one or two things you may not know about typesystems
TRANSCRIPT
one or two things
phillip calçado http://fragmental.tw
you may not know
about type systems
myths
myth:type systems are just syntax checkers
“The fundamental purpose of a type system is to prevent the occurrence of execution errors during the running of a program.”
- Luca Cardelli, Type Systems
what kind oferrors?
package org.apache.commons.lang.time;
public class DateUtils { public static boolean isSameDay(Date date1, Date date2) { if (date1 == null || date2 == null) { throw new IllegalArgumentException("The date must not be null"); } return verifySameDay(date1, date2); }}
package org.apache.commons.lang.time;
public class DateUtils { public static boolean isSameDay(Date date1, Date date2) { if (date1 == null || date2 == null) { throw new IllegalArgumentException("The date must not be null"); } Calendar cal1 = Calendar.getInstance(); cal1.setTime(date1); Calendar cal2 = Calendar.getInstance(); cal2.setTime(date2); return isSameDay(cal1, cal2); }}
but why would it be allowed to be null in
the first place?
use System;
public class DatePrinter { public static void Main(string[] args) { Print(new DateTime()); }
public static void Print(DateTime d) { Console.WriteLine(d); }}
use System;
public class DatePrinter { public static void Main(string[] args) { Print(new DateTime()); }
public static void Print(DateTime d) { Console.WriteLine(d); }}
☑
use System;
public class DatePrinter { public static void Main(string[] args) { Print(null); }
public static void Print(DateTime d) { Console.WriteLine(d); }}
use System;
public class DatePrinter { public static void Main(string[] args) { Print(null); }
public static void Print(DateTime d) { Console.WriteLine(d); }}
☒pcalcado@pcalcado:awayday2009$gmcsDatePrinter.csDatePrinter.cs(7,5):errorCS1502:Thebestoverloadedmethodmatchfor`DatePrinter.Print(System.DateTime)'hassomeinvalidargumentsDatePrinter.cs(10,22):(Locationofthesymbolrelatedtopreviouserror)DatePrinter.cs(7,5):errorCS1503:Argument`#1'cannotconvert`null'expressiontotype`System.DateTime'Compilationfailed:2error(s),0warnings
use System;
public class DatePrinter { public static void Main(string[] args) { Print(null); }
public static void Print(DateTime? d) { Console.WriteLine(d); }}
use System;
public class DatePrinter { public static void Main(string[] args) { Print(null); }
public static void Print(DateTime? d) { Console.WriteLine(d); }}
☑
“I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system [...] My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference [...] This has led to innumerable errors [...] which have probably caused a billion dollars of pain and damage in the last forty years. In recent years, a number of program analysers [...] in Microsoft have been used to check references, and give warnings if there is a risk they may be non-null. More recent programming languages like Spec# have introduced declarations for non-null references. This is the solution, which I rejected in 1965.”
- C.A.R. Hoare
dynamic means weak
myth:
what isweak?
“typeful programming advocates static typing, as much as possible, and dynamic typing when necessary; the strict observance of either or both of these techniques leads to strong typing, intended as the absence of unchecked run-time type errors.”
- Luca Cardelli, Typeful Programming
unchecked run-time type errors
pcalcado@pcalcado:~$php‐aInteractivemodeenabled<?php$i_am_a_string="see?";$weird_result=100+$i_am_a_string+20;echo$weird_result."\n";echo$i_am_a_string."\n";?>120see?
$weird_result=100+“see”+20;
unchecked run-time type errors
=>120
pcalcado@pcalcado:~$irb>>weird_result=100+"see?"+20TypeError:Stringcan'tbecoercedintoFixnum from(irb):1:in`+' from(irb):1>>
static means safe
myth:
“typeful programming advocates static typing, as much as possible, and dynamic typing when necessary; the strict observance of either or both of these techniques leads to strong typing, intended as the absence of unchecked run-time type errors.”
- Luca Cardelli, Typeful Programming
“typeful programming advocates static typing, as much as possible, and dynamic typing when necessary; the strict observance of either or both of these techniques leads to strong typing, intended as the absence of unchecked run-time type errors.”
- Luca Cardelli, Typeful Programming
$weird_result=100+“see”+20;=>120
type error:
public class NoError{ public static void main(String[] args){ Triangle t = new Triangle(); t.addVertex(0,0); t.addVertex(10,10); t.addVertex(20,20); t.addVertex(30,30); System.out.println("Your triangle has "+ t.getVertices().size() + " vertices"); }}
class Triangle{private List<int[]> vertices = new ArrayList<int[]>();
public void addVertex(int x, int y){ vertices.add(new int[]{x, y}); } public List<int[]> getVertices(){ return vertices; }}
no type error:=>Yourtrianglehas4vertices
myth: static means
bureaucratic
public class Sum {
public static int add(int a, int b) { return a + b; }
}
Is this hypothetical language dynamic or static?
add(a, b) { return a + b}
Dynamic
def add(a, b) a + bend
Ruby
Static
((a, b) => a + b)
C#
Dynamic
(defn add [a b] (+ a b))
Clojure
Static
add a b = a + b
Haskell
static can be smart
add a b = a + b
Prelude>:loadadd.hs[1of1]CompilingMain(add.hs,interpreted)Ok,modulesloaded:Main.*Main>:typeaddadd::(Numa)=>a‐>a‐>a*Main>:type(add12)(add12)::(Numt)=>t*Main>
myth:
only dynamic is flexible
>>my_func()NoMethodError:undefinedmethod`my_func'formain:Object from(irb):1>>instance_eval("defmy_func()\nputs666\nend")=>nil>>my_func()666=>nil>>
“we think that people use eval as a poor man’s substitute for higher-order functions. Instead of passing around a function and call it, they pass around a string and eval it. [...] A final use of eval that we want to mention is for partial evaluation,multi-stage programming, or meta programming. We argue that in that case strings are not really the most optimal structure to represent programs and it is much better to use programs to represent programs, i.e. C++-style templates, quasiquote/unquote as in Lisp, or code literals as in the various multi-stage programming languages.”
- The End of the Cold War Between Programming Languages, Erik Meijer and Peter Drayton
main = runBASIC $ do 10 GOSUB 1000 20 PRINT "* Welcome to HiLo *" 30 GOSUB 1000
100 LET I := INT(100 * RND(0)) 200 PRINT "Guess my number:" 210 INPUT X 220 LET S := SGN(I-X) 230 IF S <> 0 THEN 300
240 FOR X := 1 TO 5 250 PRINT X*X;" You won!" 260 NEXT X 270 STOP
300 IF S <> 1 THEN 400 310 PRINT "Your guess ";X;" is too low." 320 GOTO 200
400 PRINT "Your guess ";X;" is too high." 410 GOTO 200
1000 PRINT "*******************" 1010 RETURN
9999 END
652 lines of Haskell
what does the future hold?
does typing matter?
=>typing influences language features and tools
YES=>static typing is being wrongly bashed because of C#/Java just as dynamic was bashed because of PHP/Perl=>schools are merging (e.g. C# 4) and it’s important to know each one’s sweet spot
=>saying that something is static or dynamic doesn’t tell much about what it can do
NO=>most nice features in Python/Ruby/JavaScript are related to meta-model, not typing=>Java/C# are bureaucratic for historical reasons, not limitations on typing
refs:=>http://pico.vub.ac.be/~wdmeuter/RDL04/papers/Meijer.pdf=>http://lucacardelli.name/Papers/TypefulProg.A4.pdf=>http://sadekdrobi.com/2008/12/22/null-references-the-billion-dollar-mistake/
=>http://www.flickr.com/photos/twindx/=>http://www.flickr.com/photos/darwinbell/=>http://www.flickr.com/photos/wainwright/=>http://www.flickr.com/photos/fikirbaz/