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**Type theory & practical**

For Generic Programming Kaushik Biswas*, Dr. Partha Pratim Das, Kausik Datta 21-Aug-2010

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**Organization Genesis of Type Glimpse of Haskell**

Type inference (Haskell) Type deduction and type checking (C++) BREAK Type traits and type manipulation Type pair, Type list and usage Generic operations using types 21-Aug-2010

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**Genesis of Type: Russell’s Paradox**

Set R = { x : x is a not a rectangle } Set S = { x : x is a rectangle } Russell’s query : Is R R ? According to “naïve set theory” : TRUE 21-Aug-2010

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**Russel’s Paradox (contd.)**

Set T = { x : x is set, which does not contain x } 1) T T T T , Contradiction 2) T T T T , Contradiction Russel’s Paradox :: From naïve set theory, some set can be constructed, which leads to contradiction in set comprehension and predicate logic. There was once a barber. Some say that he lived in Seville. Wherever he lived, all of the men in this town either shaved themselves or were shaved by the barber. And the barber only shaved the men who did not shave themselves. Russel’s query: Who saved the barber? 21-Aug-2010

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**Avoiding Russell’s Paradox**

Axiomatic Set Theory ( 7 axioms on set definition) AXIOM III. Axiom of separation (Axiom der Aussonderung) "Whenever the propositional function –(x) is definite for all elements of a set M, M possesses a subset M' containing as elements precisely those elements x of M for which –(x) is true". Ernst Zermelo Ramified hierarchy and Impredicative Principle Napoleon was Corsican. => Predicative A typical Englishman is one who possesses all the properties possessed by a majority of Englishman. => Impredicative. First order properties => That do not refer to totality of properties Second order properties => Refer to totality of first order properties Third order … Bertrand Russell 21-Aug-2010

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**Simple Type Theory i is type of individuals**

( ) is type of propositions if A1, A2, .., An are types then (A1, A2, ..,An) is the type of n-ary relations over objects of respective types A1, A2, .., An Examples:- (i, i) => Binary relations over individuals (( ), ( )) => Binary connectives ((i)) => type of quantifiers over individual More general example:- (A, B, (A), (B), ((A), (B))) Formation of a type having objects, predicates, predicates of predicates. Unfortunately, simple type theory is not predicative, though it avoids Russel’s paradox. 21-Aug-2010

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**Church’s Type Theory (-calculus)**

There are two basic types i : type of individuals : type of propositions If A, B are types then AB, the type of functions from A to B is a type Church introduced function as primitive objects in 1940 using -calculus i (predicates (i) ) (i) (predicates of predicates ((i)) ) i i (type of functions) (i i ) i (type of functional ) (Haskell) Curry notation A1A2 … An B = A1,A2, ..,An B = B (A1,A2,…,An) Functional: ( Mathematics) relating to a variable quantity whose value depends upon one or more other variables 21-Aug-2010

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**Glimpse of Haskell [1,2,3] => list head [1,2,3] => 1**

tail [1,2,3] => [2,3] reverse [1,2,3] => [3,2,1] [1] ++ [2] => [1,2] (1, “rt”) => 2-tuple fst (1, “rt”) => 1 snd(1,”rt”) => “rt” let add a b = a + b let sqr a = a * a map sqr [1,2,3] => [1,4,9] zipWith add [1,2,3] [4,5,6] => [ 5, 7, 9] foldr add 0 [1,2,3] => 6 /export/home/software/ghc /bin/ghci (ssh mures) 21-Aug-2010

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**Seeing Types in Haskell**

:t “hi” :t (1, 2.1, “hi”) :t add :t sqr :t reverse :t map :t zipWith :t head :t tail :t fst :t snd 1 :: (Num t) => t 1.2 :: (Fractional t) => t [1,2,3] :: (Num t) => [t] "hi" :: [Char] (1,2.1, “hi") :: (Num t, Fractional t1) => (t, t1, [Char] ) add :: (Num a) => a -> a -> a sqr :: (Num a) => a -> a reverse :: [a] -> [a] map :: (a -> b) -> [a] -> [b] zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] head :: [a] -> a tail :: [a] -> [a] fst :: (a, b) -> a snd :: (a, b) -> b List Tuple let add a b = a + b let sqr a = a * a Explain this foldr :: (a -> b -> b) -> b -> [a] -> b 21-Aug-2010

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**Type Inference foo x y = (length x) + y length :: [a] -> Int**

Questions: :t foo = ? :t x =? :t y = ? Assign every variable and expression a new type variable foo : 1 x : 1 y : 2 (length x) + y : 2 length x : [ ] Int Create constraint set 1 = 1 2 2 2 = Int 1 = [ ] 2 = Int Solving by Hindley Milner algorithm foo :: [a] Int Int Type inference algorithm is NP-complete in general Even if we have not specified the details of type signature of function “foo”, Haskell compiler can derive the signature of the function “foo”. 21-Aug-2010

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**Type checking vs. Type inference**

Standard type checking (C++) int f ( int x) { return x + 1; } int g (int x) { return f(y+1) *2; } Examine body of each function. Use declared types of identifiers to check type agreement Type inference int f ( int x ) { return x + 1; } int g ( int y ) { return f(y+1) *2; } Examine code without type information, Infer the most general types that could have been declared. ML and Haskell are designed to make type inference feasible. C#3.0, F#, Visual Basic.Net 9.0, also uses. Soon in C++0x. 21-Aug-2010

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**Type deduction in Generic Programming**

Haskell Composition :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c Let construct a fn as follows:- fn : [1,2,3] -> [9,4,1] :t reverse reverse :: [a] -> [a] :t map map :: (a -> b) -> [a] -> [b] :t sqr sqr :: (Num a) => a -> a :t map sqr map sqr :: (Num a) => [a] -> [a] :t ((map sqr) . reverse) ((map sqr) . reverse) :: (Num a) => [a] -> [a] ((map sqr) . reverse) [1,2,3] [9,4,1] C++ compiler can not infer types like Haskell Type computation needs to be part of generic programming template <typename F1, typename F2> struct compose { typename F1::c operator()( F2::a x) { F1 ftor1; F2 ftor2; return ftor1(ftor2(x)); } }; Functor type F1 and F2 must have (b->c) and (a->b) information. To make it generic, it F1 and F2 must be encoded with their input/output types with common type names. 21-Aug-2010

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Workshop for Haskell Problem: Let (x1,y1), (x2,y2), (x3,y3) are three co-ordinate of a triangle. Find area of the triangle using following formula. = ½ { x1(y2-y3) + x2(y3-y1)+ x3(y1-y2)} area :: (Float t) => [ (t,t) ] -> t Input [(1, 1.1), (5.5, 1), (3.1, 10.3)] Generalize your implementation for n-polygon ? = ½ { x1(y2-y4) + x2(y3-y1)+ x3(y4-y2)+x4(y1-y3)} (quadrilateral) Use vector operations to compute 21-Aug-2010

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**Type traits Trait :: a distinguishing characteristic or quality**

Type trait :: a distinguishing characteristic or quality of a Type From type theory, what could be type traits? Propositions/predicates Type of lower level hierarchies Traits of “Lion” type has some common and some extended part based on observer type. 21-Aug-2010

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**Type trait implementation in C++**

// Proposition/Predicates template <typename T> struct is_object ; template <> struct is_object <module_decl> { static const bool val=true; }; Access => is_object<X>::val // Lower order types struct return_type; struct return_type <sqr> { typedef int type; }; Access => return_type<X>::type // Keeping as part of the type struct module_decl { static const bool is_object=true; typedef VeModule* type; typedef VeModule stype; static const char* name; enum { tid = 100}; }; Access => module_decl :: is_object module_decl :: type module_decl :: stype module_decl :: name module_decl :: tid 21-Aug-2010

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**Type manipulation using type-traits**

iterator <T> where T could be List List_Node List_Node :: next() List_Node :: current() Array index index ++ Array [index] template<typename T> struct iterator { typename T:: index_type current; typename T:: type vobj; inline void operator ++ () { increment <typename T:: index_type > (current); } inline typename T :: stype operator * () { return (typename T :: stype) get_info<typename T:: index_type> ( current, vobj); }; increment<T>() function could now farther be specialized on T :: index_type 21-Aug-2010

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**Type List type list set of types having similar notion**

TL = [T1, T2, T3, ….., Tn] Order is non important There is an implicit [] null list at the end of the type_list Usage: - Types forming a group - Field types of an object Required Operations on type list head (TL) = T1 tail (TL) = [T2, T3, ….., Tn] length (TL) = n remove (T2 , TL) = [T1, T3, ….., Tn] index (T2 , TL) = 2 type_of(2, TL ) = T2 is_present (Tx, TL) = { true, false} concat (TL1 , TL2) = TL1 ++ TL2 zip (TL1 , TL2) = [ (Tx , Ty) ] where Tx TL1 and Ty TL2 21-Aug-2010

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**Type Tuple type tuple n-ary relation of types**

Tu = (T1, T2, T3, ….., Tn) Order is important (T1, T2, T3, ….., Tn) = (T1, (T2, T3, ….., Tn) ) Binary relation is sufficient TPair = (TLeft , TRight) Usage: - Forming object and field relations - Forming propositions based on types Operations on tuple type:- left (TPair ) = TLeft right (TPair ) = TRight type_of ( i , TU) = Ti 21-Aug-2010

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**Type list and pair (C++)**

template <typename H, typename T> struct typeList { typedef H head; typedef T tail; }; struct null_type {}; [int, long, double ] is defined as typeList < int, typeList<long, typeList<double, null_type> > > null_type is used for recursive definition creation using type_list TYPE PAIR template <typename L, typename R> struct type_pair { typedef L left; typedef R right; }; ( module_decl, ports ) is defined as type_pair< module_decl , ports> 21-Aug-2010

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**Constructing generic type list**

Implementation of remove_null template <typename T> struct remove_null; template <class H, class T> struct remove_null<typeList<H,T> > { typedef typeList<H, typename remove_null<T>::type> type; }; template <class T> struct remove_null<typeList<null_type ,T> >{ typedef null_type type; template <> struct remove_null<null_type> { typedef null_type type; }; How do we form type_list <T1, T2, T3, T4, .., Tn > ? Template <typename T1=null_type, typename T2=null_type, typename T3=null_type, typename T4=null_type> struct type_list { typedef typeList<T1, typeList<T2, typeList<T3, typeList<T4, null_type> > > > local_type; typedef remove_null <local_type>::type type; }; 21-Aug-2010

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**Why important for Generic programming**

BNF syntaxes could be expressed in terms of type list and type tuple Rule := rule_1 | rule_2 | rule_3 | … | rule_n Implement above by type list Rule := rule_1 rule_2 rule_3 … rule_n Implement above by type tuple A higher order language based on types could be created for domain specific way An appropriate generative algorithm could be created based on this grammar to allow highly compressed expression Low level details of programming could be captured at generator level itself 21-Aug-2010

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**Non-deterministic Generic Structure**

DFA DFA NFA-Cloud Ty DFA NFA Tx Get a node Tx from T1 We know how to work within binary expression, but not how we reach there and where we are going from there. 21-Aug-2010

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**Generating human expression**

David Hilbert -Could we form mathematics on solid foundation so that there is no ambiguity? -Could we construct mechanical rules for proof? Kurt Gödel -You can ask questions that don't have answers within the system and never will. -We will never have a complete symbolic generator. Alan Turing -Created abstract symbolic computation machine to model human thinking process -Lot of deterministic things can be done even outside Halting problems -To end the session, let us be positive about symbolic computation even with its limitations. -Finally we are engineers like Turing, who needs to break Enigma code of human communication process. 21-Aug-2010

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Workshop Exercise Create print_type function to print combination of type list and type pair in haskell form. typedef type_list<T1, type_pair<T2, T4> , T3> ::type Tx; print_type<Tx>(); Output: [T1, (T2, T4), T3] Create “concat” , “index”, “type_of”” function on type_list Create “map” function on type_list so that a specific named function could be called using single line for several types. typedef type_list<T1, T2, T3, T4>::type Tx map<Tx, function>(); 21-Aug-2010

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**References http://plato.stanford.edu/entries/type-theory/**

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**Thank you for your patience!**

21-Aug-2010

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