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Type SystemsDaniel Quirino Oliveira
Tuesday, January 18, 2011
Self referencehttp://www.slideshare.net/danielqo/type-systems-6468967
Tuesday, January 18, 2011
What?
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A set of syntatic constraints and rules that classifies programming language statements according to that values they deal with by associating types to values.
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Purpose?
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Check and prove a program correctness
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Wait, wait... What’s type?
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Intuitively...A set of values something may assumeA set of operations something may do
Or both
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Label and a set of properties used by a type system to prove
program's correctness
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Types of Type Systems
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Monomorphiceach constant/variable/function may assume an unique type according to its value
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e.g.: Pascal, C
procedure foo() begin foo := 10;end
i : integer;c : character;
i := 42;c := ‘a’;
int foo() { return 10;}
int i = 42;char c = ‘a’;
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Polymorphiceach constant/variable/function may assume more than one type, given its value, according to context
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e.g.: Java, C# ...
Number toNumber(int a) { return new Integer(a);}
Number toNumber(double b) { return new Double(b);}
Number a = toNumber(42);Number b = toNumber(4.2);Double c = (Double) b;Integer d = (Integer) a;
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So, what static/dynamic... typing is about?
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Type CHECKING!
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Static Type Checking SystemA given variable is allowed assume only one TYPE in a given context
All program types are checked at least once by a “compiler” during the semantic analysis (parsing)
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Remember that...
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e.g.: Java, Scala
void foo() { Integer a = 42; String a = “hello”;}
def foo()= { var a = 0; a = “hello”;}
compile-time errorSymbol “a” already defined as being of another type.
run/compile-time errorSymbol “a” already defined as being of another type.
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Dynamic Type Checking SystemA given variable may assume values of different TYPES in a given context during the program flow
I.e., values do have TYPES whereas variables have VALUES
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e.g.: Groovy, Ruby
def foo() { def a = 42; a = “hello”;}
def foo a = 0 a = “hello”end
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Misconceptions
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Statically type checked languages require type
declarations
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So, what about C#, Scala and ML?
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All “scripting” languages are dynamically type checked
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Well, Scala is statically type checked
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All dynamically type checked languages are not compiled
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Really? So, what is Groovy?
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Dynamically type checked languages are weakly typed
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Wait! Weak?!
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Strong vs. Weak dilemma
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Strongly typed languaguesA language that defines a set of rules that restrict values of different types of interacting
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a = 1b = “1”c = a + bTypeError: String can't be coerced into Fixnum
e.g.: Ruby
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Weakly typed languaguesA language that does not define a set of rules that restrict values of different types of interacting but
automatically tries to convert somehow one value’s type into the other’s
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Dynamically type checked languages are weakly typed not
manifest typed
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a = 1b = “1”c = a + b // 11
e.g.: Javascript
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Q&A
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ReferencesBooks
Piece, Benjamin (2002). Types and Programming Languages.Scott, Michael (2009). Programming Language Pragmatics.
Wirth, Niklaus (1996). Compiler Construction.
Web
http://lucacardelli.name/Papers/TypeSystems.pdfhttp://web.archive.org/web/20080822101209/http://www.pphsg.org/cdsmith/types.html
http://web.archive.org/web/20080331215906/perl.plover.com/yak/typing/notes.html http://plato.stanford.edu/entries/type-theory/
http://www.jstor.org/stable/2266170 http://www.cs.cornell.edu/info/projects/nuprl/book/node31.html
http://research.microsoft.com/en-us/um/people/emeijer/Papers/RDL04Meijer.pdfhttp://docs.google.com/View?id=dcsvntt2_25wpjvbbhk
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Danke :)
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