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The Art of Unix Programming
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Unix Programming - Designing Minilanguages - Language or Application Protocol?

Language or Application Protocol?

Another important question you need to ask is whether your minilanguage engine will be called interactively by other programs, as a slave process. If so, your design should probably look less like a conversational language for human interaction and more like the kind of application protocols we looked at in Chapter5.

The main difference is how carefully marked the boundaries of transactions are. Human beings are good at spotting where conversational output from a CLI ends, and where the prompt for the next input is. They can use context to tell what's significant and what should be ignored. Computer programs have much more trouble with this. Without either unambiguous end markers on output or advance knowledge of the length of the output, they can't tell when to stop reading.

Even worse is when a program's input is buffered (often inadvertently, as by stdio). A program that stops overtly reading at the right place can nonetheless eat past it.

-- Doug McIlroy

Programs in which master processes are trying to do interactive things with slaved minilanguages that are not carefully designed around this problem are prone to deadlock as the master and slave fall out of synchronization (a problem we first noted in Chapter7).

There are workarounds for driving minilanguages that are not so carefully designed. The prototype for most of them is the Tcl expect package. This package assists conversation with CLIs. It's built around the following operation: read from slave until either a given regular-expression pattern is matched or a specified timeout elapses. With this (and, of course, a send-to-slave operation) it's often possible to construct master programs to do reliable dialogues with slave processes even when the latter have not been tailored for the role.

Workalikes of the expect package in other languages are available; a Web search for the name of your favorite language with the added keywords “Tcl expect” is quite likely to turn up something useful. As a minilanguage designer, however, you would be unwise to assume that all your users will be expect gurus. Even if they are, this is an extra glue layer and a place for things to go wrong.

Be aware of this issue when designing your minilanguage. It may be a good idea to add an option that changes its conversational behavior to make it respond more like an application protocol, with unambiguous end-of-output delimiters and an analog of byte stuffing.

[95] 20M is a conservative estimate based on mid-2003 figures from the Linux Counter and elsewhere.

[96] Kmail, which we looked at in Chapter6, won't even chase off-site links in HTML for this reason.

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The Art of Unix Programming
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