Compilation 2007 Static Type Checking Michael I. Schwartzbach BRICS, University of Aarhus

2 Static Type Checking The Type Checker  The type checker has several tasks: determine the types of all expressions check that values and variables are used correctly resolve certain ambiguities by transformations  Some languages have no type checker no compile-time guarantees runtime tagging and checking become necessary

3 Static Type Checking Types Describe Possible Values  The Joos types are: void (the empty set of values) byte short int char boolean C (objects of class C or any subclass) null (the polymorphic null constant) σ [] (for any type σ)

4 Static Type Checking Type Annotations  Type annotations: int x; Foo y; specify invariants about the runtime behavior: x will always contain an integer value y will always contain null or an object of class Foo or any subclass  Type annotations are not very expressive viewed as invariants

5 Static Type Checking  A program is type correct if the type annotations are valid invariants  Type correctness is trivially undecidable: int x = 0; TM(j); x = false;  The program is type correct iff the j 'th Turing machine does not halt on empty input Type Correctness

6 Static Type Checking Static Type Correctness  A program is statically type correct if it satisfies some type rules  The rules are chosen to be: simple to understand efficient to decide conservative with respect to type correctness  Type systems are not canonical  They are designed like the rest of the language

7 Static Type Checking Slack  Static type systems are necessarily flawed:  There is always slack: programs that are unfairly rejected by the type checker statically type correcttype correct slack

8 Static Type Checking Examples of Slack  Trivial example: x = 87; if (false) x = true;  Useful example: Map m = new Hashmap(); m.put("bar","foo"); String x = m.get("bar");  Java 1.5 is designed to pick up some slack

9 Static Type Checking Specifying Type Rules  Prose: "The argument of the sqrt function must be of type int, the result is of type double"  Logical rules: C |- x : int C |- sqrt(x) : double

10 Static Type Checking The Shortcomings of Prose The first expression must be of type boolean, or a compile-time error occurs. The conditional operator may be used to choose between second and third operands of numeric type, or second and third operands of type boolean, or second and third operands that are each of either reference type or the null type. All other cases result in a compile-time error. Note that it is not permitted for either the second or the third operand expression to be an invocation of a void method. In fact, it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear. The type of a conditional expression is determined as follows: If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression. Otherwise, if the second and third operands have numeric type, then there are several cases:If one of the operands is of type byte and the other is of type short, then the type of the conditional expression is short. If one of the operands is of type T where T is byte, short, or char, and the other operand is a constant expression of type int whose value is representable in type T, then the type of the conditional expression is T. Otherwise, binary numeric promotion is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands. Note that binary numeric promotion performs value set conversion. If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type. If the second and third operands are of different reference types, then it must be possible to convert one of the types to the other type (call this latter type T) by assignment conversion; the type of the conditional expression is T. It is a compile-time error if neither type is assignment compatible with the other type.

11 Static Type Checking Three Kinds of Type Rules  Declarations: this variable is declared to have this type  Propagations: if the argument is of this type, then the result is of that type  Restrictions: the argument must be of this type or that type  The logical notation handles all three uniformly

12 Static Type Checking Judging Expressions  The judgment: C, L, X, σ |- E: τ means that the expression E is statically type correct and has type τ in a context where: C is the current class L is the current local environment X is the current set of exceptions σ is the return type of the current method

13 Static Type Checking Judging Statements  The judgment: C, L, X, σ |- S means that the statement S is statically type correct in a context where: C is the current class L is the current local environment X is the current set of exceptions σ is the return type of the current method

14 Static Type Checking  The class environment is implicitly available  The notation D≤C means that D is a subclass of C  We can query a class C: C |- extends D(super class) C |- σ f(fields) C |- static σ f(static fields) C |- (σ 1,..., σ k )X(constructors) C |- σ m (σ 1,..., σ k ) X(methods) C |- static σ m (σ 1,..., σ k ) X(static methods) Class Environments return type name argument types thrown exceptions

15 Static Type Checking Assignment Compatibility  The relation σ := τ means that values of type τ may be assigned to variables of type σ: σ := σ short := byte σ :- σ int := byteint := short σ [] :- τ [], if σ :- τ int := char C := nulljava.lang.Cloneable :- σ [] σ [] := nullObject := σ [] java.io.Serializable :- σ [] java.lang.Cloneable := σ [] Object :- σ [] java.io.Serializable := σ [] C :- D, if D ≤ C σ [] := τ [], if σ :- τ C := D, if D ≤ C  It is reflexive, transitive, and anti-symmetric

16 Static Type Checking Assignment Compatibility  The relation σ := τ means that values of type τ may be assigned to variables of type σ: σ := σ short := byte σ :- σ int := byteint := short σ [] :- τ [], if σ :- τ int := char C := nulljava.lang.Cloneable :- σ [] σ [] := nullObject := σ [] java.io.Serializable :- σ [] java.lang.Cloneable := σ [] C :- D, if D ≤ C java.io.Serializable := σ [] σ [] := τ [], if σ :- τ C := D, if D ≤ C  It is reflexive, transitive, and anti-symmetric Turbo-Pascal:  reflexive  transitive  anti-symmetric  symmetric

17 Static Type Checking Statements C,L,X,σ |- S 1 C,L,X,σ |- S 2 C, L, X, σ |- S 1 S 2 C, L[n→ τ], X, σ |- S C, L, X, σ |- { τ n ; S } C, L, X, σ |- E: τ C, L, X, σ |- E ;

18 Static Type Checking Control Statements C,L,X,σ |- E: boolean C,L,X,σ |- S C, L, X, σ |- if ( E ) S C,L,X,σ |- E: boolean C,L,X,σ |- S i C, L, X, σ |- if ( E ) S 1 else S 2 C,L,X,σ |- E: boolean C,L,X,σ |- S C, L, X, σ |- while ( E ) S

19 Static Type Checking Return Statements σ = void C, L, X, σ |- return C, L, X, σ |- E: τ σ := τ C, L, X, σ |- return E

20 Static Type Checking Throw Statement C,L,X, σ |- E: τ throwok({τ},X) C, L, X, σ |- throw E throwok(A,B) ≡  α  A: Error := α  RuntimeException := α   β  B: β:=α

21 Static Type Checking Locals L(n) = τ C, L, X, σ |- n: τ

22 Static Type Checking Fields C,L,X,σ |- this: C D |- static τ f C,L,X,σ |- D. f: τ C,L,X,σ |- E: D D |- τ f C, L, X, σ |- E. f: τ

23 Static Type Checking Assignments C,L,X,σ |- E: τ 2 L(n)=τ 1 τ 1 :=τ 2 C, L, X, σ |- n = E: τ 1 C,L,X,σ |- E: τ 2 D |- static τ 1 f τ 1 :=τ 2 C, L, X, σ |- D. f = E: τ 1 C,L,X,σ |- E 2 : τ 2 C,L,X,σ |- E 1 : D D |- τ 1 f τ 1 :=τ 2 C, L, X, σ |- E 1. f = E 2 : τ 1

24 Static Type Checking Arrays C,L,X,σ |- E 1 : τ 1 [] C,L,X,σ |- E 2 : τ 2 num(τ 2 ) C, L, X, σ |- E 1 [ E 2 ] : τ 1 C, L, X, σ |- E 1 : τ 1 [] C, L, X, σ |- E 2 : τ 2 C, L, X, σ |- E 3 : τ 3 num(τ 2 ) τ 1 := τ 3 C, L, X, σ |- E 1 [ E 2 ]= E 3 : τ 1 num(σ) ≡ σ  { byte, short, int, char }

25 Static Type Checking Array Operations C,L,X,σ |- E: τ [] C,L,X,σ |- E.length : int C,L,X,σ |- E: τ [] C,L,X,σ |- E.clone() : Object C,L,X,σ |- E: τ 2 num(τ 2 ) C,L,X,σ |- new τ 1 [ E ] : τ 1 []

26 Static Type Checking Operators C,L,X,σ |- E: boolean C,L,X,σ |- ! E: boolean C,L,X,σ |- E 1 : τ 1 C,L,X,σ |- E 2 : τ 2 num(τ 1 ) num(τ 2 ) C, L, X, σ |- E 1 * E 2 : int

27 Static Type Checking Literals C,L,X,σ |- true : boolean C,L,X,σ |- "abc" : String C,L,X,σ |- null : null C,L,X,σ |- : char C,L,X,σ |- 42 : int

28 Static Type Checking Plus C,L,X,σ |- E 1 : τ 1 C,L,X,σ |- E 2 : τ 2 num(τ 1 ) num(τ 2 ) C, L, X, σ |- E 1 + E 2 : int C,L,X,σ |- E 1 : String C,L,X,σ |- E 2 : τ 2 τ 2 ≠ void C, L, X, σ |- E 1 + E 2 : String C,L,X,σ |- E 1 : τ 1 C,L,X,σ |- E 2 : String τ 1 ≠ void C, L, X, σ |- E 1 + E 2 : String

29 Static Type Checking Equality C,L,X,σ |- E 1 : τ 1 C,L,X,σ |- E 2 : τ 2 (num(τ 1 )  num(τ 2 ))  τ 1 :=τ 2  τ 2 :=τ 1 C, L, X, σ |- E 1 == E 2 : boolean

30 Static Type Checking Casts and Instanceof C,L,X,σ |- E: τ 1 (num(τ 1 )  num(τ 2 ))  τ 1 :=τ 2  τ 2 :=τ 1 C, L, X, σ |- ( τ 2 ) E: τ 2 C,L,X,σ |- E: τ τ:=D  D:=τ C, L, X, σ |- E instanceof D: boolean

31 Static Type Checking Why Restrict Casts? τ1τ1 τ2τ2 τ2τ2 τ1τ1 τ1τ1 τ2τ2 Alway succeeds, but downcasting is useful for selecting different methods τ 2 := τ 1 Really useful, the object may or may not by a subclass of τ 2 τ 1 := τ 2 Always fails, don't bother to execute τ 1 :≠ τ 2  τ 1 :≠ τ 2

32 Static Type Checking Method Invocation C,L,X,σ |- E: D C,L,X,σ |- E i : τ i D |- τ m(τ 1,...,τ k ) Y C, L, X, σ |- E. m ( E 1,..., E k ) : τ C,L,X,σ |- E i : τ i D |- static τ m(τ 1,...,τ k ) Y C, L, X, σ |- D. m ( E 1,..., E k ) : τ

33 Static Type Checking Constructor Invocation C,L,X,σ |- E i : τ i D |- (τ 1,...,τ k ) Y throwok(Y,X) C, L, X, σ |- new D ( E 1,..., E k ) : D C,L,X,σ |- E i : τ i C |- extends D D |- (τ 1,...,τ k ) Y throwok(Y,X) C, L, X, σ |- super( E 1,..., E k )

34 Static Type Checking Closest Match Method Invocation C,L,X,σ |- E: D C,L,X,σ |- E i : σ i D |- τ m(τ 1,...,τ k ) Y τ i := σ i  D |- γ m(γ 1,..., γ k ) Z : (  i : γ i := σ i )  (  i : γ i := τ i ) C, L, X, σ |- E. m ( E 1,..., E k ) : τ This rule does not describe access modifiers

35 Static Type Checking Checking Exceptions C, L, X, σ |- E: D  C, L, X, σ |- E. m ( E 1,..., E k ) : τ  throwok(  {Y i | D |- τ m(τ 1,...,τ k ) Y i }, X) Y  Z = Y  Z  Z  Y Y  Z = { y  Y | throwok({y},Z) }

36 Static Type Checking Judging Methods and Constructors C, [a i → σ i ], { X i }, σ |- S Throwable := X i C |- σ m ( σ 1 a 1,..., σ k a k )throws X 1,..., X n { S } C, [a i → σ i ], { X i }, void |- S Throwable := X i C |- C ( σ 1 a 1,..., σ k a k )throws X 1,..., X n { S }

37 Static Type Checking Different Kinds of Type Rules  Axioms:  Predicates: τ 1 :=τ 2  τ 1 :=τ 2  Inferences: C,L,X,σ |- this: C C,L,X,σ |- E: D D |- τ f C, L, X, σ |- E. f: τ

38 Static Type Checking Type Proofs  A type proof is a tree in which: nodes are inferences leaves are axioms or true predicates  A judgment is valid  it is the root of some type proof

39 Static Type Checking A Type Proof [ x → A, y → B ]( x )= A C,[ x → A, y → B ],X,σ |- x : A A  B  B  A [ x → A, y → B ]( y )= B C,[ x → A, y → B ],X,σ |- (B)x : B B := B C,[ x → A, y → B ],X, σ |- y=(B)x : B C,[ x → A, y → B ],X, σ |- y=(B)x; C,[ x → A ],X, σ |- {B y; y=(B)x;} C,[],X,σ |- {A x; {B y; y=(B)x;}}  Equal to a trace of a successful run of the type checker (Assuming that B is a subclass of A )

40 Static Type Checking Static Type Correctness  A program is statically type correct  all method and constructor judgments are valid  A type system is sound  static type correctness implies type correctness

41 Static Type Checking Java's Type System is Unsound  The following is a valid judgement: C, [ b → B,x→ A[], y → B[], c → C ], X, σ |- x=y; x[0]=c; b=y[0]; where B  A, C  A, and B and C are incomparable  Afterward, b contains an object of class C  But its declared type only allows B objects

42 Static Type Checking Greed  Java's type system is too greedy:  Runtime type checks catch the unsound cases statically type correcttype correct unsound

43 Static Type Checking Fixed Assignment Compatibility  By design, the rule for assignment of arrays has been chosen to be convenient but unsound  It should have looked like: σ := σ short := byte σ :- σ int := byteint := short σ [] :- τ [], if σ :- τ int := char C := nulljava.lang.Cloneable :- σ [] σ [] := nullObject := σ [] java.io.Serializable :- σ [] java.lang.Cloneable := σ [] C :- D, if D ≤ C java.io.Serializable := σ [] σ [] := τ [], if σ :- τ C := D, if D ≤ C  But that would then introduce annoying slack...

44 Static Type Checking Complexity of Inference Systems  Deciding valid judgments of an inference system depends on its rules and axioms and may be: undecidable (hyper)exponential time (backtracking) linear time  The Java type system is designed to be simple  A type checker performs a single traversal of the parse tree in linear time

45 Static Type Checking Type Checking in Java  Type checking is performed bottom-up  The rule to apply is uniquely determined by the types of subexpressions and the syntax  The type proof also specifies how to transform the program with respect to: address comparisons implicit string conversions concat(...) code

46 Static Type Checking Implicit Coercions  Suppose we add a coercion type rule: corresponding to the JVM bytecode i2f  Type proofs are now no longer unique! C,L,X,σ |- E: int C,L,X,σ |- E: float

47 Static Type Checking Ambiguous Type Proofs L( i )= int L( j )= int C,L,X,σ |- i : int C,L,X,σ |- j : int C,L,X,σ |- i : float C,L,X,σ |- j : float L( f )= float C,L,X,σ |- i+j : float C,L,X,σ |- f : float C,L,X,σ |- i+j==f : boolean L( i )= int L( j )= int C,L,X,σ |- i : int C,L,X,σ |- j : int C,L,X,σ |- i+j : int L( f )= float C,L,X,σ |- i+j : float C,L,X,σ |- f : float C,L,X,σ |- i+j==f : boolean

48 Static Type Checking Semantic Coherence  Different type proofs generate different transformed programs  The type system is semantically coherent if they always have the same semantics  The previous hypothetical type system is not: i = j = f = E9  Java chooses the proof with coercions as near the root as possible