Understanding Type Checking in Programming Languages

1. Programming Language Concepts (CIS 635) Elsa L Gunter 4303 GITC NJIT, www.cs.njit.edu/~elsa/635

2. Type Checking • When is op(arg1,…,argn) allowed? • Type checking assures that operations are applied to the right number of arguments of the right types • Right type may mean same type as was specified, or may mean that there is a predefined implicit coercion that will be applied • Used to resolve overloaded operations

3. Type Checking • Type checking may be done statically at compile timeor dynamically at run time • Untyped languages (eg LISP, Prolog) do only dynamic type checking • Typed languages can do most type checking statically

4. Dynamic Type Checking • Performed at run-time before each operation is applied • Types of variables and operations left unspecified until run-time • Same variable may be used at different types

5. Dynamic Type Checking • Data object must contain type information • Errors aren’t detected until violating application is executed (maybe years after the code was written)

6. Static Type Checking • Performed after parsing, before code generation • Type of every variable and signature of every operator must be known at compile time

7. Static Type Checking • Can eliminate need to store type information in data object if no dynamic type checking is needed • Catches many programming errors at earliest point

8. Strongly Typed Language • When no application of an operator to arguments can lead to a run-time type error, language is strongly typed • Depends on definition of “type”

9. Strongly Typed Language • C is “strongly typed” but type coercions may cause unexpected (undesirable) effects; no array bounds check (in fact, no runtime checks at all) • SML “strongly typed” but still must do dynamic array bounds checks, arithmetic overflow checks

10. How to Handle Type Mismatches • Type checking to refuse them • Apply implicit function to change type of data • Coerce int into real • Coerce char into int

11. Conversion Between Types: • Explicit: all conversions between different types must be specified • Implicit: some conversions between different types implied by language definition • Implicit conversions called coercions

12. Coercion Examples Example in Pascal: var A: real; B: integer; A := B • Implicit coercion - an automatic conversion from one type to another

13. Coercions Versus Conversions • When A has type int and B has type real, many languages allow coercion implicit in A := B • In the other direction, often no coercion allowed; must use explicit conversion: • B := round(A); Go to integer nearest A • B := trunc(A); Delete fractional part of A

14. Subtypes • A is a subtype of B if every value of A is a value of B (or more accurately it appears to be) • Frequently candidate for coercion • In C many things are subtypes of integer – many things can be implicitly coerced into a integers

15. Type Equality (aka Type Compatibility) • When are two types “the same”? • Name equivalence: two types equal only if they have the same name • Simple but restrictive • Usually loosened to allow two types to be equal when one is defined with the name of the other (declaration equivalence)

16. Type Equality • Structure equivalence: Two types are equivalent if the underlying data structures for each type are the same • Problem: how far to go – are two records with the same number of fields of same type, but different labels equivalent?

17. Scope of Variable • Range of program that can reference that variable (ie access the corresponding data object by the variable’s name) • Variable is local to program or block if it is declared there • Variable is nonlocal to program unit if it is visible there but not declared there

18. Static Scoping • Scope computed at compile time, based on program text • To determine the variable of a name used must find statement declaring variable

19. Static Scoping • Subprograms and blocks generate hierarchy of scopes • Subprogram or block that declares current subprogram or contains current block is its static parent

20. Static Scoping • General procedure to find declaration: • First see if variable is local; if yes, done • If nonlocal to current subprogram or block recursively search static parent until declaration is found • If no declaration is found this way, undeclared variable error detected

21. program main; var x : integer; procedure sub1; var x : integer; begin { sub1 } … x … end; { sub1 } begin { main } … x … end; { main } Example

22. Dynamic Scope • Now generally thought to have been a mistake • Main example of use: original versions of LISP • Common LISP uses static scope • Perl allows variables to be decalred to have dynamic scope

23. Dynamic Scope • Determined by the calling sequence of program units, not static layout • Name bound to corresponding variable most recently declared among still active subprograms and blocks

24. program main; var x : integer; procedure sub1; begin { sub1 } … x … end; { sub1 } procedure sub2; var x : integer; begin { sub2 } … call sub1 … end; { sub2 } … call sub2… end; { main } Example

25. program main; var x : integer; procedure sub1; begin { sub1 } … x … end; { sub1 } procedure sub2; var x : integer; begin { sub2 } … call sub1 … end; { sub2 } … call sub2… end; { main } Example: Static Scope

26. program main; var x : integer; procedure sub1; begin { sub1 } … x … end; { sub1 } procedure sub2; var x : integer; begin { sub2 } … call sub1 … end; { sub2 } … call sub2… end; { main } Example: Dynamic Scope

27. Referencing Environment • The referencing environment of a program point is the set of all variables (names bound to data objects) visible to that program point • In a static scoped language referencing environment is all local variables (declared so far) together with all declared variables of all static ancestors minus those that have been hidden

28. program main; var x, y : integer; procedure sub1; var z : integer begin { sub1 } … point1 … end; { sub1 } procedure sub2; var w, x : integer; begin { sub2 } … point2 … end; { sub2 } …point3 … end; { main } Example (Static Scope)

29. Example • Referencing Envirnoments: • Point1: main.x main.y sub1.z • Point2: main.y sub2.x sub2.w • Point3: main.x main.y