Ch Ch jcmt CSE 3302 Programming Languages CSE3302 Programming Languages (n-n-n-notes) Summer 2003 Dr. Carter Tiernan

Ch Ch jcmt CSE 3302 Programming Languages Functional Programming List Processing - LISP Complex interrelationships among data Recursion in conjunction with conditional expressions Primitive list-handling subroutines Applicative language

Ch Ch jcmt CSE 3302 Programming Languages LISP John McCarthy Function applications –Prefix (Polish) notation : flexibility –Fully parenthesized : no precedence rules Symbolic data –Lists of symbols called atoms –List is ONLY data structure in LISP

Ch Ch jcmt CSE 3302 Programming Languages Lists S-expressions –Function applications are evaluated – Quoted lists are treated as data Programs and data represented the same way –Convenient for one program to generate and call for execution of another –Simple to write program manipulators

Ch Ch jcmt CSE 3302 Programming Languages LISP Execution Usually interpreted and often interactive Functions – Pure : compute a value only eq, plus, difference, etc. – Pseudo : have side effects on computer state set, defun, etc.

Ch Ch jcmt CSE 3302 Programming Languages Data Structures Constructor : list Primitives : atoms –Numeric (integer and floating point) Many ops provided: arithmetic, increment, decrement, max, min, relational, predicates –Nonnumeric character strings Limited ops: comparisons for equality nil (false, empty set) null is the test op

Ch Ch jcmt CSE 3302 Programming Languages Constructor List –Surrounded by parentheses –Separated by blanks –Zero, one or more elements –Can contain lists –() equivalent to nil ; called empty or null list

Ch Ch jcmt CSE 3302 Programming Languages Accessing Parts of a List Car –Selects the first element of a list Cdr –Returns all of a list except its first element Car and Cdr are selectors Car and Cdr can be combined to access interior parts of a list: ex. caadr

Ch Ch jcmt CSE 3302 Programming Languages Building a list Cons –Creates a list from a first element and a list –Is the inverse of car and cdr –Example: (car ‘(to be or not))= to (cdr ‘(to be or not))= (be or not) (cons ‘to ‘(be or not))= (to be or not)

Ch Ch jcmt CSE 3302 Programming Languages Info Representation Property lists –( p1 v1 p2 v2 … pn vn) where pn is the nth property and vn is the value of that property –P-lists give flexibility –Easy to define a function to get properties

Ch Ch jcmt CSE 3302 Programming Languages Info Representation Association lists –(( a1 v1) (a2 v2) … ( an vn )) –Handles properties which are flags or properties with multiple values

Ch Ch jcmt CSE 3302 Programming Languages Recursive list construction Append two lists together –Identify simplest cases (append ‘() L) = L (append L ‘()) = L –Reduce other cases to the simplest cases (append L M) = (cons (car L) (append (cdr L) M) –(defun append (L M) (if (null L)) M (cons (car L) (append (cdr L) M) ))

Ch Ch jcmt CSE 3302 Programming Languages Atoms Created by mentioning them Have properties and relations (p-list) –Print name / pname – putprop adds a property to an atom – apval denotes the binding of an atom to a value – set binds an atom – symbol-plist shows property list of an atom

Ch Ch jcmt CSE 3302 Programming Languages List representation Linked list Cells in the list have a right and a left –Right part points to next cell in list –Left part points to value of the cell Data and programs both represented the same way –Program linked list called an expression tree

Ch Ch jcmt CSE 3302 Programming Languages List Primitives Efficient –Car returns a left part –Cdr returns a right part –Cons requires storage allocation for new cell –Car, Cdr, and Cons do not change values Sublists can be shared Pseudo-functions alter lists –rplaca, rplacd –Have same drawbacks as aliasing

Ch Ch jcmt CSE 3302 Programming Languages Conditional expression Cond –Mimics mathematical notation –Logical ops are defined in terms of the conditional And & Or –Operands are evaluated sequentially – Conditional interpretation vs. strict

Ch Ch jcmt CSE 3302 Programming Languages Iteration Performed by recursion Reduction –Perform some op on every element of a list –Uses a binary function to reduce a list to a single value Mapping –Apply a function to every element of a list –Returns a list of the results Filtering –Forms a sublist containing all elements that satisfy some property

Ch Ch jcmt CSE 3302 Programming Languages Functional arguments Abstracting out pattern Mapcar Filter Reduce Suppress details Simplify combination

Ch Ch jcmt CSE 3302 Programming Languages Recursive Interpreters Arranged by cases –Atoms Numeric Nonnumeric –Quote –Conditional –Functions

Ch Ch jcmt CSE 3302 Programming Languages Interpreters Primitive ops performed explicitly User-defined functions –Evaluate parameters –Bind formals to actuals –Add bindings to environment –Evaluate function in environment

Ch Ch jcmt CSE 3302 Programming Languages Environment of evaluation Arguments going to ( apply f x a) (p.380) – f the function - lambda expression – x the parameters - bound formals – a the environment - existing environment with the addition of the bound formals Universal function

Ch Ch jcmt CSE 3302 Programming Languages Static scoping Closure to indicate environment of definition - function keyword – Instruction part (ip) - program, lambda expr. – Environment part (ep) - definition environment ( closure ip ep) User-defined functions can be –Dynamically scoped lambda expressions –Statically scoped function closures

Ch Ch jcmt CSE 3302 Programming Languages Incompatible scope rules Options –Adopt uniform static scoping –Use default static scoping but allow “special” dynamic scoping

Ch Ch jcmt CSE 3302 Programming Languages Storage reclamation Explicit erasure –Work for programmers –Security problems (dangling pointers) Automatic reclamation –Reference counts –Garbage collection

Ch Ch jcmt CSE 3302 Programming Languages Reference counts Accessible cells are referenced Reference count keeps track of how many other cells reference the current one Count must be incremented and decremented correctly Cells with zero count are added to the free list

Ch Ch jcmt CSE 3302 Programming Languages Reference counts Cycles defeat reference counts –Cyclic reference boosts the count of each member plus one member has an outside reference. –If outside reference goes away, cycle still has references internally but no access from anywhere else in program –Allowing cycles is an open issue

Ch Ch jcmt CSE 3302 Programming Languages Garbage Collection When storage becomes low, system starts up garbage collection Mark and sweep process –Starting at roots mark all cells that can be reached recursively moving from each cell to its children –When all are marked, sweep through memory from top, free unmarked cells and reset flags on marked cells

Ch Ch jcmt CSE 3302 Programming Languages Garbage Collection Wreaks havoc on execution time Non uniform and unpredictable Approaches to more efficiency include –Continuous garbage collection –Parallel

Ch Ch jcmt CSE 3302 Programming Languages LISP AI - represent and manipulate complex interrelationships among symbolic data Suited to ill-specified problems Easy to manipulate LISP programs in LISP