Clojure Deep
Dive
Howard M. Lewis Ship
TWD Consulting
hlship@gmail.com
1 © 2009 Howard Lewis Ship

Agenda
• Core Language
• Standard Tools
• Clojure Compilation
• Clojure Pitfalls
2 © 2009 Howard Lewis Ship

Core Language
3 © 2009 Howard Lewis Ship

What's A Form?
Constants: Keywords: Symbols:
42 :foo map
"hello" :tag strutils2/take
Data Structures:
[…]
{…}
#{ … }
Callables:
Special Forms: Function Call: Macros Expansion:
(if …) (map …) (and …)
(var …) (or …)
(do …) (ns …)
4 © 2009 Howard Lewis Ship

defn: Define a function
Symbol (in current namespace)
Optional doc string
(defn to-seq
"Converts the object to a sequence (a vector)
unless it is already sequential."
[obj]
(if (sequential? obj) obj [obj]))
List of parameters
user=> (doc to-seq)
-------------------------
user/to-seq
([obj])
Converts the object to a sequence (a vector)
unless it is already sequential.
nil
user=>
5 © 2009 Howard Lewis Ship

defn: Multiple Arities
One doc string
Series of argument lists / forms
(defn not=
"Same as (not (= obj1 obj2))"
([x] false)
([x y] (not (= x y)))
([x y & more]
(not (apply = x y more))))
& symbol ➠ provide
remaining values as a
seq
user=> (doc not=)
-------------------------
clojure.core/not=
([x] [x y] [x y & more])
Same as (not (= obj1 obj2))
nil
user=>
6 © 2009 Howard Lewis Ship

let: local symbols
(let [center (calc-center-for-frame frame)
x (get-x center)
y (get-y center)]
…)
•Even number: symbol1 form1 symbol2 form2 …
•Not variables: symbols are read-only
•Can re-define symbols
7 © 2009 Howard Lewis Ship

do: evaluate for side-effects
(if value-found
(do
(println "Found the value.")
value-found))
Last form is the result
; Broken
(if value-found
(println "Found the value.") If true, side effect & return nil
value-found)
If false, return value (false or nil)
8 © 2009 Howard Lewis Ship

Branching: if, when
(if test then-form)
(if test then-form else-form)
• Special form: not a function
• Evaluates test, then evaluates & returns either then-form or
else-form
• false and nil ➠ logical false
• anything else ➠ logical true
(when test then-forms) (if-not test then-form)
(if-not test then-form else-form)
Any number of forms in an
implicit do Like (if (not test) then else)
(when-not test then-forms)
9 © 2009 Howard Lewis Ship

if-let, when-let
(if-let [symbol test] then-form)
(if-let [symbol test] then-form else-form)
Symbol can be referenced
(when-let [symbol test] then-forms)
Any number of forms in an
implicit do
10 © 2009 Howard Lewis Ship

or, and
(or form…)
(and form…)
• or: first value from list that evaluates to true
• Will return value of last expression (false or nil)
• (or) returns nil
• and: first logical false (nil or false)
• Returns value of last expression (if prior are logical true)
• (and) return true
• Can be used as a "guard" Only invoke symbol and find-ns
(defn dispatch-named-function-to-pipeline if ns-name is non-nil
[env pipeline]
(let [split-path (-> env :cascade :split-path)
[_ ns-name fn-name] split-path
fn-namespace (and ns-name (find-ns (symbol ns-name)))
named-function (and fn-namespace fn-name (ns-resolve fn-namespace (symbol fn-name)))
new-env (assoc-in env [:cascade :extra-path] (drop 3 split-path))]
(call-pipeline pipeline new-env named-function)))
11 © 2009 Howard Lewis Ship

Destructuring
Bu
N wo
sy
o rk
• Automatically extract values from data structures
!
• Binding forms:
• Map to extract keys
• Vector to extract indexed values
• Function parameters
One un-named parameter
• let, if-let, when-let, etc. destructured to two symbols
user=> (defn natural-name
[{fname :first-name lname :last-name}]
(str lname ", " fname))
#'user/natural-name
user=> (natural-name (persons 1))
"Simon, Scott"
user=>
12 © 2009 Howard Lewis Ship

Destructuring
• Binding Forms:
• symbol (let [x (get-x center)] … )
• vector of binding forms (let [[x y] center] … )
• map of binding form to map key
(let [{fname :first-name lname :last-name} person] …)
13 © 2009 Howard Lewis Ship

Vector Destructuring
center must be sequential
(let [[x y] center] …)
May be nil if not enough values in center
(let [[first-born second-born & other-children] children] …)
& symbol gets the remaining values
(let [[first-born second-born & other-children :as children] (find-children parent)] …)
Nested binding forms! :as symbol gets the full list
user=> (let [[[x1 y1][x2 y2]] [[1 2] [3 4]]]
[x1 y1 x2 y2])
[1 2 3 4]
user=>
14 © 2009 Howard Lewis Ship

Map Destructuring
(defn natural-name [{fname :first-name lname :last-name}] (str lname ", " fname))
(defn natural-name [{:keys [first-name last-name]}] (str last-name ", " first-name))
:keys maps a keyword key for each symbol
:or sets defaults for missing keys
user=> (defn natural-name [{:keys [first-name last-name]
:or {first-name "(none)" last-name "(none)"}}]
(str last-name ", " first-name))
#'user/natural-name
user=> (natural-name {:first-name "Gromit"})
"(none), Gromit"
user=> (natural-name {:first-name "Gromit" :last-name nil})
", Gromit"
user=>
:as symbol to get the original map
:strs ➠ like :keys, but maps to string keys, not keyword keys
:syms ➠ like :keys, but maps to symbol keys, not keyword keys
15 © 2009 Howard Lewis Ship

ns: define namespaces
(ns example.utils)
cascade/dispatcher.clj
(ns cascade.dispatcher
(:import (javax.servlet ServletResponse))
File: example/utils.clj (:use (cascade config dom logging path-map pipeline)
(cascade.internal utils)))
(ns namespace
(:import …) Optional directives
(:use …))
user=> (defn natural-name
[{fname :first-name lname :last-name}]
(str lname ", " fname))
#'user/natural-name
user=> (natural-name (persons 1))
"Simon, Scott"
user=>
user/natural-name
Namespace: user
Symbol: natural-name
16 © 2009 Howard Lewis Ship

:import Java Classes
Package name, then any number of classes/interfaces
(ns cascade.dispatcher
(:import (javax.servlet ServletResponse))
…)
Repeat for other packages
Can also list fully qualify class names
17 © 2009 Howard Lewis Ship

:use Import Namespaces
(:use clojure.contrib.pprint)
Symbols defined by pprint are available, i.e.
(pprint [1 2 3])
(:use (clojure.contrib pprint monads))
Add multiple namespaces under the root
namespace
(:use (clojure.contrib (str-utils2 :only [join map-str]))
Uses just join and map-str from str-utils2
(:use (clojure.contrib (str-utils2 :exclude [take replace drop]))
Exclude just some of the symbols
(:use (clojure.contrib (str-utils2 :rename {take strtake}))
Use str-utils2, mapping take as strtake
18 © 2009 Howard Lewis Ship

Other ns directives
• :require ➠ load a namespace but don't import it
• :gen-class ➠ create a Java class from functions in the
namespace
• :refer-clojure ➠ import selected clojure.core symbols
• :load ➠ load Clojure scripts (not namespaces)
19 © 2009 Howard Lewis Ship

Standard
Tools
20 © 2009 Howard Lewis Ship

Anonymous Functions
#() ➠ implicit anonymous function
(filter #(not (.startsWith % ".")) names)
% is replaced by the function's parameter
%n is parameter #n
(map
#(assoc %1 :sort-index %2)
(sort-by :age persons)
(iterate inc 0))
inline anonymous function
(map
21
(fn [person-map sort-index]
(assoc person-map :sort-index sort-index))
(sort-by :age persons)
(iterate inc 0))
© 2009 Howard Lewis Ship

What are seqs?
Seqable
seq() : ISeq
IPersistentCollection Sequential
count() : int
cons(Object): IPersistentCollection
empty() : IPersistentCollection
equiv(Object) : boolean
PersistentSet ISeq
first() : Object
next(): ISeq
more(): ISeq
cons(Object): ISeq Returned from map, for,
filter, remove, etc.
PersistentList PersistentVector LazySeq
22 © 2009 Howard Lewis Ship

assoc-in / update-in
user=> (def person { :first-name "Howard" :last-name "Lewis Ship"
:address { :street "123 NW 12th Ave. #541" :city "Portland" :state "OR" :zip 97309 }})
#'user/person
user=> (assoc-in person [:address :street] "2647 SE 97th Ave.")
{:first-name "Howard", :last-name "Lewis Ship",
:address {:street "2647 SE 97th Ave.", :city "Portland", :state "OR", :zip 97309}}
user=> (update-in person [:address :zip] + 5)
{:first-name "Howard", :last-name "Lewis Ship",
:address {:street "123 NW 12th Ave. #541", :city "Portland", :state "OR", :zip 97314}}
user=>
:first-name "Howard"
:last-name "Lewis Ship"
:address :street "123 NW 12th Ave."
:city "Portland"
:state "OR"
:zip 97309
(update-in person [:address :zip] + 5)
(+ 97309 5)
:first-name "Howard"
:last-name "Lewis Ship"
:address :street "123 NW 12th Ave."
:city "Portland"
:state "OR"
:zip 97314
23 © 2009 Howard Lewis Ship

map
La
zy
f
:age acts as a function
user=> (map :age persons)
(42 44 29)
user=> (map #(apply str (reverse (% :first-name))) persons)
("drawoH" "ttocS" "ylloM")
user=> (map identity (persons 0))
([:first-name "Howard"] iterate a map ➠ key/value pairs
[:last-name "Lewis Ship"]
[:age 42])
user=> (seq (persons 0))
([:first-name "Howard"]
[:last-name "Lewis Ship"]
[:age 42])
user=>
24 © 2009 Howard Lewis Ship

map
La
zy
f
N seqs ➠ N parameters
user=> (map #(assoc %1 :sort-index %2)
(sort-by :age persons)
(iterate inc 0))
({:sort-index 0, :first-name "Molly", :last-name "Newman", :age 29}
{:sort-index 1, :first-name "Howard", :last-name "Lewis Ship", :age 42}
{:sort-index 2, :first-name "Scott", :last-name "Simon", :age 44})
user=>
25 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
26 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
(reduce + '(42 44 29 38))
27 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
(reduce + '(42 44 29 38))
(+ (+ (+ 42 44) 29) 38)
28 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
(reduce + '(42 44 29 38))
(+ (+ (+ 42 44) 29) 38)
(+ (+ 86 29) 38)
29 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
(reduce + '(42 44 29 38))
(+ (+ (+ 42 44) 29) 38)
(+ (+ 86 29) 38)
(+ 115 38)
30 © 2009 Howard Lewis Ship

reduce
f
user=> (map :age persons)
(42 44 29 38)
user=> (reduce + (map :age persons))
153
(reduce + (map :age persons))
(reduce + '(42 44 29 38))
(+ (+ (+ 42 44) 29) 38)
(+ (+ 86 29) 38)
(+ 115 38)
153
31 © 2009 Howard Lewis Ship

reduce
f
user=> (def input-string "Clojure is a fascinating language with unique
capabilities and total integration with Java.")
#'user/input-string
user=> (seq input-string)
(C l o j u r e space i s space a space f a s c i n a
t i n g space l a n g u a g e space w i t h space u
n i q u e space c a p a b i l i t i e s space a n d
space t o t a l space i n t e g r a t i o n space w
i t h space J a v a .)
user=> (reduce
(fn [m k] (update-in m [k] #(if (nil? %) 1 (inc %))))
Optional initial value {}
(seq input-string))
{space 12, a 12, b 1, C 1, c 2, d 1, e 5, f 1, g 4, h 2, i 11,
J 1, j 1, l 4, . 1, n 7, o 3, p 1, q 1, r 2, s 3, t 8, u 4,
v 1, w 2}
user=>
32 © 2009 Howard Lewis Ship

filter / remove
La
zy
f
?
user=> (remove #(< (% :age) 30) persons)
({:first-name "Howard", :last-name "Lewis Ship", :age 42}
{:first-name "Scott", :last-name "Simon", :age 44})
user=> (filter #(< (% :age) 30) persons)
({:first-name "Molly", :last-name "Newman", :age 29})
user=>
33 © 2009 Howard Lewis Ship

apply
f
user=> (map :age persons)
(42 44 29)
user=> (apply + (map :age persons))
115
user=>
(apply + (map :age persons))
(apply + '(42 44 29))
(+ 42 44 29)
115
34 © 2009 Howard Lewis Ship

Clojure Compilation
35 © 2009 Howard Lewis Ship

Function Calls
(remove nil? coll)
IFn
invoke() : Object
invoke(Object) : Object
Namespace invoke(Object, Object) : Object
…
applyTo(ISeq) : Object
Var
get() clojure.core$remove__4782 PersistentMap Keyword
36 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn first-non-nil
"Returns the first non-nil value from the collection."
[coll]
(first (remove nil? coll)))
package com.howardlewisship.cascade.internal;
import clojure.lang.*;
public class utils$first_non_nil__24 extends AFunction
{
public Object invoke(Object coll)
throws Exception
{
return ((IFn)const__0.get()).invoke(((IFn)const__1.get()).
invoke(const__2.get(), coll = null));
}
public static final Var const__0 = (Var)RT.var("clojure.core", "first");
public static final Var const__1 = (Var)RT.var("clojure.core", "remove");
public static final Var const__2 = (Var)RT.var("clojure.core", "nil?");
public utils$first_non_nil__24()
{
}
}
37 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn first-non-nil
"Returns the first non-nil value from the collection."
[coll]
(first (remove nil? coll)))
package com.howardlewisship.cascade.internal;
import clojure.lang.*;
public class utils$first_non_nil__24 extends AFunction
{
public Object invoke(Object coll)
throws Exception
{
return ((IFn)const__0.get()).invoke(((IFn)const__1.get()).
invoke(const__2.get(), coll = null));
}
public static final Var const__0 = (Var)RT.var("clojure.core", "first");
public static final Var const__1 = (Var)RT.var("clojure.core", "remove");
public static final Var const__2 = (Var)RT.var("clojure.core", "nil?");
public utils$first_non_nil__24()
{
}
}
38 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn first-non-nil
"Returns the first non-nil value from the collection."
[coll]
(first (remove nil? coll)))
package com.howardlewisship.cascade.internal;
import clojure.lang.*;
public class utils$first_non_nil__24 extends AFunction
{
public Object invoke(Object coll)
throws Exception
{
return ((IFn)const__0.get()).invoke(((IFn)const__1.get()).
invoke(const__2.get(), coll = null));
}
public static final Var const__0 = (Var)RT.var("clojure.core", "first");
public static final Var const__1 = (Var)RT.var("clojure.core", "remove");
public static final Var const__2 = (Var)RT.var("clojure.core", "nil?");
public utils$first_non_nil__24()
{
}
}
39 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn first-non-nil
"Returns the first non-nil value from the collection."
[coll]
(first (remove nil? coll)))
package com.howardlewisship.cascade.internal;
import clojure.lang.*;
public class utils$first_non_nil__24 extends AFunction
{
public Object invoke(Object coll)
throws Exception
{
return ((IFn)const__0.get()).invoke(((IFn)const__1.get()).
invoke(const__2.get(), coll = null));
}
public static final Var const__0 = (Var)RT.var("clojure.core", "first");
public static final Var const__1 = (Var)RT.var("clojure.core", "remove");
public static final Var const__2 = (Var)RT.var("clojure.core", "nil?");
public utils$first_non_nil__24()
{
}
}
40 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn format-date
[env params]
(let [#^Date date (params :date)
package app1; #^DateFormat fmt (DateFormat/getDateTimeInstance
DateFormat/MEDIUM
import clojure.lang.*; DateFormat/MEDIUM)]
import java.text.DateFormat; (.format fmt date)))
import java.util.Date;
public class fragments$format_date__21 extends AFunction
{
public Object invoke(Object env, Object params)
throws Exception
{
Object date = ((IFn)params).invoke(const__1);
Object fmt = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
env = null;
params = null;
date = null;
fmt = null;
return ((DateFormat)fmt).format((Date)date);
}
public static final Var const__0 = (Var)RT.var("clojure.core", "let");
public static final Object const__1 = Keyword.intern(Symbol.create(null, "date"));
public fragments$format_date__21()
{
}
}
41 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn format-date
[env params]
(let [#^Date date (params :date)
package app1; #^DateFormat fmt (DateFormat/getDateTimeInstance
DateFormat/MEDIUM
import clojure.lang.*; DateFormat/MEDIUM)]
import java.text.DateFormat; (.format fmt date)))
import java.util.Date;
public class fragments$format_date__21 extends AFunction
{
public Object invoke(Object env, Object params)
throws Exception
{
Object date = ((IFn)params).invoke(const__1);
Object fmt = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
env = null;
params = null;
date = null;
fmt = null;
return ((DateFormat)fmt).format((Date)date);
}
public static final Var const__0 = (Var)RT.var("clojure.core", "let");
public static final Object const__1 = Keyword.intern(Symbol.create(null, "date"));
public fragments$format_date__21()
{
}
}
42 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn format-date
[env params]
(let [#^Date date (params :date)
package app1; #^DateFormat fmt (DateFormat/getDateTimeInstance
DateFormat/MEDIUM
import clojure.lang.*; DateFormat/MEDIUM)]
import java.text.DateFormat; (.format fmt date)))
import java.util.Date;
public class fragments$format_date__21 extends AFunction
{
public Object invoke(Object env, Object params)
throws Exception
{
Object date = ((IFn)params).invoke(const__1);
Object fmt = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
env = null;
params = null;
date = null;
fmt = null;
return ((DateFormat)fmt).format((Date)date);
}
public static final Var const__0 = (Var)RT.var("clojure.core", "let");
public static final Object const__1 = Keyword.intern(Symbol.create(null, "date"));
public fragments$format_date__21()
{
}
}
43 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn format-date
[env params]
(let [#^Date date (params :date)
package app1; #^DateFormat fmt (DateFormat/getDateTimeInstance
DateFormat/MEDIUM
import clojure.lang.*; DateFormat/MEDIUM)]
import java.text.DateFormat; (.format fmt date)))
import java.util.Date;
public class fragments$format_date__21 extends AFunction
{
public Object invoke(Object env, Object params)
throws Exception
{
Object date = ((IFn)params).invoke(const__1);
Object fmt = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
env = null;
params = null;
date = null;
fmt = null;
return ((DateFormat)fmt).format((Date)date);
}
public static final Var const__0 = (Var)RT.var("clojure.core", "let");
public static final Object const__1 = Keyword.intern(Symbol.create(null, "date"));
public fragments$format_date__21()
{
}
}
44 © 2009 Howard Lewis Ship

Compiled to Bytecode
(defn format-date
[env params]
(let [#^Date date (params :date)
package app1; #^DateFormat fmt (DateFormat/getDateTimeInstance
DateFormat/MEDIUM
import clojure.lang.*; DateFormat/MEDIUM)]
import java.text.DateFormat; (.format fmt date)))
import java.util.Date;
public class fragments$format_date__21 extends AFunction
{
public Object invoke(Object env, Object params)
throws Exception
{
Object date = ((IFn)params).invoke(const__1);
Object fmt = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
env = null;
params = null;
date = null;
fmt = null;
return ((DateFormat)fmt).format((Date)date);
}
public static final Var const__0 = (Var)RT.var("clojure.core", "let");
public static final Object const__1 = Keyword.intern(Symbol.create(null, "date"));
public fragments$format_date__21()
{
}
}
45 © 2009 Howard Lewis Ship

Ahead of Time Compilation
app/fragments.clj
(ns app.fragments
(:import (java.util Date)
(java.text DateFormat)))
(defn format-date
[env params]
(…))
Compiler
app/fragments__init.class
app/fragments$format_date__21.class
46 © 2009 Howard Lewis Ship

Ahead Of Time Compilation
build.xml
<target name="compile" description="Compile Clojure sources.">
<mkdir dir="${classes.dir}" />
<pathconvert pathsep=" " property="compile.namespaces">
<fileset dir="${src.dir}" includes="**/*.clj" />
<chainedmapper>
<packagemapper from="${basedir}/${src.dir}/*.clj" to="*" />
<filtermapper>
<replacestring from="_" to="-" />
</filtermapper>
</chainedmapper>
</pathconvert>
<java classname="clojure.lang.Compile">
<classpath>
<path refid="libs.path" />
<path location="${classes.dir}" />
<path location="${src.dir}" />
</classpath>
<sysproperty key="clojure.compile.path" value="${classes.dir}" />
<arg line="${compile.namespaces}" />
</java>
</target>
47 © 2009 Howard Lewis Ship

Ahead Of Time Compilation
build.xml
<target name="compile" description="Compile Clojure sources.">
<mkdir dir="${classes.dir}" />
<pathconvert pathsep=" " property="compile.namespaces">
<fileset dir="${src.dir}" includes="**/*.clj" />
<chainedmapper>
<packagemapper from="${basedir}/${src.dir}/*.clj" to="*" />
<filtermapper>
<replacestring from="_" to="-" />
</filtermapper> Locate *.clj under
</chainedmapper> ${src.dir} and convert to
</pathconvert>
namespace names
<java classname="clojure.lang.Compile">
<classpath>
<path refid="libs.path" />
<path location="${classes.dir}" />
<path location="${src.dir}" />
</classpath>
<sysproperty key="clojure.compile.path" value="${classes.dir}" />
<arg line="${compile.namespaces}" />
</java>
</target>
48 © 2009 Howard Lewis Ship

Ahead Of Time Compilation
build.xml
<target name="compile" description="Compile Clojure sources.">
<mkdir dir="${classes.dir}" />
<pathconvert pathsep=" " property="compile.namespaces">
<fileset dir="${src.dir}" includes="**/*.clj" />
<chainedmapper>
<packagemapper from="${basedir}/${src.dir}/*.clj" to="*" />
<filtermapper>
<replacestring from="_" to="-" />
</filtermapper>
</chainedmapper>
</pathconvert> clojure.jar, source
<java classname="clojure.lang.Compile">
directory and output
<classpath> directory must be on
<path refid="libs.path" /> classpath
<path location="${classes.dir}" />
<path location="${src.dir}" />
</classpath>
<sysproperty key="clojure.compile.path" value="${classes.dir}" />
<arg line="${compile.namespaces}" />
</java>
</target>
49 © 2009 Howard Lewis Ship

Ahead Of Time Compilation
build.xml
<target name="compile" description="Compile Clojure sources.">
<mkdir dir="${classes.dir}" />
<pathconvert pathsep=" " property="compile.namespaces">
<fileset dir="${src.dir}" includes="**/*.clj" />
<chainedmapper>
<packagemapper from="${basedir}/${src.dir}/*.clj" to="*" />
<filtermapper>
<replacestring from="_" to="-" />
</filtermapper>
</chainedmapper>
</pathconvert>
Output directory for
<java classname="clojure.lang.Compile"> generated classes
<classpath>
<path refid="libs.path" />
<path location="${classes.dir}" />
<path location="${src.dir}" />
</classpath>
<sysproperty key="clojure.compile.path" value="${classes.dir}" />
<arg line="${compile.namespaces}" />
</java>
</target>
List of namespaces to
compile
50 © 2009 Howard Lewis Ship

Clojure Pitfalls
51 © 2009 Howard Lewis Ship

IDE Support
52 © 2009 Howard Lewis Ship

Laziness
RuntimeException:
Divide by zero
53 © 2009 Howard Lewis Ship

Laziness
Clojure 1.0.0-
1:1 user=> (def nums (map / (range 0 10) (reverse (range 0 10))))
#'user/nums
1:2 user=> nums
java.lang.ArithmeticException: Divide by zero
(0 1/8 2/7 1/2 4/5 5/4 2 7/2 1:3 user=> (.. *e getCause printStackTrace)
java.lang.ArithmeticException: Divide by zero
at clojure.lang.Numbers.divide(Numbers.java:138)
at clojure.core$_SLASH___3350.invoke(core.clj:575)
at clojure.core$map__3815$fn__3822.invoke(core.clj:1508)
at clojure.lang.LazySeq.seq(LazySeq.java:41)
at clojure.lang.Cons.next(Cons.java:37)
at clojure.lang.RT.next(RT.java:560)
at clojure.core$next__3117.invoke(core.clj:50)
at clojure.core$nthnext__4405.invoke(core.clj:2531)
at clojure.core$print_sequential__5354.invoke(core_print.clj:53)
at clojure.core$fn__5439.invoke(core_print.clj:136)
at clojure.lang.MultiFn.invoke(MultiFn.java:161)
at clojure.core$pr_on__4145.invoke(core.clj:2020)
at clojure.core$pr__4148.invoke(core.clj:2030)
at clojure.lang.AFn.applyToHelper(AFn.java:173)
at clojure.lang.RestFn.applyTo(RestFn.java:137)
at clojure.core$apply__3243.doInvoke(core.clj:390)
at clojure.lang.RestFn.invoke(RestFn.java:428)
at clojure.core$prn__4159.doInvoke(core.clj:2053)
at clojure.lang.RestFn.invoke(RestFn.java:413)
at clojure.main$repl__5813$read_eval_print__5825.invoke(main.clj:177)
at clojure.main$repl__5813.doInvoke(main.clj:193)
at clojure.lang.RestFn.invoke(RestFn.java:876)
at clojure.contrib.repl_ln$repl__84.doInvoke(repl_ln.clj:259)
at clojure.lang.RestFn.invoke(RestFn.java:426)
at clojure.contrib.repl_ln$_main__44.doInvoke(repl_ln.clj:136)
at clojure.lang.RestFn.applyTo(RestFn.java:142)
at clojure.contrib.repl_ln.main(Unknown Source)
nil
1:7 user=>
54 © 2009 Howard Lewis Ship

Lack of Type Checking
55 © 2009 Howard Lewis Ship

Spot the Error
view_manager.clj
(defn to-dom-node-seq
[any]
"Converts the result of a render function to a seq as needed."
(cond
(nil? any) nil
(sequential? any) any
; A map is assumed to be a DOM node, wrap it in a vector
(map? any) [map]
(string? any) [(struct-map dom-node :type :text :value any)]
true (throw (RuntimeException. (format "A rendering function returned %s. ↵
Rendering functions should return nil, a string, a seq of DOM nodes, or a single ↵
DOM node."
(pr-str any))))))
56 © 2009 Howard Lewis Ship

Exception
ERROR in (simple-view-and-fragment) (test_is.clj:657)
Uncaught exception, not in assertion.
expected: nil
actual: java.lang.IllegalArgumentException: Wrong number of args passed to: core$map
at clojure.lang.AFn.throwArity (AFn.java:449)
clojure.lang.RestFn.invoke (RestFn.java:417)
com.howardlewisship.cascade.dom$fn__960.doInvoke (dom.clj:58)
clojure.lang.RestFn.invoke (RestFn.java:443)
clojure.lang.MultiFn.invoke (MultiFn.java:165)
com.howardlewisship.cascade.dom/render_xml_with_ns (dom.clj:118)
com.howardlewisship.cascade.dom/render_xml (dom.clj:125)
com.howardlewisship.cascade.test_views/render (test_views.clj:27)
com.howardlewisship.cascade.test_views/test_view (test_views.clj:56)
com.howardlewisship.cascade.test_views/test_view (test_views.clj:52)
com.howardlewisship.cascade.test_views/fn (test_views.clj:62)
dom.clj
(defmulti render-node-xml
(fn [node & rest]
(node :type)))
57 © 2009 Howard Lewis Ship

Spot the Error
view_manager.clj
(defn to-dom-node-seq
[any]
"Converts the result of a render function to a seq as needed."
(cond
(nil? any) nil
(sequential? any) any
; A map is assumed to be a DOM node, wrap it in a vector
(map? any) [map]
(string? any) [(struct-map dom-node :type :text :value any)]
true (throw (RuntimeException. (format "A rendering function returned %s. ↵
Rendering functions should return nil, a string, a seq of DOM nodes, or a single ↵
DOM node."
(pr-str any))))))
Returning function clojure.core/map;
should return any (the parameter)
58 © 2009 Howard Lewis Ship

Bus Factor
59 © 2009 Howard Lewis Ship

GitHub Factor
60 © 2009 Howard Lewis Ship

Performance
61 © 2009 Howard Lewis Ship

Performance
(defn shorts-to-bytes [#^shorts src #^bytes dst words]
(loop [src-offset (int 0)
dst-offset (int 0)]
(when (< src-offset words)
(let [sample (short (aget src src-offset))]
(aset-byte dst dst-offset (byte sample))
(aset-byte dst (inc dst-offset) (byte (bit-shift-right sample 8))))
(recur (inc src-offset) (unchecked-add 2 dst-offset)))))
public static void shortsToBytes(short[] src, byte[] dst, int len)
{
int idx = 0; 900 ms
short s;
while (len-- > 0) {
s = src[idx];
dst[idx*2] = (byte)s; 675 ms
dst[idx*2+1] = (byte)(s>>>8);
idx++;
}
}
450 ms
225 ms
0 ms
Java Clojure
62 © 2009 Howard Lewis Ship

API Docs
63 © 2009 Howard Lewis Ship

Wrap Up
64 © 2009 Howard Lewis Ship

What's Not Covered?
• multimethods ➠ inheritance-like function
dispatch
• Concurrency Support ➠ atoms, agents, refs
& vars
http://www.clojure.org
• loop & recur ➠ iterative-style coding
• clojure.contrib ➠ Community library
• Macros & Meta-Programming ➠ Create the language you want inside
Clojure
• Tuning Clojure Performance
• More ….
65 © 2009 Howard Lewis Ship

Stuart Halloway
Pragmatic Bookshelf
http://pragprog.com/titles/shcloj/programming-clojure
66 © 2009 Howard Lewis Ship

http://jnb.ociweb.com/jnb/jnbMar2009.html
67 © 2009 Howard Lewis Ship

Image Credits
© 2005 Jean-Philippe Daigle
http://www.flickr.com/photos/jpdaigle/59942231/
© 2007 Howard Gees
http://www.flickr.com/photos/cyberslayer/952121271/
© 2006 Simon Law
http://www.flickr.com/photos/sfllaw/222795669/
© 2008 II-cocoy22-II
http://www.flickr.com/photos/30954876@N07/3090155264/
© 2009 Martin Biskoping
http://www.flickr.com/photos/mbiskoping/3388639698/
© 2008 sea turtle
http://www.flickr.com/photos/sea-turtle/3049443478/
© 2009 Tulio Bertorini
http://www.flickr.com/photos/tuliobertorini/3159130251/
© 2007 Adam Balch
http://www.flickr.com/photos/triplemaximus/795758146/
© 2008 jessamyn west
http://www.flickr.com/photos/iamthebestartist/2636607001/
68 © 2009 Howard Lewis Ship