Application-Specific Languages

1. Application-Specific Languages

2. General-purpose languages • A general-purpose language is designed for a wide variety of programming problems • Examples:C, C++, Fortran, Algol, Pascal, Java • To meet conflicting demands, a general-purpose language must make many compromises

3. ASLs • An Application-Specific Language (ASL) is designed for a particular class of problems • ZIL (Zork Implementation Language) • MOO (an O-O language for MUDs) • Bard (for manipulating Abstract Syntax Trees) • ML (originally designed for just one program--Edinburgh LCF, Logic for Computable Functions)

4. Advantages of an ASL • An ASL is not a "Jack of all trades, master of none" -- it's very focused • An ASL can often avoid compromises, or at least bias them in favor of the application • An ASL can use very high-level constructs • An ASL can be simpler to learn and to use • An ASL can boost productivity up to 10x

5. Disadvantages of an ASL • You have to find or implement the ASL • It's yet another language to learn • It may be much worse for programming problems that the designer didn't anticipate • I've found this to be true of JavaScript

6. Deciding to use an ASL • Consider using an ASL if • The problem area has unique features • There are suitable high-level operations • The high-level operations are ubiquitous • There is a lot of very similar code to write • You can find a suitable existing ASL, or... • …you know how to write compilers

7. Zork

8. Sample ZIL Object -> tasty_food "tasty food" with description "Sure looks yummy!", article "some", initial "There is tasty food here.", name "food" "ration" "rations", after [; Eat: "Delicious!"; ], has edible;

9. More sample ZIL Object -> wicker_cage "wicker cage" with description "It's a small wicker cage.", initial "There is a small wicker cage discarded nearby.", name "cage" "small" "wicker", after [; Open: if (little_bird notin self) rfalse; print "(releasing the little bird)^"; <>; ], has container open openable transparent;

10. Telephone scenario (MOO) KhufuMongo dial #350 The phone is ringing.The other phone is ringing. The other phone is ringing. The phone is ringing.answer phone It sounds like you have It sounds like you havea connection. a connection. say helloMongo says, "hello" You say, "hello"

11. MOO Example 1 @program generic_telephone:dial . . . fork (1) while (this_phone.off_hook && connected == 0 && nrings < maxrings) other_party = other_phone.location; . . . suspend(1); endwhile

12. MOO Example 2 @program generic_telephone:answer this_phone = this; if (player != this_phone.location) player:tell("You must first take the phone."); else this_phone.off_hook = 1; other_phone = this_phone.connected_to; player.is_on_phone = this_phone; if (other_phone == 0) player:tell("You hear a dial tone."); endif endif .

13. How do you write an ASL? • Compilers generally consist of three parts: • A lexical scanner, to turn an input stream of characters into tokens. • A parser, to construct an Abstract Syntax Tree (AST) from the token stream. • Either a code generator, to walk the AST and translate it into some other computer language, or • An interpreter, to walk the AST and perform the operations indicated therein

14. The lexical scanner • Input to the ASL compiler is just a stream of characters • The characters have to be grouped into tokens that are meaningful for the particular language • Example: • Input: i f ( m i n < 0 ) m i n + + ; • Tokens: if(min<0)min++;

15. The parser • The parser constructs an Abstract Syntax Tree (AST) from the token stream • The AST defines the operations to be performed and the order in which to perform them • The AST is abstract because it leaves out information that may have been needed to construct the tree, but is no longer needed

16. IF LESS_THAN INCREMENT min 0 min Example Abstract Syntax Tree • if (min < 0 ) min++ ;

17. Compilers and interpreters • Once you have the AST, you have two choices: • Compile it: Generate code in some other computer language that does the same thing as the program in the ASL • Interpret it: Walk the tree and do what it says • Compiling gives you faster code • Interpreting is much easier and more flexible

18. lex and yacc • One of the easiest and best ways to write a compiler or interpreter is to use a pair of UNIX programs called lex and yacc • lex is an ASL for writing lexical scanners • yacc is an ASL for writing code generators • These ASLs have been ported to most platforms

19. Sample lex code • Definitions • D [0-9] OD [0-7] HD [a-fA-F0-9] • Tokens • {D}+ {return Make_Constant;} "16#"{HD}+ {return Make_Hex_Constant;} "and" {return Return_Token (AND_OP);} "array" {return Return_Token (ARRAY_TERM);} ":=" {return Return_Token (ASSIGN_OP);}

20. Sample yacc code • Token Definitions • %token AND_OP ARRAY_TERM ASSIGN_OP %token BEGIN_TERM %token IDENTIFIER_TERM IF_TERM IN_TERM • Nonterminal Definitions • assignment_statement : variable ASSIGN_OP expression { $$ := Apply (Assignment, $1, $3); } ; • while_statement : WHILE_TERM expression DO_TERM statement { $$ := Apply (While_Op, $2, $4); } ;

21. Bard example idiom remove_null_statements; -- Remove null statements, except empty loop bodies. procedure remove_null_statements (1, Null_Statement); go to first child of parent; not match While_Op; not match Do_While; not match C_For_Loop; commit; go to @trigger; delete; end procedure; end idiom;

22. Summary I • A programmer's goal should be to write the best code with the least time and effort. • General-purpose languages provide tools to let you expand the program's "vocabulary": • functions (procedures, methods...) • structs, objects, tables and databases, etc. • Much of the work is customizing the language • ...so why not start with a better language?

23. Summary II • Application-specific languages are most useful if: • You can find suitable high-level operations. • The high-level operations are unusual. • The high-level operations are ubiquitous. • There is a lot of very similar code to write. Laymen need to be able to do the programming. • You know how to write compilers.

24. The End