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1.1       root        1: .\"    @(#)tt08        6.1 (Berkeley) 5/23/86
                      2: .\"
                      3: .NH
                      4: Introduction to Macros
                      5: .PP
                      6: Before we can go much further in
                      7: .UL troff ,
                      8: we need to learn a bit about the
                      9: macro
                     10: facility.
                     11: In its simplest form, a macro is just a shorthand notation
                     12: quite similar to a string.
                     13: Suppose we want every paragraph to start
                     14: in exactly the same way _
                     15: with a space and a temporary indent of two ems:
                     16: .P1
                     17: ^sp
                     18: ^ti +2m
                     19: .P2
                     20: Then to save typing, we would like to collapse these into
                     21: one shorthand line,
                     22: a
                     23: .UL troff
                     24: `command' like
                     25: .P1
                     26: ^PP
                     27: .P2
                     28: that would be treated by
                     29: .UL troff
                     30: exactly as
                     31: .P1
                     32: ^sp
                     33: ^ti +2m
                     34: .P2
                     35: .BD .PP
                     36: is called a
                     37: .ul
                     38: macro.
                     39: The way we tell
                     40: .UL troff
                     41: what
                     42: .BD .PP
                     43: means is to
                     44: .ul
                     45: define
                     46: it with the
                     47: .BD .de
                     48: command:
                     49: .P1
                     50: ^de PP
                     51: ^sp
                     52: ^ti +2m
                     53: ^^
                     54: .P2
                     55: The first line names the macro
                     56: (we used
                     57: .BD .PP ' `
                     58: for `paragraph',
                     59: and upper case so it wouldn't conflict with
                     60: any name that
                     61: .UL troff
                     62: might
                     63: already know about).
                     64: The last line
                     65: .BD ..
                     66: marks the end of the definition.
                     67: In between is the text,
                     68: which is simply inserted whenever
                     69: .UL troff
                     70: sees the `command'
                     71: or macro call
                     72: .P1
                     73: ^PP
                     74: .P2
                     75: A macro
                     76: can contain any mixture of text and formatting commands.
                     77: .PP
                     78: The definition of
                     79: .BD .PP
                     80: has to precede its first use;
                     81: undefined macros are simply ignored.
                     82: Names are restricted to one or two characters.
                     83: .PP
                     84: Using macros for commonly occurring sequences of commands
                     85: is critically important.
                     86: Not only does it save typing,
                     87: but it makes later changes much easier.
                     88: Suppose we decide that the paragraph indent is too small,
                     89: the vertical space is much too big,
                     90: and roman font should be forced.
                     91: Instead of changing the whole document,
                     92: we need only change the definition of
                     93: .BD .PP
                     94: to
                     95: something like
                     96: .P1
                     97: ^de PP \e" paragraph macro
                     98: ^sp 2p
                     99: ^ti +3m
                    100: ^ft R
                    101: ^^
                    102: .P2
                    103: and the change takes
                    104: effect everywhere we used
                    105: .BD .PP .
                    106: .PP
                    107: .BD \e"
                    108: is a
                    109: .UL troff
                    110: command that causes the rest of the line to be ignored.
                    111: We use it here to add comments to the macro
                    112: definition
                    113: (a wise idea once definitions get complicated).
                    114: .PP
                    115: As another example of macros,
                    116: consider these two which start and end a block of offset,
                    117: unfilled text, like most of the examples in this paper:
                    118: .P1
                    119: ^de BS \e" start indented block
                    120: ^sp
                    121: ^nf
                    122: ^in +0.3i
                    123: ^^
                    124: ^de BE \e" end indented block
                    125: ^sp
                    126: ^fi
                    127: ^in \(mi0.3i
                    128: ^^
                    129: .P2
                    130: Now we can surround text like
                    131: .P1
                    132: Copy to
                    133: John Doe
                    134: Richard Roberts
                    135: Stanley Smith
                    136: .P2
                    137: by the commands
                    138: .BD .BS
                    139: and
                    140: .BD .BE ,
                    141: and it will come out as it did above.
                    142: Notice that we indented by
                    143: .BD .in\ +0.3i
                    144: instead of
                    145: .BD .in\ 0.3i .
                    146: This way we can nest our uses of
                    147: .BD .BS
                    148: and
                    149: .BD BE
                    150: to get blocks within blocks.
                    151: .PP
                    152: If later on we decide that the indent
                    153: should be 0.5i, then it is only necessary to
                    154: change the definitions of
                    155: .BD .BS
                    156: and
                    157: .BD .BE ,
                    158: not the whole paper.

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