Alphard: Form and Content by Mary Shaw (auth.), Mary Shaw (eds.)

By Mary Shaw (auth.), Mary Shaw (eds.)

Alphard is a layout for a programming approach that helps the abstraction and verification concepts required by way of sleek program'ming method. in the course of the language layout procedure, we have been involved at the same time with difficulties of method, correctness, and potency. Methodological issues are addressed via amenities for outlining new, task·specific abstractions that catch advanced notions when it comes to their meant homes, with no explicating them by way of particular low· point implementations. ideas for verifying convinced homes of those courses handle the correctness issues. eventually, the language has been designed to allow compilation to effective item code. even supposing a compiler was once no longer carried out, the examine make clear specification matters and on programming method. an abstraction, specifying its habit Alphard language constructs let a programmer to isolate publicly whereas localizing wisdom approximately its implementation. The verification of such an abstraction involves exhibiting that its implementation behaves according to the general public specification. Given one of these verification, the abstraction can be used with self assurance to build higher·level, extra summary, courses. the most typical form of abstraction in Alphard corresponds to what's now known as an summary facts kind. An summary facts kind contains a collection of values for parts of the kind and a collection of operations on these values. a brand new language build, the shape, offers the way to encapsulate the definitions of information buildings and operations in the sort of method that merely public details should be accessed via the remainder of the program.

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P. eles in 0. m Once the stack form is defined, programs may declare and use stacks. The following program uses a stack as defined by this form to traverse a (finite) binary tree and count its tips. It also uses iteration and an explicit stack of binary trees [BurstaIl74, London75]. A binary tree is defined recursively to be either nil or to have a left son and a right son. which are both binary trees. f. t-nil then 1 else tips(ieftson(t»+tips(rightson(t» We shall not define a binary tree form explicitly, but shall presume that it meets at least the specifications isleaf(t:binarytree) returns b:boolean post b ..

Buxton and B. ttee. Rome. Itcl,. October 27-31, 1969, NATO, April 1970. Ole-Johan Dahl and C. A. R. -J. Dahl, E. W. R. Hoare), Academic Pres~ 1972 (pp. 175-220~ [OahI72] [OataConference76] Proc.

Imagine that while designing some program we found it desirable to use the notion of a stack -- in particular, a stack whose elements are integers. We presume that our language does not contain stacks as a primitive concept, as indeed Alphard does not, so we want to introduce it as a new abstraction. Suppose further that an a priori depth limit is known or desired, so we need not define a general stack mechanism, only one which behaves like a stack so long as its depth does not exceed some predetermined maximum.

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