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rar e n hi-e.s e.> A-7 l-u-m.L11\a-( <7,, mbols 1VI tlit'- ru-,\ es : TopHat Q1 - Q2 Variables Variables • Functional languages use variables as names (where the association name-value is stored in an environment). • We can remember the association, or read the value, but we cannot change it. • Imperative languages are abstractions of von Neumann architecture • A variable abstracts the concept of memory location • Understanding how variables are managed is an important part to understand the semantics of a programming language. 5 Mutable vs Immutable Variables • When we consider variables as names we are working with immutable variables (e.g. the part of OCaml we studied) • When we consider variables as memory locations we are working with mutable variables (e.g. Python, c, etc.) • Understanding how variables are managed is an important part to understand the semantics of a programming language. 6 Learning Goals for today • Understandingtheconceptofscopeandhowlanguages can differ in their scoping rules. • Understandingtheconceptofbindingandbinding-time. Scoping rules Variable names How shall we evaluate this expression? Is it referring to this definition? let k=3 in (let k=2 in k+5) Is it referring to this definition? What is the value of k here? Scope of a variable • The scope of a variable is the range of statements over which it is visible • The scope rules of a language determine how references to names are associated with variables let k=3 in (let k=2 in k+5) OCaml scoping rule says that a variable name is statically associated with the closest definition in the abstract syntax tree. Back to our example This is the first declaration we find Start from here let k=3 in (let k=2 in k+5) To find the value of k we look/search declarations, first locally, then in increasingly larger enclosing scopes Another example This is the first declaration we find Start from here let k=3 in (let z=2 in k+5) To find the value of k we look/search declarations, first locally, then in increasingly larger enclosing scopes Another example This is the first declaration we find Start from here let k=3 in k + (let k=2 in k+5) This is the first declaration we find Start from here Another example This is the first declaration we find Start from here let k=3 in (let k=2 in k+5) + k This is the first declaration we find Start from here Static Scope • Based on program text • To connect a name reference to a variable, we (or the compiler) must find the declaration • Some languages allow nested subprogram definitions, which create nested static scopes • Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name Static Scope • Variables can be hidden from a unit by having a "closer" variable with the same name declaration of x declaration of x use of x Static Scope • Variables can be hidden from a unit by having a "closer" variable with the same name declaration of x declaration of x use of x use of x Static Scope • Search process: search declarations, first locally, then in increasingly larger enclosing scopes, until one is found for the given name • Most of the modern languages are statically scoped: Python, Java, Scala, etc. TopHat Q3-Q10 Scope Blocks A method of creating static scopes inside program units (ALGOL 60) void sub() { int count; while (...) { int count; count++; ... } ... } Program constructs (“blocks”) create scopes Scope Block Example: int main() { int x=5; { int x=4; printf("The value of x in the block is %d\n", x); } printf("The value of x in outside the block is %d", x); return 0; } In C we can write a program like the one above Scope Block Example: int main() { int x=5; { int x=4; printf("The value of x in the block is %d\n", x); } printf("The value of x in outside the block is %d", x); return 0; } Dynamic Scope • Based on calling sequences of program units, not their textual layout, • You can think about it more as temporal rather than spatial, • References to variables are connected to declarations by searching back through the chain of subprogram calls that brought execution to this point. 24 Dynamic Scope Example function big() { function sub1(){ var x = 7; sub2(); } function sub2() { var y = x; } var x = 3; sub1(); } big calls sub1 sub1 calls sub2 sub2 uses x • Static scoping -- Ref to x in sub2 is to big's x • Dynamic scoping-- Ref to x in sub2 is to sub1's x 25 Dynamic Scope Example in bash $ x=1 $ function g () { echo $x ; x=2 ; } $ function f () { x=3 ; g ; } $ f # does this print 1, or 3? $ g # does this print 1, 2 or 3? $ echo $x # does this print 1,2 or 3? echo $x corresponds to printing the value of the variable x. What does this program print? 26 Another Example in bash $ x=1 $ function h () { x=2 ; echo $x ; } $ function g () { x=3 ; echo $x ; h;} $ function f () { x=4 ; echo $x; g ; } $ f # What does this print? $ g # What does this print? $ h # What does this print? $ echo $x # What does this print? What does this program print? 27 TopHat Q11 - Q13 Variables classification • The local variables of a program unit are those that are declared in that unit • The nonlocal variables of a program unit are those that are visible in the unit but not declared there • Global variables are a special category of nonlocal variables Local Variables in Ocaml - examples let k=3 in 6 + k fun k -> 8 * k
let x=3 in (let k=2 in k + x) + x
The variable k is local to the body of the let
The variable k is local to the body of the function
The variable k is local to the body of the internal let, and the variable x is local to the body of the external let.