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Example research essay topic: Programming Language Target Language - 1,622 words

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... n slowly or are not very space efficient and then reliably transformed into programs which are smaller and run much faster -- and are also correct. Code transformations become second nature to experienced Scheme programmers. When a function similar to the recursive function fact is seen, it tends to get written down in the iterative form. Scheme has several important advantages. Its elegantly simple, regular structure and trivial syntax avoids "special case" confusion.

Its expressiveness means that one spends little time trying to work around the language -- it lets users concentrate on what they want to say rather than on how to say it. Its support of a variety of styles (including OO, which has not been emphasized here) allows users to better match their solution style to the style of the problems to be solved. Its formal underpinnings make reasoning about programs much easier. Its attractive power makes it easy to separate system specific optimizations from reusable code.

Its comparability makes it easy to construct systems from well-tested components. If you want to write complex, correct programs quickly, scheme for; ; MACROS Just as functions are semantic abstractions over operations, macros are textual abstractions over syntax. Managing complex software systems frequently requires designing specialized "languages" to focus on areas of interest and hide superfluous details. Macros have the advantage of expanding the syntax of the base language without making the native compiler more complex Macros have always been a source of great power and confusion. Scheme has perhaps the most sophisticated macro system of any programming language in wide use today. How has macro technology advanced?

What are the problems which have been solved? Macros are frequently considered an advanced topic. Non-trivial macros are notoriously hard to get right. Because macros can be used to extend the base language, doing macro design can also be In the early days, macros were based on simple text substitution. A problem with text substitution is that token ization rules of the programming language are not respected. Indeed the token ization rules of the preprocessor may not match the rules of the target language.

For example, using the m 4 preprocessor: the token guelph followed by the non-token glop" In addition to token ization confusion, there are problems where macro expansion does not respect the structure of source language expressions. For example, a well known problem with the C and is interpreted by the compiler as There are frequently problems relating to the accidental collision of introduced names. Even obscure names may collide where there are multiple macro writers or recursive macros. #define swap (a, b) { int temp; temp = a; a = b; b = temp } {int temp; temp = temp; temp = b; b = temp} Free names may collide with those used locally: #define clear (addr, len) fill (addr, len, 0) clear (mem ptr, mem size); / local fill shadows hidden global fill / In general, macro systems have done poorly on name conflicts where lexical scoping and recursion are involved. So what do we want out of a macro system? We want respect for the syntax, expressions, and name bindings. We want error checking.

We want a convenient notation for specifying syntactic transcriptions. And of course we want this in linear processing One thing to notice is that as macro systems have become more sophisticated, they have moved closer to the semantic analysis phase of the compiler. LISP systems achieved respect for target language syntax by doing macro transformations on parse trees rather than source text. Scheme's system takes things a step further by respecting lexical scoping and name bindings. In addition, the standard high-level part of Scheme's macro system specifies syntactic rewrite rules in a pattern directed fashion To contrast Scheme's macro system with those of older LISPs, here is a brief historical view of the LET macro. LET is a construct for introducing new name bindings.

It has the form: The semantics of LET is to evaluate the expressions in an environment extended by the names which have initial values obtained by evaluating the expressions. An example is: (let ( (a 1) (b 2) ) (+ a b) ), which binds the value 1 to a, 2 to b and then evaluates the expression (+ a b). LET is So (let ( (a 1) (b 2) ) (+ a b) ) rewrites to Early LISP systems operated directly on the list structures of the parsed source expression using low-level operations: (cons (map car (cadr form) ) (car form) ) ) Later, arguments and "backquote's" were added, making things much more readable, but without error checking. The back quote (') indicates an "almost constant" list expression, where comma (, ) or comma-splice (, @) are used to force evaluation of a sub expression. The comma replaces the evaluated expression directly, where comma-splice splices it into the list. So ' (lambda, (cdr (cons 'a 'b) ), @ (cdr (cons 'a 'b) ) ) Here is LET with argument binding and back quote: (define-syntax (LET def-list.

expressions) ' ( (lambda, (map car def-list), @expressions) And here is the Scheme version using pattern machine with error ( (let ( ()... )... ); before ( (lambda (... )... )... ); after This next example demonstrates both macro names shadowing local variables and locals shadowing macros. The outer binding of CAR is to a function which returns the first element of a list. (let-syntax ( (first (syntax-rules () ( (first? x) (car? x) ) ) ) (second (syntax-rules () ( (second? x) (car (cdr? x) ) ) ) ) (let-syntax ( (classic (syntax-rules () ( (classic) car) ) ) ) (let-syntax ( (affordable (syntax-rules () ( (affordable) car) ) ) ) (let ( (cars (list (classic) (affordable) ) ) ) (list (second cars) (first cars) ); result The above evaluates to: ("Matchbox"Packard") PRACTICUM: extending core Scheme to standard scheme Scheme can remain such a small language because one can extend the syntax of the language without making the compiler more complex.

This allows the compiler to know a great deal about the few (7) basic forms. Most compiler optimizations then fall out as general rather than special cases. The following syntax definitions from the current Scheme standard are directly (and simply) implemented. Example: (or ( = divisor 0) (/ number divisor) ) Semantics: OR evaluates the expressions from left to right. The value of the first true (not #f) expression is returned. Any remaining expressions are not evaluated.

Example: (and (input-port? p) (read p) ) Semantics: AND evaluates the expressions from left to right. The value of the first false expression is returned. Any remaining expressions are not evaluated. (let ( (a 2) (b (+ a 5) ) ) ( a b) ); = 84 (let loop ( (count N) (accum 0) ) (loop (- count 1) ( count accum) ) Semantics: LET evaluates the s in the enclosing environment in some unspecified order and then extends the environment by binding the values to the s and evaluates the expressions from left to right, returning the value of the last expression as the value of the LET. LET can be thought of as a "parallel assignment." Note that the value of B in the first example depends on the value of A in the outer environment. The second form is known as NAMED LET and allows recursion within the let form.

For the example above, the call to LOOP acts as a goto which rebinds the s to fresh values and "starts over at the top of the loop." Note that this is a functional construct: there are no unbound variables introduced and no (lambda (... )... ) ) (let ( (a 2) (b (+ a 5) ) ) ( a b) ); = 14 Semantics: Like un-named LET except that the expressions are evaluated sequentially and each "sees" the value of the (if (zero? n) #t (odd? (sub 1 n) ) ) ) ) (if (zero? n) #f (even? (sub 1 n) ) ) ) ) Semantics: Mutually recursive form of let. All name bindings are visible to all s. Because the order of evaluation of the s is unspecified, it must be possible to evaluate each init without refering to the value of any. Note that if the values are all lambda expressions, this condition is always (cond (... )... (else... ) ) (cond ( (as key alist) = cdr) (else search-failure-value) ) Semantics: Each expression is evaluated in turn.

The first which evaluates to true causes the s to be evaluated. The value of the last is returned in this case, or the value of if there are no s. If no is true, the s of the else clause are evaluated and the value of the last is the value of the COND expression. If not is true and there is no else clause, the result is unspecified. If a is true and followed by ' = then the following must evaluate to a procedure of one argument and the procedure is called with the value of the (syntax-rules (else = ); ; 'else and ' = must match exactly (if (begin... ) (cond... ) ) Example: (case (peak-char (current-input-port) ) ( (#h #? ) (print-help-text) ) ( (#space #newline) (keep-looking) ) (else (read-command) ) ) Semantics: The expression is evaluated and compared in turn to each.

If it is equivalent (e? ), then the s of the first clause containing the datum are evaluated and the value of the last one is returned as the value of the CASE expression. If no match is found, then the else expression (s) are evaluated and the last value returned, otherwise the value of the CASE is Bibliography: None


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