Evaluation of string to erlang’s term:
-module(evaluator_utils).
-export([eval_string/1]).
eval_string([]) -> [];
eval_string(String) ->
{ok, Tokens, _} = erl_scan:string(lists:concat([String, "."])),
{Status, Term} = erl_parse:parse_term(Tokens),
case Status of
ok -> {ok, Term};
error ->
{_, _, ErrorMessage} = Term,
{error, ErrorMessage}
end.
This is how it can be done in clojure:
user=> (defmulti fib (fn [x] x) :default :default) #'user/fib user=> (defmethod fib 0 [x] 0) #<MultiFn clojure.lang.MultiFn@de1b8a> user=> (defmethod fib 1 [x] 1) #<MultiFn clojure.lang.MultiFn@de1b8a> user=> (defmethod fib :default [n] (+ (fib (- n 2)) (fib (- n 1)))) #<MultiFn clojure.lang.MultiFn@de1b8a> user=> (fib 32) 2178309 user=> (time (fib 32)) "Elapsed time: 3406.246008 msecs" 2178309 user=>
Factorial pattern matching:
(defmulti fac (fn [x] x) :default :default) (defmethod fac 1 [x] 1) (defmethod fac :default [n] (* n (fac (- n 1))))
Context: when remote module pushing is needed.
Initial setup:
1. 2 nodes (physical nodes, each one having one erlang instance started): node1@10.0.0.1 and node2@10.0.0.2
2. both erlang instances must be started with the same cookie, in order to be able to successfully communicate
Steps:
(On node1, create test module)
-module(test).
-export([test_fun/0]).
test_fun() ->
io:format("Hello ~s~n",["World"]).
(On node1, start erlang instance)
$ erl -name node1@10.0.0.1 -setcookie testcookie Erlang R13B (erts-5.7.1) [source] [smp:2:2] [rq:2] [async-threads:0] [kernel-poll:false] Eshell V5.7.1 (abort with ^G) (node1@10.0.0.1)1>
(On node2, start erlang instance)
$ erl -name node2@10.0.0.2 -setcookie testcookie Erlang R13B (erts-5.7.1) [source] [smp:2:2] [rq:2] [async-threads:0] [kernel-poll:false] Eshell V5.7.1 (abort with ^G) (node1@10.0.0.2)1>
(On node1, compile/load/remote load test module)
(node1@10.0.0.1)1> c(test).
{ok,test}
(node1@10.0.0.1)2> {Mod, Bin, _} = code:get_object_code(test).
{test,<<70,79,82,49,0,0,1,252,66,69,65,77,65,116,111,109,
0,0,0,63,0,0,0,7,4,116,101,...>>,
"/home/alin/test.beam"}
(node1@10.0.0.1)3> rpc:call('node2@10.0.0.2',erlang,load_module, [Mod, Bin]).
{module,test}
(node1@10.0.0.1)4> spawn('node2@10.0.0.2', test, test_fun, []).
Hello World
<8592.45.0>
(node1@10.0.0.1)5>
(On node1, edit test.erl)
Instead of
... io:format("Hello ~s~n",["World"]).
set
... io:format("Hello ~s~n",["World2"]).
(On node1, go back to opened erlang shell)
(node1@10.0.0.1)5> f().
ok
(node1@10.0.0.1)6> c(test).
{ok,test}
(node1@10.0.0.1)7> {Mod, Bin, _} = code:get_object_code(test).
{test,<<70,79,82,49,0,0,1,252,66,69,65,77,65,116,111,109,
0,0,0,63,0,0,0,7,4,116,101,...>>,
"/home/alin/test.beam"}
(node1@10.0.0.1)8> rpc:call('node2@10.0.0.2',code,purge, [Mod]).
false
(node1@10.0.0.1)9> rpc:call('node2@10.0.0.2',erlang,load_module, [Mod, Bin]).
{module,test}
(node1@10.0.0.1)10> spawn('node2@10.0.0.2', test, test_fun, []).
<8592.48.0>
Hello World2
(node1@10.0.0.1)11>
Note: node2 was not touched for anything else than starting (erl -name node2@10.0.0.2 -setcookie testcookie)
Done.
Just run:
$ erl -man <module>
for instance:
$ erl -man erl $ erl -man rpc $ erl -man erlang
etc.
OS: Ubuntu 9.04
sudo apt-get install yaws
Create some document folder:
mkdir /home/alin/public_html
Add data to an test.html file, in order to be rendered:
echo '<html><body><div>Hello from Yaws!</div></body></html>' > /home/alin/public_html/test.html
erlang module for embedded yaws:
-module(test_yaws).
-export([start/1, stop/0]).
-define(YAWS_LIB,"/usr/lib/erlang/lib/yaws-1.80/ebin").
start(Port) ->
code:add_path(?YAWS_LIB),
SL=[{listen,{0,0,0,0}},
{port,Port},
{servername,"localhost"},
{flags,[{dir_listings,true},{access_log,false}]}],
GL=[{trace,false},{tmpdir,"/tmp"},{flags,[{auth_log,false},{copy_errlog,false}]}],
yaws:start_embedded("/home/alin/public_html", SL, GL).
stop() ->
code:add_path(?YAWS_LIB),
yaws:stop().
Start embedded yaws:
$ erl Erlang R13B (erts-5.7.1) [source] [smp:2:2] [rq:2] [async-threads:0] [kernel-poll:false] Eshell V5.7.1 (abort with ^G) 1> c(test_yaws). 2> test_yaws:start(5559). =INFO REPORT==== 24-May-2009::18:28:35 === Yaws: Listening to 0.0.0.0:5559 for servers - http://localhost:5559 under /home/alin/public_html ok 3>
To check if everything is ok, go to: http://localhost:5559/ (listing of public_html folder will be done if no other resource is specified).
If accessing http://localhost:5559/test.html, “Hello from Yaws!” message will be displayed.
Few days ago I was stuck at some issue:
I wanted to do some profiling of ordinary fibonacci function written in Haskell but I didn’t wanted to display the result (using main function, from Main module). Guys from #haskell irc channel (irc.freenode.net) were very helpful and they guided me to evaluate and rnf
module Main where import Control.Parallel.Strategies import Control.Exception fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib(n - 1) + fib(n - 2) main = do evaluate(rnf (fib 32))
or, using function composition:
module Main where import Control.Parallel.Strategies import Control.Exception fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib(n - 1) + fib(n - 2) eval = evaluate . rnf main = do eval(fib 32)
ghc --make Fib.hs time ./Fib
and for better performance:
ghc --make -O2 Fib.hs time ./Fib
Haskell:
[1..]
Clojure:
(iterate inc 1)
Haskell:
[1..109]
Clojure:
(range 1 110)
user=> (def x (lazy-seq (cons (do (println "evaluated") (+ 1 2)) (list 1 2 3)))) #'user/x user=> (def y (cons (do (println "evaluated") (+ 1 2)) (list 1 2 3))) evaluated #'user/y
(Include clojure.jar in your lib path)
List list = new PersistentList("t1");
try {
list.add("t2");
} catch (UnsupportedOperationException e) {
System.out.println("It seems that this operation is not supported.");
}
System.out.println("List size: " + list.size());
Map map = new PersistentArrayMap(new Object[]{"key1", "value1"});
try {
map.put("key2", "value2");
} catch (UnsupportedOperationException e) {
System.out.println("It seems that this operation is not supported.");
}
System.out.println("Map size: " + map.size());
This can be used as an alternative for
java.util.Collections.unmodifiableList(…)
Now, to demonstrate lazy vs. non-lazy behavior in clojure:
(defn create-lazy [i]
(if (zero? i)
()
(lazy-seq (cons nil (create-lazy (dec i))))))
(defn create-non-lazy [i]
(if (zero? i)
()
(cons nil (create-non-lazy (dec i)))))
(create-lazy 1000000)
(try (create-non-lazy 1000000)
(catch java.lang.StackOverflowError e
(println "StackOverflow error")))
Of course, in real life situations, easier methods can be used, like
user=> (replicate 7 nil) (nil nil nil nil nil nil nil) user=>
