1. Organization of Programming Languages
Meeting 21
March 7, 2016

2. Planning Ahead Quiz on Wednesday Multiple assignment statements
Program functions
Control structures
Sequence
If-then-else
While-do
For
Associated functions: range, next

3. Functions, Methods, etc.
Segmenting programs is an old technique producing program parts known by many names:
Function
Subroutine
Procedure
Method
Operator
Subprogram
See Sebesta, Chapter 9, for his exposition of the topic

4. Functions (2) All segmentation strategies involve: Input values
Local variables
Output values
We look in detail at the mechanisms provided by various programming languages using the semantic categories from before: Env, Ide, Loc, Store, V*, M, …

5. Functions (3) Syntax issues: Defining the function
Using (invoking) the function

6. Defining Functions Need to specify: Name Parameters
Kind: function, procedure, operator, …
Type of output
Parameters
Type
Mode

7. Output Values Produced in several ways Appear as parameter values:
foofun(x,y) takes x and computes y
Modes of y: result, reference
Can handle arbitrary type as output

8. Output Values (2) Return in function name
Follows structure in mathematics
log(x)
Easy to work with in expressions
Implemented with
RETURN statement
Assignment to function name

9. Output Values (3) Side effect to global variable
Action but no state change
Print
Write to file

10. Simple Functions Specialize to
One or two arguments (parameters) of same type
One value of same type as arguments
Monadic or dyadic operators

11. Simple Functions (2) Implementation strategies
Mix operators and functions
Typical, mimics mathematics: 1+log(4)
Operators only
Use symbols for functions
Monadic functions only
Use list structure for several arguments

12. Subprogram Calls and Returns
Need to specify for calls:
Parameter passing mechanism (mode)
Allocation of storage for local variables and binding to names
Making global variables visible
Saving execution status of calling program unit
Transfer of control to subprogram code

13. Calls and Returns (2) Need to specify for return
Moving values to output parameters
Deallocating storage
Resetting variable access
Returning control to calling program

14. Semantics of Parameter Passing
Consider five ways of parameter passing:
Value
Result
Value-Result
Reference (aka Location)
Name

15. Semantics (2)
In each case, we’ll assume a parameter list specified in the header of the procedure as (x1, x2, …). These are the formal parameters. We’ll assume a call foofun(a1,a2, …). The a’s are the actual parameters. Here’s what happens in each scenario.

16. Pass by Value Bind local parameters to locations:
Element of Env becomes
x1 -> loc 1
x2 -> loc 2
If x1 or x2 or … were bound to locations before invocation, those bindings are saved as part of the transfer of control

17. Pass by Value (2) Bind values of actual parameters to these locations.
Assume a1 -> loc 50 -> v1
Then bind loc 1 to v1, changing the element of the Store
Execute subprogram

18. Pass by Value (3)
Break the bindings of the local parameters to locations.
Either the element of Env shows
x1 -> unbound or
x1 -> location saved on entry

19. Pass by Result Bind local parameters to locations:
Element of Env becomes
x1 -> loc 1
x2 -> loc 2
If x1 or x2 or … were bound to locations before invocation, those bindings are saved as part of the transfer of control

20. Pass by Result (2) Execute subprogram
Copy values of local parameters to locations of actual parameters
x1 -> loc 1 -> v1
a1 -> loc 50
Bind loc 50 -> v1

21. Pass by Result (3)
Break the bindings of the local parameters to locations.
Either the element of Env shows
x1 -> unbound or
x1 -> location saved on entry

22. Pass by Value-Result Bind local parameters to locations:
Element of Env becomes
x1 -> loc 1
x2 -> loc 2
If x1 or x2 or … were bound to locations before invocation, those bindings are saved as part of the transfer of control

23. Pass by Value-Result (2)
Bind values of actual parameters to these locations.
Assume a1 -> loc 50 -> v1
Then bind loc 1 to v1, changing the element of the Store
Execute subprogram

24. Pass by Value-Result (3)
Copy values of local parameters to locations of actual parameters
x1 -> loc 1 -> v1
a1 -> loc 50
Bind loc 50 -> v1

25. Pass by Value-Result (4)
Break the bindings of the local parameters to locations.
Either the element of Env shows
x1 -> unbound or
x1 -> location saved on entry

26. Pass by Reference
Bind local parameters to locations of actual parameters:
If a1 -> loc 50, a2 -> loc 51, … then
Element of Env becomes
x1 -> loc 50
x2 -> loc 51, etc
If x1 or x2 or … were bound to locations before invocation, those bindings are saved as part of the transfer of control

27. Pass by Reference (2)
Execute subprogram

28. Pass by Reference (3)
Break the bindings of the local parameters to locations.
Either the element of Env shows
x1 -> unbound or
x1 -> location saved on entry

29. Pass by Name Bind local parameters to rexp of actual parameters
If a1 is given as string1 then
x1 is assumed to be string1
etc
If x1 or x2 or … were bound to locations before invocation, those bindings are saved as part of the transfer of control.

30. Pass by Name (2)
Execute subprogram

31. Pass by Name (3)
Break the bindings of the local parameters to strings (of code)
Either the element of Env shows
x1 -> unbound or
x1 -> location saved on entry