one or two things you may not know about typesystems

one or two things you may not know about type systems phillip calçado http://fragmental.tw

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 of errors?

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 { but why would it be public static boolean isSameDay(Date date1, Date date2) { if (date1 == null || date2 == null) { allowed to be null in throw new IllegalArgumentException("The date must not be null"); } the first place? Calendar cal1 = Calendar.getInstance(); cal1.setTime(date1); Calendar cal2 = Calendar.getInstance(); cal2.setTime(date2); return isSameDay(cal1, cal2); } }

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$ gmcs DatePrinter.cs } DatePrinter.cs(7,5): error CS1502: The best overloaded method match for  `DatePrinter.Print(System.DateTime)' has some invalid arguments DatePrinter.cs(10,22): (Location of the symbol related to previous error) DatePrinter.cs(7,5): error CS1503: Argument `#1' cannot convert `null'  expression to type `System.DateTime' Compilation failed: 2 error(s), 0 warnings

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

myth: dynamic means weak

what is weak?

“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 ‐a Interactive mode enabled <?php  $i_am_a_string = "see?";  $weird_result = 100 + $i_am_a_string + 20;  echo $weird_result . "n"; echo $i_am_a_string . "n"; ?> 120 see?

unchecked run-time type errors $weird_result = 100 + “see” + 20;  => 120

pcalcado@pcalcado:~$ irb >> weird_result = 100 + "see?" + 20 TypeError: String can't be coerced into  Fixnum   from (irb):1:in `+'   from (irb):1 >> 

myth: static means safe

“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

type error: $weird_result = 100 + “see” + 20;  => 120

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: => Your triangle has 4 vertices

myth: static means bureaucratic

public class Sum { public static int add(int a, int b) { return a + b; } }

add(a, b) { return a + b } Is this hypothetical language dynamic or static?

Ruby def add(a, b) a + b end Dynamic

C# ((a, b) => a + b) Static

Clojure (defn add [a b] (+ a b)) Dynamic

Haskell add a b = a + b Static

static can be smart

add a b = a + b Prelude> :load add.hs [1 of 1] Compiling Main           ( add.hs, interpreted ) Ok, modules loaded: Main. *Main> :type add add :: (Num a) => a ‐> a ‐> a *Main> :type (add 1 2) (add 1 2) :: (Num t) => t *Main> 

myth: only dynamic is flexible

>> my_func() NoMethodError: undefined method  `my_func' for main:Object   from (irb):1 >> instance_eval("def my_func()n puts  666n end") => 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 =>static typing is YES 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 =>most nice features in NO 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/

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one or two things you may not know about typesystems

by Phil Calçado on Aug 12, 2009

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