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Thus far we have talked mainly about interfaces among the components of a program or between programs But there is another important kind of interface, between a program and its human users Most of the example programs in this book are text-based, so their user interfaces tend to be straightforward As we discussed in the previous section, errors should be
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detected and reported, and recovery attempted where it makes sense Error output should include all available information and should be as meaningful as possible out of context; a diagnostic should not say estrdup f a i 1ed when it could say markov: estrdup("Derrida") f a i l e d : Memory l i m i t reached It costs nothing to add the extra information as we did in estrdup, and it may help a user to identify a problem or provide valid input Programs should display information about proper usage when an error is made, as shown in functions like
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/n usage: p r i n t usage message and e x i t */ voi d usage (voi d)
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f p r i n t f ( s t d e r r , "usage: %s [-dl [-n nwordsl" " [-s seed] [ f i l e s l \ n W , progname0); e x i t (2) ;
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The program name identifies the source of the message which is especially important if this is part of a larger process If a program presents a message that just says syntax e r r o r or estrdup f a i l e d , the user might have no idea who said it The text of error messages, prompts, and dialog boxes should state the form of valid input Don't say that a parameter is too large; report the valid range of values When possible, the text should be valid input itself, such as the full command line with the parameter set properly In addition to steering users toward proper use, such output can be captured in a file or by a mouse sweep and then used to run some further process This points out a weakness of dialog boxes: their contents are hard to grab for later use One effective way to create a good user interface for input is by designing a specialized language for setting parameters, controlling actions and so on; a good notation can make a program easy to use while it helps organize an implementation Language-based interfaces are the subject of 9 Defensive programming, that is, making sure that a program is invulnerable to bad input, is important both for protecting users against themselves and also as a security mechanism This is discussed more in 6 which talks about program testing For most people graphical interfaces are the user interface for their computers Graphical user interfaces are a huge topic, so we will say only a few things that are germane to this book First, graphical interfaces are hard to create and make "right" since their suitability and success depend strongly on human behavior and expectations Second, as a practical matter, if a system has a user interface, there is usually more code to handle user interaction than there is in whatever algorithms do the work
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Nevertheless, familiar principles apply to both the external design and the internal implementation of user interface software From the user's standpoint, style issues like simplicity, clarity, regularity, uniformity, familiarity, and restraint all contribute to an interface that is easy to use; the absence of such properties usually goes along with unpleasant or awkward interfaces Uniformity and regularity are desirable including consistent use of terms units, formats, layouts fonts, colors, sizes, and all the other options that a graphical system makes available How many different English words are used to exit from a program or close a window The choices range from Abandon to control-Z, with at least a dozen between This inconsistency is confusing to a native speaker and baffling for others Within graphics code interfaces are particularly important, since these systems are large, complicated and driven by a very different input model than scanning sequential text Object-oriented programming excels at graphical user interfaces, since it provides a way to encapsulate all the state and behaviors of windows, using inheritance to combine similarities in base classes while separating differences in derived classes
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