one or two things you may not know about typesystems

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one or two things you may not know about typesystems

  1. 1. one or two things you may not know about type systems phillip calçado http://fragmental.tw
  2. 2. myths
  3. 3. myth:type systems are just syntax checkers
  4. 4. “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
  5. 5. what kind of errors?
  6. 6. 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); } }
  7. 7. 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); } }
  8. 8. use System; public class DatePrinter { public static void Main(string[] args) { Print(new DateTime()); } public static void Print(DateTime d) { Console.WriteLine(d); } }
  9. 9. ☑ use System; public class DatePrinter { public static void Main(string[] args) { Print(new DateTime()); } public static void Print(DateTime d) { Console.WriteLine(d); } }
  10. 10. use System; public class DatePrinter { public static void Main(string[] args) { Print(null); } public static void Print(DateTime d) { Console.WriteLine(d); } }
  11. 11. ☒ 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
  12. 12. use System; public class DatePrinter { public static void Main(string[] args) { Print(null); } public static void Print(DateTime? d) { Console.WriteLine(d); } }
  13. 13. ☑ use System; public class DatePrinter { public static void Main(string[] args) { Print(null); } public static void Print(DateTime? d) { Console.WriteLine(d); } }
  14. 14. “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
  15. 15. myth: dynamic means weak
  16. 16. what is weak?
  17. 17. “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
  18. 18. unchecked run-time type errors
  19. 19. 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?
  20. 20. unchecked run-time type errors $weird_result
=
100
+
“see”
+
20;
 =>
120
  21. 21. pcalcado@pcalcado:~$
irb >>
weird_result
=
100
+
"see?"
+
20 TypeError:
String
can't
be
coerced
into
 Fixnum 
 from
(irb):1:in
`+' 
 from
(irb):1 >>

  22. 22. myth: static means safe
  23. 23. “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
  24. 24. “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
  25. 25. type error: $weird_result
=
100
+
“see”
+
20;
 =>
120
  26. 26. 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; } }
  27. 27. no type error: =>
Your
triangle
has
4
vertices
  28. 28. myth: static means bureaucratic
  29. 29. public class Sum { public static int add(int a, int b) { return a + b; } }
  30. 30. add(a, b) { return a + b } Is this hypothetical language dynamic or static?
  31. 31. Ruby def add(a, b) a + b end Dynamic
  32. 32. C# ((a, b) => a + b) Static
  33. 33. Clojure (defn add [a b] (+ a b)) Dynamic
  34. 34. Haskell add a b = a + b Static
  35. 35. static can be smart
  36. 36. 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>

  37. 37. myth: only dynamic is flexible
  38. 38. >>
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 >>

  39. 39. “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
  40. 40. 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
  41. 41. 652 lines of Haskell
  42. 42. what does the future hold?
  43. 43. does typing matter?
  44. 44. =>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
  45. 45. =>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
  46. 46. 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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