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The Art of Unix Programming
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Unix Programming - Applying Minilanguages - Case Study: bc and

Case Study: bc and dc

We first examined bc(1) and dc(1) in Chapter7 as a case study in shellouts. They are examples of domain-specific minilanguages of the imperative type.

dc is the oldest language on Unix; it was written on the PDP-7 and ported to the PDP-11 before Unix [itself] was ported.

-- Ken Thompson

The domain of these two languages is unlimited-precision arithmetic. Other programs can use them to do such calculations without having to worry about the special techniques needed to do those calculations.

In fact, the original motivation for dc had nothing to do with providing a general-purpose interactive calculator, which could have been done with a simple floating-point program. The motivation was Bell Labs' long interest in numerical analysis: calculating constants for numerical algorithms, accurately is greatly aided by being able to work to much higher precision than the algorithm itself will use. Hence dc's arbitrary-precision arithmetic.

-- Henry Spencer

Like SNG and Glade markup, one of the strengths of both of these languages is their simplicity. Once you know that dc(1) is a reverse-Polish-notation calculator and bc(1) an algebraic-notation calculator, very little about interactive use of either of these languages is going to be novel. We'll return to the importance of the Rule of Least Surprise in interfaces in Chapter11.

These minilanguages have both conditionals and loops; they are Turing-complete, but have a very restricted ontology of types including only unlimited-precision integers and strings. This puts them in the borderland between interpreted minilanguages and full scripting languages. The programming features have been designed not to intrude on the common use as a calculator; indeed, most dc/bc users are probably unaware of them.

Normally, dc/bc are used conversationally, but their capacity to support libraries of user-defined procedures gives them an additional kind of utility — programmability. This is actually the most important advantage of imperative minilanguages, one that we observed in the case study of the Documenter's Workbench tools to be very powerful whether or not a program's normal mode is conversational; you can use them to write high-level programs that embody task-specific intelligence.

Because the interface of dc/bc is so simple (send a line containing an expression, get back a line containing a value) other programs and scripts can easily get access to all these capabilities by calling these programs as slave processes. Example8.6 is one famous example, an implementation of the Rivest-Shamir-Adelman public-key cipher in Perl that was widely published in signature blocks and on T-shirts as a protest against U.S. export retrictions on cryptography, c. 1995; it shells out to dc to do the unlimited-precision arithmetic required.


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The Art of Unix Programming
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