Daily CS103 Doggie Bag 10
The most important topics and details of the day. February 23, 2006
Topic 1. Records. In addition to variants, records are another sort of user-dened datatype in OCaml. Intuitively, records are collections of named values, very much like a struct in C/C++, except that basic record elds are immutable. Record types must be declared before they are constructed, by specifying eld names and their associated types. Field values are accessed (i.e., records are deconstructed), via familiar dot notation; i.e., if r is a record, and l is a eld name in r, then r.l accesses the associated eld value. Examples: type student = { name : string; email : string; gpa : float} let bob = { name = bob; email = [e-mail]; gpa = 3.2} bob.email [e-mail] type rt = { a : int; b : bool } let rv = { a = 1 + 2; b = true} rv : rt rv.a : int rv.a 3 rv.a = 2 false Topic 2. Mutation and state. So far, we have considered the purely functional subset of OCaml. Now, we turn out consideration to mutable features in OCaml. Mutation is a form of side effect, where something happens behind the scenes, rather than as a direct result of evaluation. With mutable variables, what happens behind the scenes is that the store, where mutable values are saved, is changed, affecting the state of the computer. Reference cells are the most basic sort of mutable variables. All reference cells have a ref type, and may be dereferenced and updated. Reference cells are in fact memory locations, analogous to a pointer. There are three basic operations on reference cells: creation: ref v c, where c is a fresh reference cell; stores v at store location referenced by c dereferencing: !e v returns the contents of c iff e c assignment: e := e unit iff e c, and stores v in c where e v. NB: Assignment does not affect variable bindings. Also, now we see the usefulness of type unit, as the dummy result of a primarily side-effecting computation. example: let a = ref 5 a : int ref !a 5 := a 4 !a 4 a = 3 not well typed 1
Topic 3. Sequencing and evaluation order. Side effects make evaluation order really matter. For example, suppose x is a value of type int ref and suppose we perform the operations x := 1 and x := 2; the order in which we perform these operations has an effect on the evaluation of subsequent expressions such as !x + 2. Sequencing allows you to explicitly evaluate one expression before the other; an expression of the form (e1;e2) will result in the evaluation of e1 before e2. For example: let x = ref 0 in (x := 1; x := 2; !x + 2) 4 let x = ref 0 let store_and_return y = (x := y; y) store_and_return 1 1 !x 1 Note that the order of evaluation for compound expressions becomes very important. For example, the order in which operands of an addition expression are evaluated will effect the outcome of the following expression: let x = ref 0 in (x := 1; 1) + (x := 2; 1) + !x does it evaluate to 2, or to 4? Topic 4. Using mutation. Some languages are purely functional and contain no mutability. Some functional purists maintain that state is uneccessary! Indeed, it is easier to reason, via induction, about purely functional programs, and so correctn...