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1.1     ! root        1: .\"    @(#)e3  6.1 (Berkeley) 5/22/86
        !             2: .\"
        !             3: .NH
        !             4: Language Design
        !             5: .PP
        !             6: The fundamental principle upon which we based our language design
        !             7: is that the language should be easy to use
        !             8: by people (for example, secretaries) who know neither mathematics nor typesetting.
        !             9: .PP
        !            10: This principle implies
        !            11: several things.
        !            12: First,
        !            13: ``normal'' mathematical conventions about operator precedence,
        !            14: parentheses, and the like cannot be used,
        !            15: for to give special meaning to such characters means
        !            16: that the user has to understand what he or she
        !            17: is typing.
        !            18: Thus the language should not assume, for instance,
        !            19: that parentheses are always balanced,
        !            20: for they are not in
        !            21: the half-open interval $(a,b]$.
        !            22: Nor should it assume that
        !            23: that $sqrt{a+b}$ can be replaced by
        !            24: $(a+b) sup roman \(12$,
        !            25: or that $1/(1-x)$ is better written as $1 over 1-x$
        !            26: (or
        !            27: vice versa).
        !            28: .PP
        !            29: Second, there should be relatively few rules,
        !            30: keywords,
        !            31: special symbols and operators, and the like.
        !            32: This keeps the language easy to learn and remember. Furthermore, there should be few exceptions to
        !            33: the rules that do exist: 
        !            34: if something works in one situation,
        !            35: it should work everywhere.
        !            36: If a variable can have a subscript,
        !            37: then a subscript can have a subscript, and so on without limit.
        !            38: .PP
        !            39: Third, ``standard'' things should happen automatically.
        !            40: Someone who types ``x=y+z+1'' should get ``$x=y+z+1$''.
        !            41: Subscripts and superscripts should automatically
        !            42: be printed in an appropriately smaller size,
        !            43: with no special intervention.
        !            44: Fraction bars have to be made the right length and positioned at the
        !            45: right height.
        !            46: And so on.
        !            47: Indeed a mechanism for overriding default actions has to exist,
        !            48: but its application is the exception, not the rule.
        !            49: .PP
        !            50: We assume
        !            51: that the typist has a reasonable picture
        !            52: (a two-dimensional representation)
        !            53: of the desired final form, as might be handwritten
        !            54: by the author of a paper.
        !            55: We also assume that
        !            56: the input is typed on a computer terminal much like an ordinary typewriter.
        !            57: This implies an input alphabet
        !            58: of perhaps 100 characters,
        !            59: none of them special.
        !            60: .PP
        !            61: A secondary, but still important, goal in our design
        !            62: was that the system should be easy to implement,
        !            63: since neither of the authors had any desire to make
        !            64: a long-term project of it.
        !            65: Since our design was not firm,
        !            66: it was also necessary that the program be easy to change
        !            67: at any time.
        !            68: .PP
        !            69: To make the program easy to build and to change,
        !            70: and to guarantee regularity
        !            71: (``it should work everywhere''),
        !            72: the language is defined by a
        !            73: context-free grammar, described in Section 5.
        !            74: The compiler for the language was built using a compiler-compiler.
        !            75: .PP
        !            76: A priori,
        !            77: the grammar/compiler-compiler approach seemed the right thing to do.
        !            78: Our subsequent experience leads us to believe
        !            79: that any other course would have been folly.
        !            80: The original language was designed in a few days. 
        !            81: Construction of a working system
        !            82: sufficient to try significant examples
        !            83: required perhaps a person-month.
        !            84: Since then, we have spent a modest amount of additional time
        !            85: over several years
        !            86: tuning, adding facilities,
        !            87: and occasionally changing the language as users
        !            88: make criticisms and suggestions.
        !            89: .PP
        !            90: We also decided quite early that
        !            91: we would let
        !            92: .UC TROFF
        !            93: do our work for us whenever possible.
        !            94: .UC TROFF
        !            95: is quite a powerful program, with
        !            96: a macro facility, text and arithmetic variables, numerical computation and testing,
        !            97: and conditional branching.
        !            98: Thus we have been able to avoid writing
        !            99: a lot of mundane but tricky software.
        !           100: For example, we store no text strings,
        !           101: but simply pass them on to
        !           102: .UC TROFF .
        !           103: Thus we avoid having to write a storage management package.
        !           104: Furthermore, we have been able to isolate ourselves
        !           105: from most details of the particular device and character set
        !           106: currently in use.
        !           107: For example, we let
        !           108: .UC TROFF
        !           109: compute the widths of all strings of characters;
        !           110: we need know nothing about them.
        !           111: .PP
        !           112: A third design goal is special to our environment.
        !           113: Since our program is only useful for typesetting mathematics,
        !           114: it is necessary that it interface cleanly with the underlying typesetting language
        !           115: for the benefit of users
        !           116: who want to set intermingled mathematics and text
        !           117: (the usual case).
        !           118: The standard mode of operation
        !           119: is that when a document is typed,
        !           120: mathematical expressions are input as part of the text,
        !           121: but marked by user settable delimiters.
        !           122: The program reads this input and treats as comments
        !           123: those things which are not mathematics,
        !           124: simply passing them through untouched.
        !           125: At the same time it converts the mathematical input
        !           126: into the necessary
        !           127: .UC TROFF
        !           128: commands.
        !           129: The resulting ioutput is passed directly to
        !           130: .UC TROFF
        !           131: where the comments and the mathematical parts both become
        !           132: text and/or
        !           133: .UC TROFF
        !           134: commands.

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