hot programming in java cos 441 princeton university fall 2004
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HOT Programming in Java
COS 441
Princeton University
Fall 2004
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What’s HOT
Term coined by Bob Harper
Higher-Order and Typed
Some HOT languages
SML, Haskell, Java, C#
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Features of HOT Languages
• Type-Safe
• Lexical Closures
• Garbage Collected
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A HOT Programming Style
SML is arguably “HOTer” than most language– Immutable by default– Supports inductive datatypes and pattern
matching– Advanced module system– Second-Class Polymorphism with Type
Inference
How does Java stack up?
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Object Orientated vs HOT Programming
Mitchell asked the question (10.2.5)“Can you program in an Object Oriented way in ML?”
Today’s question is“Can you program in a HOT way in and Object Oriented language?”
AlternativelyHow can I use what I learned about programming in SML and use it in a “real language”
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A Simple Example
datatype nat = Zero | Succ of nat
fun add(m,Zero) = m | add(m,Succ(n)) = Succ(add(m,n))
fun fib(Zero) = Succ(Zero) | fib(Succ(Zero)) = Succ(Zero) | fib(Succ(Succ(n))) = add(fib(Succ(n),
fib(n))
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Encoding a Datatype
datatype nat = Zero | Succ of nat
abstract class Nat { }class Zero extends Nat {
Zero(){ /* nop */ } }class Succ extends Nat { final Nat n; Nat(Nat n) { this.n = n; }}
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Creating a Value
val one = Succ(Zero)
final Nat one = new Succ (new Zero ());
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Creating a Value
val one = Succ(Zero)
import static Nat.*;final Nat one = Succ (Zero);
abstract class Nat { static Nat Zero = new Zero(); static Nat Succ(Nat n) {
return new Succ(n); } }
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The General Pattern
When encoding a type of the form
t = A + B + C + D
Create an abstract class of type “t” and have each case inherit from the abstract base class
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Encoding a Function
Several ways to do this
Will try obvious pure OO way first
Later approach using type casts
Finally Visitor Pattern
fun add(m,Zero) = m | add(m,Succ(n)) = Succ(add(m,n))
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Encoding a Function
Swap order of arguments so that add is inductively defined on its first argument
Now think of add(n,m) as n.add(m) where n is either a Zero or Succ object
Define methods appropriately
fun add(Zero,m) = m | add(Succ(n),m) = Succ(add(n,m))
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Encoding a Function
fun add(Zero,m) = m | add(Succ(n),m) = Succ(add(n,m))
abstract class Nat { abstract Nat add(Nat n); }
class Zero extends Nat { … Nat add(Nat m) { return m; } }class Succ extends Nat { … Nat add(Nat m) {
return new Succ(this.n.add(m)); }}
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A General Pattern?
Technique we described was not a general solution relies on having simple inductive definition of first argument
How do we handle binary methods like “eq”
fun eq(Zero,Zero) = true | eq(Succ(n),Succ(m)) = eq(n,m) | eq(_,_) = false
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Double Dispatchfun eq(Zero,Zero) = true | eq(Succ(n),Succ(m)) = eq(n,m) | eq(_,_) = false
abstract class Nat { … abstract boolean eq(Nat m); abstract boolean zeroEq(); abstract boolean succEq(Nat n);}
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Double Dispatchfun eq(Zero,Zero) = true | eq(Zero,_) = false
class Zero extends Nat { … boolean eq(Nat m){return m.zeroEq();} boolean zeroEq() {return true; } boolean succEq(Nat n) {return false;}}
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Double Dispatchfun eq(Succ(n),Succ(m)) = eq(n,m) | eq(_,_) = false
class Succ extends Nat { … boolean eq(Nat m){
return m.succEq(this.n); } boolean zeroEq() {return false; } boolean succEq(Nat n){
return n.eq(this.n); }}
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What about Fib?
Encoding Fib not easy to encode for other reasons besides double dispatch!
fun fib(Zero) = Succ(Zero) | fib(Succ(Zero)) = Succ(Zero) | fib(Succ(Succ(n))) = add(fib(Succ(n),
fib(n))
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What about Fib?
fun fib(Zero) = Succ(Zero) | fib(Succ(Zero)) = Succ(Zero) | fib(Succ(Succ(n))) = add(fib(Succ(n),
fib(n))
Nat fib(Nat n) {if(n instanceof Zero) {
return new Succ(new Zero());}Succ m = (Succ) n;…
}
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InstanceOf
Java provides instanceof operator to test for class at runtime and dynamic type casts to cast appropiately
Requires runtime type info (not available in C++)
Error Prone and Hard to read
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Reducing to Primitive Matching
fun fib(Zero) = Succ(Zero) | fib(Succ(Zero)) = Succ(Zero) | fib(Succ(Succ(n))) = add(fib(Succ(n),
fib(n))fun fib(Zero) = Succ(Zero) | fib(Succ(m)) = fibSucc(m)and fibSucc(Zero) = Succ(Zero) | fibSucc(Succ(n)) = add(fib(Succ(n),
fib(n))
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Visitor Design Patternabstract class Nat { abstract Nat accept(NatVisitor v); }
interface NatVisitor { Nat vZero(); Nat vSucc(Nat n);}
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Visitor Design Patternclass Zero extends Nat { … Nat accept(NatVisitor v) { return v.vZero(); }} class Succ extends Nat { … final Nat n; Nat accept(NatVisitor v) { return v.vSucc(this.n); }}
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Visitor for Computing Fibclass Fib implements NatVisitor { static Nat fib(Nat n) {
return n.accept(new Fib()); } public vZero() { return new Succ(new Zero()); } public vSucc(Nat n) { return FibSucc.fibSucc(n); }}
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class FibSucc implements NatVisitor { static Nat fibSucc(Nat n) {
return n.accept(new FibSucc()); } public vZero() { return new Succ(new Zero()); } public vSucc(Nat n) { return Fib.fib(new Succ(n)). add(Fib.fib(n)); }}
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Reducing to Primitive Matching
fun fib(Zero) = Succ(Zero) | fib(Succ(Zero)) = Succ(Zero) | fib(Succ(Succ(n))) = add(fib(Succ(n),
fib(n))fun fib(Zero) = Succ(Zero) | fib(Succ(n)) = (case n of Zero => Succ(Zero) | Succ(m) => fib(n,m))
Alternate way that avoids extra function
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Using An Anonymous Inner Class
class Fib implements NatVisitor { … public vSucc(final Nat n) { return n.accept(new NatVisitor() { public vZero { return …;} public vSucc(Nat m) { return fib(n,m); }
} ); }}
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A Generic Visitorabstract class Nat { abstract <T> accept(Visitor<T> v); }
interface Visitor<T> { T vZero(); T vSucc(Nat n);}
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How Painful Is This?Lines of
SMLLines of
JavaJava/
SML
Name Library 33 26 0.79
MinML Syntax 30 188 6.27
Static Semantics 48 86 1.79
Dynamic Semantics 36 84 2.33
Shape Library 62 109 1.76
Ratio is ~2.5 Lines of Java for Every Line of SML
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How Fast Is This?
20 Loops of fib(25)
SML Java
Native 0.25 sec 0.12 sec
MinML Interp 33 sec 60 sec
Java is about 2x slower but our scheme is 4x more expensive when compare to SML/NJ
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Dimensions of Feature Extensions
Add Mul Fact Fib …
Zero … … … … …
Succ … … … … …
Extensible number of functions or attributes
Fix
ed s
et o
f O
bjec
ts
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Dimensions of Feature Extensions
Weight Desnsity Cost
Bricks … … …
Stone … … …
Wood … … …
Glass … … …
…
Fixed set of attributes or functions
Ext
ensi
ble
num
ber
kind
s of
obj
ects
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I Want Both
We have been extending MinML interpreter in both dimensions!
Different operational semantics
M-Machine, C-Machine, E-Machine, …
Different expressions and types
Pairs, Sums, Recursive, Polymorphic, …
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Language Design Challenge
• A good language should support extensions in both dimensions
• ML clunky in the object extension direction
• Java is clunky in the functionality extension dimension
• New languages being designed as we speak to support both well– Nice, Scala, MultiJava, …
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An Old Saying
A FORTRAN programmer can program FORTRAN in any language!
i.e. Program in any language as if it were FORTRAN!
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Summary
A HOT programmer can program HOTly in any language!
i.e. Never let your programming language stop you from being too clever!