Question regarding example in Stuart Halloway's book (page 120)
arasoft
(take-while (complement #{\a\e\i\o\u}) "the-quick-brown-fox")
When I do something similar, like
(take-while (complement #(Character/isWhitespace %)) "the-quick-brown-
fox")
I have to deal with the parameter explicitly ("%"). How is the
parameter hidden in the set/function? Could I do something like that
in my own code (not sure I'd want to, just curious what magic is at
work here)?
Richard Newman
>
> (take-while (complement #{\a\e\i\o\u}) "the-quick-brown-fox")
>
> When I do something similar, like
>
> (take-while (complement #(Character/isWhitespace %)) "the-quick-brown-
> fox")
>
> I have to deal with the parameter explicitly ("%"). How is the
> parameter hidden in the set/function?
The set *is* a function (it implements IFn). complement accepts a
function as input, so that works.
Character/isWhitespace is a Java static method, which is not a first-
class function, and thus can't be passed directly to complement.
The #(...) syntax is shorthand for (fn [...] ...), which introduces a
new anonymous function. What you've written there is equivalent to
(defn whitespace? [c]
(Character/isWhitespace c))
(take-while (complement whitespace?) "the quick")
... i.e., you've made a function that calls a Java static method on
its input.
> Could I do something like that
> in my own code (not sure I'd want to, just curious what magic is at
> work here)?
In your Java code, implement IFn. That's the magic. In your Clojure
code, just hand around functions.
Stephen C. Gilardi
In addition to functions, several of Clojure's other objects are
"invokable"--they can operate successfully as the first item in a call
expression (a list in code).
Invoking a set returns the element in the set that's = to the argument
or nil
Invoking a map returns the value in the map whose corresponding key is
= to the argument or nil
Invoking a keyword requires a map as its argument and returns the
value in the map corresponding to the key that's = to the keyword or nil
"Being invokable" in Clojure is equivalent to implementing the IFn
interface. You can see in this diagram which objects in Clojure
implement IFn:
(IFn is at the lower right).
The core function "ifn?" can also let you know if something is
invokable:
Clojure 1.1.0-alpha-SNAPSHOT
user=> (ifn? #{:a :b :c})
true
user=> (ifn? 3)
false
user=> (fn? #{:a :b :c})
false
user=>
--Steve
arasoft
Rich Hickey
For all those who want to understand how the class hierarchies map to
various Clojure abstractions, I have broken down that (rather
daunting) graph into separate areas in various slides from the
tutorial I gave at ILC09:
http://clojure.googlegroups.com/web/tutorial.pdf
There you can much more clearly see how the collections, seqs, refs,
and various concepts like
sequential/associative/counted/reversible/metadata/callability/java
collection/java interop are organized.
Rich
Rowdy Rednose
familiar with all the function names.
There's however a small mistake on page 28. The 'd' doesn't belong
there in the 2nd line of this example:
(let [{a :a, b :b, c :c, :as m :or {a 2 b 3}} {:a 5 :c 6}]
[a b c d m])
-> [5 3 6 {:c 6, :a 5}]
Rowdy
On Jun 25, 9:52 am, Rich Hickey <richhic...@gmail.com> wrote:
>
> > On Jun 24, 2009, at 7:02 PM, arasoft wrote:
>
> >> Why does this work?
>
> >> (take-while (complement #{\a\e\i\o\u}) "the-quick-brown-fox")
>
> >> When I do something similar, like
>
> >> (take-while (complement #(Character/isWhitespace %)) "the-quick-brown-
> >> fox")
>
> >> I have to deal with the parameter explicitly ("%"). How is the
> >> parameter hidden in the set/function? Could I do something like that
> >> in my own code (not sure I'd want to, just curious what magic is at
> >> work here)?
>
> > In addition to functions, several of Clojure's other objects are
> > "invokable"--they can operate successfully as the first item in a call
> > expression (a list in code).
>
> > Invoking a set returns the element in the set that's = to the argument or
> > nil
> > Invoking a map returns the value in the map whose corresponding key is = to
> > the argument or nil
> > Invoking a keyword requires a map as its argument and returns the value in
> > the map corresponding to the key that's = to the keyword or nil
>
> > "Being invokable" in Clojure is equivalent to implementing the IFn
> > interface. You can see in this diagram which objects in Clojure implement
> > IFn:
>