clojure is a good book, I
just don't think I fit within the target audience.
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Lee Spector <mailto:lspec...@hampshire.edu>
July 9, 2014 at 7:41 PM
Or -- I now see, instead of adding the "do" you could just remove the
outermost parentheses after the parameter list. But as Sam and I said
this is a bad way to go anyway -- you want to avoid the nested defs.
-Lee
Lee Spector <mailto:lspec...@hampshire.edu>
July 9, 2014 at 7:31 PM
On Jul 9, 2014, at 8:48 PM, Stephen Feyrer<stephen.fey...@gmail.com> wrote:
Hi,
I tried to create the function below in a Lighttable instarepl. In lieu of any
better idea for formatting, the< > statements below indicate instarepl
output.
(defn avged ([x]
((def sumed (reduce + x))< 10>
(def counted (count x))< 4>
(def result (/ sumed counted))< 5/2>
result
)))
(avged [1 2 3 4])< java.lang.ClassCastException: java.lang.Long cannot be cast
to clojure.lang.IFn
Var.java:392 clojure.lang.Var.fn
Var.java:423 clojure.lang.Var.invoke
(Unknown Source) user/avged>
The objective of this function is just a learning exercise.
I have three questions:
1. Why doesn't it work, return a value?
Hi Stephen,
The main reason it doesn't work is that the expression after the parameter list [x] is a
list and will be interpreted as a function call, with the function being whatever is
returned from the first thing in the list -- which in this case is the "def
sumed" definition, which returns the defined var.
You could patch (not recommended!) this by adding "do" to the beginning of that
list:
(defn avged
([x]
(do
(def sumed (reduce + x))
(def counted (count x))
(def result (/ sumed counted))
result)))
That will work, but it's not idiomatic clojure -- it is almost never idiomatic
clojure to have a def or defn within another defn. Also, you've used the syntax
that allows for multiple arglist/body pairs, although you only need one so the
extra layer of parentheses isn't necessary.
Here's how I might write it somewhat more idiomatically:
(defn avged
[x]
(let [sumed (reduce + x)
counted (count x)
result (/ sumed counted)]
result))
=> (avged [1 2 3 4])
5/2
=> (float (avged [1 2 3 4]))
2.5
2. What does the error message mean? and seeing this or similar again, how do
I investigate to get a meaningful Clojure solution?
I think it means it's trying to call the value of sumed as a function. I'll
leave it to others to suggest ways of understanding Clojure error messages (and
just note for any of those others who have the ability to do something about
this that it sure would be great to be able to get stack backtraces with values
of locals regardless of IDE etc.).
3. Code Style, what can I do to improve readability and form?
Myself, I'd probably do it without any variables at all, and name everything
more meaningfully, e.g.:
(defn average-numbers
"Returns the average of the provided numbers."
[numbers]
(/ (reduce + numbers)
(count numbers)))
=> (average-numbers [1 2 3 4])
5/2
-Lee
Stephen Feyrer <mailto:stephen.fey...@gmail.com>
July 9, 2014 at 6:48 PM
Hi,
I tried to create the function below in a Lighttable instarepl. In
lieu of any better idea for formatting, the < > statements below
indicate instarepl output.
(defn avged ([x]
((def sumed (reduce + x)) < 10 >
(def counted (count x)) < 4 >
(def result (/ sumed counted)) < 5/2 >
result
)))
(avged [1 2 3 4]) < java.lang.ClassCastException: java.lang.Long
cannot be cast to clojure.lang.IFn
Var.java:392 clojure.lang.Var.fn
Var.java:423 clojure.lang.Var.invoke
(Unknown Source) user/avged >
The objective of this function is just a learning exercise.
I have three questions:
1. Why doesn't it work, return a value?
2. What does the error message mean? and seeing this or similar
again, how do I investigate to get a meaningful Clojure solution?
3. Code Style, what can I do to improve readability and form?
Thanks.
--
Stephen Feyrer.
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