The actions of the pretty printer when a piece of output is too large to fit in the space available can be precisely controlled. Three concepts underlie the way these operations work—logical blocks , conditional newlines , and sections . Before proceeding further, it is important to define these terms.
The first line of Figure 22–3 shows a schematic piece of output. Each of
the characters in the output is represented by "-". The positions of
conditional newlines are indicated by digits. The beginnings and ends of
logical blocks are indicated by "<" and ">" respectively.
The output as a whole is a logical block and the outermost section. This
section is indicated by the 0’s on the second line of Figure 1. Logical
blocks nested within the output are specified by the macro
pprint-logical-block. Conditional newline positions are specified
by calls to pprint-newline. Each conditional newline defines
two sections (one before it and one after it) and is associated with a
third (the section immediately containing it).
The section after a conditional newline consists of: all the output up to, but not including, (a) the next conditional newline immediately contained in the same logical block; or if (a) is not applicable, (b) the next newline that is at a lesser level of nesting in logical blocks; or if (b) is not applicable, (c) the end of the output.
The section before a conditional newline consists of: all the output back to, but not including, (a) the previous conditional newline that is immediately contained in the same logical block; or if (a) is not applicable, (b) the beginning of the immediately containing logical block. The last four lines in Figure 1 indicate the sections before and after the four conditional newlines.
The section immediately containing a conditional newline is the shortest
section that contains the conditional newline in question. In Figure 22–3,
the first conditional newline is immediately contained in the section
marked with 0’s, the second and third conditional newlines are immediately
contained in the section before the fourth conditional newline, and the
fourth conditional newline is immediately contained in the section after
the first conditional newline.
<-1---<--<--2---3->--4-->->
000000000000000000000000000
11 111111111111111111111111
22 222
333 3333
44444444444444 44444
Whenever possible, the pretty printer displays the entire contents of a section on a single line. However, if the section is too long to fit in the space available, line breaks are inserted at conditional newline positions within the section.