Programming Languages Chapter 6: Other Types
Definition of Records in COBOL • COBOL uses level numbers to show nested records; others use recursive definition 01 EMP-REC. 02 EMP-NAME. 05 FIRST PIC X(20). 05 MID PIC X(10). 05 LAST PIC X(20). 02 HOURLY-RATE PIC 99V99. • EMP-REC record consists of the EMP-NAME record and the HOURLY-RATE field
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Garbage Collection Algorithms • Whose responsibility is garbage collection? • C and C++ default the garbage collection to the programmer - Provide explicit tools, such as the delete function - Recent versions provide more options • Java, Lisp, and other languages provide automatic garbage collection and take the responsibility for heap management out of the hands of programmers
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List Types • F# Lists - Like those of ML, except elements are separated by semicolons and hd and tl are methods of the List class • Python Lists - The list data type also serves as Python's arrays - Unlike Scheme, Common Lisp, ML, and F#, Python's lists are mutable - Elements can be of any type - Create a list with an assignment myList = [3, 5.8, "grape"
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List Types • Haskell's List Comprehensions - The original [n * n | n <- [1..10]] • F#'s List Comprehensions let myArray = [|for i in 1 .. 5 -> [i * i) |] • Both C# and Java supports lists through their generic heap-dynamic collection classes, List and ArrayList, respectively
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List Types • List Operations in ML - Lists are written in brackets and the elements are separated by commas - List elements must be of the same type - The Scheme CONS function is a binary operator in ML, :: 3 :: [5, 7, 9] evaluates to [3, 5, 7, 9] - The Scheme CAR and CDR functions are named hd and tl, respectively
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List Types • List Operations in Scheme - CAR returns the first element of its list parameter (CAR ′(A B C)) returns A - CDR returns the remainder of its list parameter after the first element has been removed (CDR ′(A B C)) returns (B C) - CONS puts its first parameter into its second parameter, a list, to make a new list (CONS ′A (B C)) returns (A B C) - LIST returns a new list of its parameters (LIST ′A ′B ′(C D)) returns (A B (C D))
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List Types • Python Lists (continued) - List elements are referenced with subscripting, with indices beginning at zero x = myList[1] Sets x to 5.8 - List elements can be deleted with del del myList[1] - List Comprehensions - derived from set notation [x * x for x in range(7) if x % 3 == 0] range(7) creates [0, 1, 2, 3, 4, 5, 6] Constructed list: [0, 9, 36]
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Look at slides 24 - 26 JonJon for Unions in F#
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Pointer Arithmetic in C and C++ float stuff[100]; float *p; p = stuff; *(p+5) is equivalent to stuff[5] and p[5] *(p+i) is equivalent to stuff[i] and p[i
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Reference Counting • When a pointer assignment occurs in the program, such as q = p the following book-keeping on the RCs must be done: 1. The RC for p's node is increased by 1 2. The RC for q's node is decreased by 1 3. If the RC for q's node becomes 0, the RC for each of its descendants is decreased by 1, q's node is returned to the free_list, and this step is repeated for each of q's node's descendants 4. The pointer q is assigned the reference value of p
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References to Records • Record field references 1. In COBOL field_name OF record_name_1 OF ... OF record_name_n MID of EMP-NAME OF EMP-REC 2. In Others (dot notation) record_name_1.record_name_2. ... record_name_n.field_name • Fully qualified references must include all record names • Elliptical references allow leaving out record names as long as the reference is unambiguous, for example in COBOL FIRST, FIRST OF EMP-NAME, and FIRST of EMP-REC are elliptical references to the employee's first name
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Strong Typing More Language examples: - Scheme and Perl (both dynamically typed) are strongly typed 1. The typing of values themselves, and the run time checking of those values by the interpreter prevent undetected type errors - ML and F# are
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Structure Type in C • Example: C struct employeeType { int id; char name[26]; int age; float salary; char dept; } struct employeeType employee; ... employee.age = 45;
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Type Checking 0100 0000 0101 1000 0000 0000 0000 0000 • The floating point number 3.375 • The 32-bit integer 1,079,508,992 • Two 16-bit integers 16472 and 0 • Four ASCII characters: @ X NUL NUL
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Type Checking • Machine data carries no type information (it's typeless ) • Basically, just a sequence of bits. • Example: 0100 0000 0101 1000 0000 0000 0000 0000 • What does this binary string represent?
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Union Types • Example: C/C++ union myUnion { int i; float r; } union myUnion u; • Under the u.i variant, the storage location is interpreted as an integer • Under the u.r variant, the storage location is interpreted as a floating point value
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look a the example on slide 68 JonJon
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look at picture on slide 38
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Implementation of Record Type (look on slide 11 JonJon) Individual fields within a structure are usually allocated in a contiguous block. Number of bytes allocated and order of fields is machine and compiler dependent. Offset address relative to the beginning of the records is associated with each field
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Look at slides 55- 57 JonJon
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Mark-Sweep Mark-Sweep Algorithm • Makes 2 passes on the heap • Uses a mark bit (MB) which is attached to each heap node and initialized to 0 Pass I (the mark pass): Mark all nodes that are (directly or indirectly) accessible from the stack by setting their MB=1. Pass II (the sweep pass): Sweep through the entire heap and return all unmarked (MB=0) nodes to the free list. • Note: all orphans are detected and returned to the free list.
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look at slide 40 for JonJon for garbage example
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Heap Management *Contains value that are dynamically allocated and structured while the program is...... -fixed..... -variable.... -allocated at......
The Heap • Contains values that are dynamically allocated and structured while the program is running, such as strings, arrays, objects, and various dynamic data structures like linked lists - Fixed size - Variable content - Allocated at run-time
Reference Type *C++ includes a special kind of pointer type called a -advantages of both.... *Java extends C++'s reference variables and allows..... -references are references to objects, rather than..... *C# includes both the references of......
• C++ includes a special kind of pointer type called a reference type that is used primarily for formal parameters - Advantages of both pass-by-reference and pass-by-value • Java extends C++'s reference variables and allows them to replace pointers entirely - References are references to objects, rather than being addresses • C# includes both the references of Java and the pointers of C++
Copy Collection *Called only when.... *Make only one pass on the..... *Partitions the heap into 2 halves, called the ( ) and ( ) -only is one is....... *Requires significantly more memory because only half the entire....... *Does not use a ( ), but maintains a ( )
• Called only when the heap is full • Makes only one pass on the heap, so it is faster than mark-sweep • Partitions the heap into 2 halves, called the from_space and the to_space (only one is active) • Requires significantly more memory because only half the entire heap space is actively available for allocating new memory blocks to the program at any one time • Does not use a free_list, but maintains a pointer free to the end of the allocated area (the active area)
Mark-Sweep *Called only when the heap is..... -Invoked whenever a new mode is requested by the.... *So, several thousand heap block allocations and deallocations can..... *Once mark-sweep is invoked, a more elaborate and.......
• Called only when the heap is full (free_list is empty) - Invoked whenever a new node is requested by the program and the heap is full • So, several thousand heap block allocations and deallocations can take place with no garbagecollection overhead occurring • Once mark-sweep is invoked, a more elaborate and time-consuming process interrupts the program
Problems with Pointers *Dangling pointers (dangerous) -A pointer points to a .... *Lost heap-dynamic variable -An allocated heap-dynamic variable that is no longer.... 1. example.....
• Dangling pointers (dangerous) - A pointer points to a heap-dynamic variable that has been deallocated • Lost heap-dynamic variable - An allocated heap-dynamic variable that is no longer accessible to the user program (often called garbage ) • Pointer p1 is set to point to a newly created heapdynamic variable • Pointer p1 is later set to point to another newly created heap-dynamic variable • The process of losing heap-dynamic variables is called memory leakage
Evaluation of Pointers *Dangling pointers and dangling objects are.... *Pointers are like goto's they widen the..... *Pointers or references are necessary for......
• Dangling pointers and dangling objects are problems as is heap management • Pointers are like goto's--they widen the range of cells that can be accessed by a variable • Pointers or references are necessary for dynamic data structures--so we can't design a language without them
Discriminated vs. Free Unions *Design Issue -should type..... *C and C++ provide union constructs in which there is no language....... -the union in these languages is called.... *Type checking of unions require that each union include a..... -supported by....
• Design issue - Should type checking be required? • C and C++ provide union constructs in which there is no language support for type checking - The union in these languages is called free union • Type checking of unions require that each union include a type indicator called a discriminant - Supported by ML, Haskell, and F#
Pointers in C and C++ *Extremely flexible but.... *Pointers can point at any variable regardless of.... *Used for dynamic..... *Pointer...... *Explicit dereferencing and.... *Domain type need not.....
• Extremely flexible but must be used with care • Pointers can point at any variable regardless of when or where it was allocated • Used for dynamic storage management and addressing • Pointer arithmetic is possible • Explicit dereferencing and address-of operators • Domain type need not be fixed (void *) void * can point to any type and can be type checked (cannot be de-referenced)
Record Types *First appeared in... *First abstract form of a structure originated in both..... *Java omits scutures (why) *C++ has structures.... -why
• First appeared in Cobol and PL/I • First abstract form of a structure originated in both Pascal-style languages and C-style languages • Java omits structures entirely (considered redundant because of classes) • C++ has structures in addition to classes - retains backward compatibility with C
Evaluation of Unions *Free unions are..... -why? *Java and C# do not support.... -Why?
• Free unions are unsafe - Do not allow type checking • Java and C# do not support unions - Reflective of growing concerns for safety in programming language
Heap Management *Garbage is any block in heap memory that.... *Garbage can occur when either: -An allocated block of.... -A reference exists to a block of memory.... Example:......
• Garbage is any block in heap memory that cannot be accessed by the program. • Garbage can occur when either: 1. An allocated block of heap memory has no reference to it (an "orphan"), or 2. A reference exists to a block of memory that is no longer allocated (a "widow"). Example: a dynamically allocated array has its descriptor removed from the stack (when stack frame is popped) before its heap block is returned via a delete
Type Checking *I all type binding are static, nearly all..... *If type bindings are dynamic,....... *A programming language is strongly typed if...... -Advantage of strong typing: allows the detection of....
• If all type bindings are static, nearly all type checking can be static • If type bindings are dynamic, type checking must be dynamic • A programming language is strongly typed if type errors are always detected - Advantage of strong typing: allows the detection of the misuses of variables that result in type errors
Type Equivalence *Important to clearly define type equivalence -To be able to properly.... -To be able to properly implement....
• Important to clearly define type equivalence - To be able to properly implement assignment operation - To be able to properly implement parameter passing of nonbasic types
List Types *List in Lisp and Scheme are delimited by.... *Data and code have the same form -As data, (A,B,C) is literally.... -As code, (A B C) is the.... *The interpreter needs to know which a list is, so if it is data, we......
• Lists in Lisp and Scheme are delimited by parentheses and use no commas (A B C D) and (A (B C) D) • Data and code have the same form As data, (A B C) is literally what it is As code, (A B C) is the function A applied to the parameters B and C • The interpreter needs to know which a list is, so if it is data, we quote it with an apostrophe ′(A B C) is data
Garbage Collection Summary *Modern algorithms are more elaborate -Most are...... -May select between ( ) and ( ) depending on the number..... *In java, garbage collection is built-in -Runs as a low-priority thread- executes whenever....... 1. Reduces the number of.... -Also, the garbage collector ( ) may be explicitly called by the program- invokes..... *Functional languages have.... *C/C++ default garbage collection to the.....
• Modern algorithms are more elaborate. - Most are hybrids/refinements of the above three. - May select between mark-sweep and copy collection depending on the number of active nodes compared to the heap size • In Java, garbage collection is built-in. - Runs as a low-priority thread - executes whenever demand on processing time from other threads is low • Reduces the number of garbage collection calls needed during peak processing times. - Also, the garbage collector System.gc() may be explicitly called by the program - invokes garbage collection immediately • Functional languages have garbage collection built-in. • C/C++ default garbage collection to the programmer
Evaluation and Comparison to Arrays *Records are used when collection of..... *Access to array elements is much slower than access to... *Dynamic subscripts could be used with.....
• Records are used when collection of data values is heterogeneous • Access to array elements is much slower than access to record fields, because subscripts are dynamic (field names are static) • Dynamic subscripts could be used with record field access, but it would disallow type checking and it would be much slower
Reference Counting *Reference counting algorithm is applied dynamically -Whenever a new or delete operation occurs on a pointer,..... -An orphan is a node whose RC becomes 0, or any of.... -When a new operation occurs, the algorithm allocates a.....
• Reference counting algorithm is applied dynamically - Whenever a new or delete operation occurs on a pointer, the algorithm is activated and as many orphans as possible are returned to the free_list - An orphan is a node whose RC becomes 0, or any of its descendants (in the chain) whose RC becomes 0 as a result of a delete operation - When a new operation occurs, the algorithm allocates a heap node from the free_list and initializes its RC to 1
Strong Typing *Strong Typing -promotes more.... -is viewed as a virtue in.... *Union types are hole in the....
• Strong typing - promotes more reliable programs - is viewed as a virtue in programming language design • Union types are a hole in the type system of many languages.
Garbage Collection Algorithms *The heap contains both ( ) and ( ) -Inactive objects are those blocks that had been..... -altogether, inactive objects are..... *Garbage collection is any strategy that reclaims unused heap....
• The heap contains both active and inactive objects - Inactive objects are those blocks that had been allocated during an earlier stage of program execution and then relegated to the status of "no longer needed" - Altogether, inactive objects are known as garbage • Garbage collection is any strategy that reclaims unused heap blocks for later use by the program
Garbage Collection Algorithms *Reference Counting -occurs whenever... -doesn't detect all *Mark-Sweep -occurs only... -detect all garbage, but.... *Copy Collection -faster than mark-sweep, but...
• Three classic garbage collection strategies: - Reference Counting • occurs whenever a heap block is allocated • doesn't detect all garbage. - Mark-Sweep • occurs only on heap overflow • detects all garbage, but makes two passes on the heap. - Copy Collection • faster than mark-sweep, but reduces the size of the heap space.
Pointer Operations *Two fundamental operations: (what) *Assignment is used to..... *Dereferencing yields the value stored at the...... -Dereferencing can be.... -C++ uses an.....
• Two fundamental operations: assignment and dereferencing • Assignment is used to set a pointer variable's value to some useful address • Dereferencing yields the value stored at the location represented by the pointer's value - Dereferencing can be explicit or implicit - C++ uses an explicit operation via * j = *ptr sets j to the value located at ptr
Type checking *Type checking is the activity of ensuring that the........ -A compatible type is one that is either.......... 1. the automatic conversion is called.......
• Type checking is the activity of ensuring that the operands of an operator are of compatible types - A compatible type is one that is either legal for the operator, or is allowed under language rules to be implicitly converted, by compiler- generated code, to a legal type • This automatic conversion is called a coercion
Type Equivalence • Under name equivalence, two types are the same if they have the.... look at example on slide 75 JonJon
• Under name equivalence, two types are the same if they have the same name - In the previous example, variables c and d have the same type, because their types share the same type name: struct complex. - a and c are not the same type, since their types have different type names (a is an anonymous struct) - d and e are not the same type either - a and b are the same type under name equivalence
Type Equivalence *Under structural equivalence, two types are the same if.....
• Under structural equivalence, two types are the same if they have the same structure - For records, having the same structure means having the same number and order of fields, as well as the name and type of each field. - Under structural equivalence, a, b, c, and d are the same type - d and e have different types, since the two field names are different (even though both have two floating point fields
Design Issues of Pointers *What are the scope and.... *What is the lifetime of a.... *Are pointers are restricted as to the type.... *Are pointers used for dynamic storage..... *Should the language support.....
• What are the scope of and lifetime of a pointer variable? • What is the lifetime of a heap-dynamic variable? • Are pointers restricted as to the type of value to which they can point? • Are pointers used for dynamic storage management, indirect addressing, or both? • Should the language support pointer types, reference types, or both?
Pointer and Reference Types *A pointer type variable has a range of values that consist of.... *provide the power of..... *provide a way to manage.... *A pointer can be used to access a location in the area where......
• A pointer type variable has a range of values that consists of memory addresses and a special value, nil • Provide the power of indirect addressing • Provide a way to manage dynamic memory • A pointer can be used to access a location in the area where storage is dynamically created (usually called a heap)
Record Types *A record is a possibly heterogeneous aggregate of data elements in which the.... -also called...... *Design issues: -What is the syntactic form of references to.... -Are elliptical.....
• A record is a possibly heterogeneous aggregate of data elements in which the individual elements are identified by names - Also called structures • Design issues: - What is the syntactic form of references to the field? - Are elliptical references allowed?
Strong Typing *A strongly typed language can be either..... -Ada and Java are.... -C and C++ are not..... 1. Parameter type checking can be.... -Java and C#...;.
• A strongly typed language can be either statically or dynamically typed. - Ada and Java are strongly typed - C and C++ are not strongly typed (even though they are statically typed) 1. parameter type checking can be avoided; unions are not type checked - Java and C# are, almost (because of explicit type casting)
Tuple Types *A tuple is a data type that is similar to a record, except that..... *Used in Python, ML, and F# to allow functions to.... -python 1. closely related to its list, but..... 2. example....
• A tuple is a data type that is similar to a record, except that the elements are not named • Used in Python, ML, and F# to allow functions to return multiple values - Python • Closely related to its lists, but immutable • Create with a tuple literal myTuple = (3, 5.8, ′apple′) Referenced with subscripts (begin at 1) Catenation with + and deleted with del
Type error *A type error is any error that arises because an operation is...... -Type errors are common in..... -High level languages reduce the....
• A type error is any error that arises because an operation is attempted on a data type for which it is undefined. - Type errors are common in assembly language programming. - High level languages reduce the number of type errors
Type system *A type system provides a basis for..... *A type system is a precise definition of the binding between the......
• A type system provides a basis for detecting type errors. • A type system is a precise definition of the bindings between the type of a variable, its values, and the possible operations on those values.
Unions Types *A union is a type whose variables are allowed to store different type values at..... -two or more different fields share -only one field holds a valid value.... -the other fields will contain..... *C:....... *Pacal:.......
• A union is a type whose variables are allowed to store different type values at different times during execution - Two or more different fields share the same block of memory - Only one field holds a valid value at any given time - The other fields will contain nonsense • C: Union • Pascal: Case-variant record
Reference Counting *Advantage -it occurs dynamically, whenever a pointer assignment or other heap action is triggered by... *Disadvantages -It fails to detect.... -There is storage overhead due to..... -performance overhead is created by the algorithm whenever a......
• Advantage - It occurs dynamically, whenever a pointer assignment or other heap action is triggered by the program; thus, the overhead associated with garbage collection is naturally distributed over the run-time life of the program • Disadvantages - It fails to detect inaccessible circular chains - There is storage overhead due to appending an integer reference count to every node in the heap - Performance overhead is created by the algorithm whenever a pointer is assigned or a heap block is allocated or deallocated
Mark-Sweep *Advantages -reclaims all.... -it is only invoked when.... *Disadvantage -the algorithm is more..... -storage overhead......
• Advantages - Reclaims all garbage in the heap - It is only invoked when the heap is full • Disadvantage - The algorithm is more time-consuming when it does run - Storage overhead from the mark bit
Reference Counting *Assumes the initial heap is a.... *Each node has a reference count(RC)-...... *example of garbage of occuring look at picture slide 44
• Assumes the initial heap is a chain of nodes (the free_list) • Each node has a reference count (RC) - a count of the number of pointers referencing that node (initially 0) • For an assignment, like q = p, garbage can occur
