3:: Chapter Names, Scopes, and Bindings
Name, Scope, and Binding
A name is exactly what you think it is
Most names are identifiers symbols (like '+') can also be names
Programming Language Pragmatics
Michael L. Scott
A binding is an association between two things, such as a name and the thing it names The scope of a binding is the part of the program (textually)in which the binding is active
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Binding
Binding
Binding Time is the point at which a binding is created or, more generally, the point at which any implementation decision is made
language design time
program structure, possible type
Implementation decisions (continued ):
program writing time
algorithms, names
compile time
plan for data layout
language implementation time
I/O, arithmetic overflow, type equality (if unspecified in manual)
link time
layout of whole program in memory
load time
choice of physical addresses
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Binding
Binding
Implementation decisions (continued):
run time
value/variable bindings, sizes of strings subsumes
program start-up time module entry time elaboration time (point a which a declaration is first "seen") procedure entry time block entry time statement execution time
The terms STATIC and DYNAMIC are generally used to refer to things bound before run time and at run time, respectively
static is a coarse term; so is "dynamic"
IT IS DIFFICULT TO OVERSTATE THE IMPORTANCE OF BINDING TIMES IN PROGRAMMING LANGUAGES
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Binding
Binding
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In general, early binding times are associated with greater efficiency Later binding times are associated with greater flexibility Compiled languages tend to have early binding times Interpreted languages tend to have later binding times Today we talk about the binding of identifiers to the variables they name
Scope Rules - control bindings
Fundamental to all programming languages is the ability to name data, i.e., to refer to data using symbolic identifiers rather than addresses Not all data is named! For example, dynamic storage in C or Pascal is referenced by pointers, not names
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Lifetime and Storage Management
Lifetime and Storage Management The period of time from creation to destruction is called the LIFETIME of a binding If object outlives binding it's garbage If binding outlives object it's a dangling reference The textual region of the program in which the binding is active is its scope In addition to talking about the scope of a binding, we sometimes use the word scope as a noun all by itself, without an indirect object
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Key events
creation of objects creation of bindings references to variables (which use bindings) (temporary) deactivation of bindings reactivation of bindings destruction of bindings destruction of objects
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Lifetime and Storage Management
Lifetime and Storage Management
Storage Allocation mechanisms
Static Stack Heap
Static allocation for
code globals static or own variables explicit constants (including strings, sets, etc) scalars may be stored in the instructions
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Lifetime and Storage Management
Lifetime and Storage Management
Central stack for
parameters local variables temporaries
Contents of a stack frame (cf., Figure 3.2)
arguments and returns local variables temporaries bookkeeping (saved registers, line number static link, etc.)
Why a stack?
allocate space for recursive routines (not necessary in FORTRAN no recursion) reuse space (in all programming languages)
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Local variables and arguments are assigned fixed OFFSETS from the stack pointer or frame pointer at compile time
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Lifetime and Storage Management
Lifetime and Storage Management
Maintenance of stack is responsibility of calling sequence and subroutine prolog and epilog
space is saved by putting as much in the prolog and epilog as possible time may be saved by
putting stuff in the caller instead or combining what's known in both places (interprocedural optimization)
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Lifetime and Storage Management
Scope Rules
Heap for dynamic allocation
A scope is a program section of maximal size in which no bindings change, or at least in which no re-declarations are permitted (see below) In most languages with subroutines, we OPEN a new scope on subroutine entry:
create bindings for new local variables, deactivate bindings for global variables that are redeclared (these variable are said to have a "hole" in their scope) make references to variables
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Scope Rules
Scope Rules
On subroutine exit:
destroy bindings for local variables reactivate bindings for global variables that were deactivated
With STATIC (LEXICAL) SCOPE RULES, a scope is defined in terms of the physical (lexical) structure of the program
The determination of scopes can be made by the compiler All bindings for identifiers can be resolved by examining the program Typically, we choose the most recent, active binding made at compile time Most compiled languages, C and Pascal included, employ static scope rules
Algol 68:
ELABORATION = process of creating bindings when entering a scope
Ada (re-popularized the term elaboration):
storage may be allocated, tasks started, even exceptions propagated as a result of the elaboration of declarations
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Scope Rules
Scope Rules
The classical example of static scope rules is the most closely nested rule used in block structured languages such as Algol 60 and Pascal
An identifier is known in the scope in which it is declared and in each enclosed scope, unless it is re-declared in an enclosed scope To resolve a reference to an identifier, we examine the local scope and statically enclosing scopes until a binding is found
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We will see classes - a relative of modules - later on, when discussing abstraction and objectoriented languages
These have even more sophisticated (static) scope rules
Euclid is an example of a language with lexically-nested scopes in which all scopes are closed
rules were designed to avoid ALIASES, which complicate optimization and correctness arguments
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Scope Rules
Scope Rules
Note that the bindings created in a subroutine are destroyed at subroutine exit
The modules of Modula, Ada, etc., give you closed scopes without the limited lifetime Bindings to variables declared in a module are inactive outside the module, not destroyed The same sort of effect can be achieved in many languages with own (Algol term) or static (C term) variables (see Figure 3.5)
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Access to non-local variables STATIC LINKS
Each frame points to the frame of the (correct instance of) the routine inside which it was declared In the absence of formal subroutines, correct means closest to the top of the stack You access a variable in a scope k levels out by following k static links and then using the known offset within the frame thus found
More details in Chapter 8
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Scope Rules
Scope The Rules
key idea in static scope rules is that bindings are defined by the physical (lexical) structure of the program. With dynamic scope rules, bindings depend on the current state of program execution
They cannot always be resolved by examining the program because they are dependent on calling sequences To resolve a reference, we use the most recent, active binding made at run time
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Scope Rules
Scope Rules Example: Static vs. Dynamic
Dynamic scope rules are usually encountered in interpreted languages
early LISP dialects assumed dynamic scope rules.
Such languages do not normally have type checking at compile time because type determination isn't always possible when dynamic scope rules are in effect
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program scopes (input, output ); var a : integer; procedure first; begin a := 1; end; procedure second; var a : integer; begin first; end; begin a := 2; second; write(a); end.
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Scope Rules Example: Static vs. Dynamic
Scope Rules Example: Static vs. Dynamic
If static scope rules are in effect (as would be the case in Pascal), the program prints a 1 If dynamic scope rules are in effect, the program prints a 2 Why the difference? At issue is whether the assignment to the variable a in procedure first changes the variable a declared in the main program or the variable a declared in procedure second
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Static scope rules require that the reference resolve to the most recent, compile-time binding, namely the global variable a Dynamic scope rules, on the other hand, require that we choose the most recent, active binding at run time
Perhaps the most common use of dynamic scope rules is to provide implicit parameters to subroutines This is generally considered bad programming practice nowadays
Alternative mechanisms exist
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static variables that can be modified by auxiliary routines default and optional parameters
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Scope Rules Example: Static vs. Dynamic
Binding of Referencing Environments
At run time we create a binding for a when we enter the main program. Then we create another binding for a when we enter procedure second
This is the most recent, active binding when procedure first is executed Thus, we modify the variable local to procedure second, not the global variable However, we write the global variable because the variable a local to procedure second is no longer active
Accessing variables with dynamic scope:
(1) keep a stack (association list) of all active variables
When you need to find a variable, hunt down from top of stack This is equivalent to searching the activation records on the dynamic chain
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Binding of Referencing Environments
Binding of Referencing Environments
Accessing variables with dynamic scope:
(2) keep a central table with one slot for every variable name
If names cannot be created at run time, the table layout (and the location of every slot) can be fixed at compile time Otherwise, you'll need a hash function or something to do lookup Every subroutine changes the table entries for its locals at entry and exit.
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(1) gives you slow access but fast calls (2) gives you slow calls but fast access In effect, variable lookup in a dynamicallyscoped language corresponds to symbol table lookup in a statically-scoped language Because static scope rules tend to be more complicated, however, the data structure and lookup algorithm also have to be more complicated
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Binding of Referencing Environments
Binding of Referencing Environments
REFERENCING ENVIRONMENT of a statement at run time is the set of active bindings A referencing environment corresponds to a collection of scopes that are examined (in order) to find a binding
SCOPE RULES determine that collection and its order BINDING RULES determine which instance of a scope should be used to resolve references when calling a procedure that was passed as a parameter
they govern the binding of referencing environments to formal procedures
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6
Binding within a Scope
Binding within a Scope
Aliasing
What are aliases good for? (consider uses of FORTRAN equivalence)
space saving - modern data allocation methods are better multiple representations - unions are better linked data struc...