1prettypr(3)                Erlang Module Definition                prettypr(3)
2
3
4

NAME

6       prettypr - A generic pretty printer library.
7

DESCRIPTION

9       A  generic pretty printer library. This module uses a strict-style con‐
10       text passing implementation of John Hughes algorithm, described in "The
11       design  of  a Pretty-printing Library". The paragraph-style formatting,
12       empty documents, floating documents, and null strings are my own  addi‐
13       tions to the algorithm.
14
15       To  get  started,  you  should read about the document() data type; the
16       main constructor functions: text/1, above/2, beside/2,  nest/2,  sep/1,
17       and par/2; and the main layout function format/3.
18
19       If  you  simply  want to format a paragraph of plain text, you probably
20       want to use the text_par/2 function, as in the following example:
21
22         prettypr:format(prettypr:text_par("Lorem ipsum dolor sit amet"), 20)
23

DATA TYPES

25         document():
26
27
28           An abstract character-based "document"  representing  a  number  of
29           possible  layouts,  which can be processed to produce a single con‐
30           crete layout. A concrete layout can then be rendered as a  sequence
31           of  characters  containing  linebreaks,  which  can  be passed to a
32           printer or terminal that uses a fixed-width font.
33
34           For example, a document sep([text("foo"), text("bar")])  represents
35           the two layouts
36
37              foo bar
38
39           and
40
41              foo
42              bar
43
44           Which  layout  is chosen depends on the available horizontal space.
45           When processing a document, the main parameters are the paper width
46           and  the  line  width  (also  known  as the "ribbon width"). In the
47           resulting layout, no text should be printed beyond the paper  width
48           (which  by  default is 80 characters) as long as it can be avoided,
49           and each single line of text (its indentation  not  counted,  hence
50           "ribbon")  should  preferably  be  no wider than the specified line
51           width (which by default is 65).
52
53           Documents can be joined into a single new document using  the  con‐
54           structor functions of this module. Note that the new document often
55           represents a larger number of possible layouts than just the sum of
56           the components.
57

EXPORTS

59       above(D1::document(), D2::document()) -> document()
60
61              Concatenates documents vertically. Returns a document represent‐
62              ing the concatenation of the documents D1 and D2 such  that  the
63              first line of D2 follows directly below the last line of D1, and
64              the first character of D2 is in the same  horizontal  column  as
65              the first character of D1, in all possible layouts.
66
67              Examples:
68
69                   ab  cd  =>  ab
70                               cd
71
72                                  abc
73                   abc   fgh  =>   de
74                    de    ij      fgh
75                                   ij
76
77       beside(D1::document(), D2::document()) -> document()
78
79              Concatenates  documents  horizontally. Returns a document repre‐
80              senting the concatenation of the documents D1 and D2  such  that
81              the  last  character of D1 is horizontally adjacent to the first
82              character of D2, in all possible layouts. (Note: any indentation
83              of D2 is lost.)
84
85              Examples:
86
87                   ab  cd  =>  abcd
88
89                   ab  ef      ab
90                   cd  gh  =>  cdef
91                                 gh
92
93       best(D::document(),   PaperWidth::integer(),  LineWidth::integer())  ->
94       empty | document()
95
96              Selects a "best" layout for a document, creating a corresponding
97              fixed-layout  document. If no layout could be produced, the atom
98              empty is returned instead.  For  details  about  PaperWidth  and
99              LineWidth, see format/3. The function is idempotent.
100
101              One  possible  use of this function is to compute a fixed layout
102              for a document, which can then be included as part of  a  larger
103              document. For example:
104
105                   above(text("Example:"), nest(8, best(D, W - 12, L - 6)))
106
107              will  format  D as a displayed-text example indented by 8, whose
108              right margin is indented by 4 relative to the paper width  W  of
109              the  surrounding  document,  and  whose  maximum individual line
110              length is shorter by 6 than the line length L of the surrounding
111              document.
112
113              This function is used by the format/3 function to prepare a doc‐
114              ument before being laid out as text.
115
116       break(D::document()) -> document()
117
118              Forces a line break at the end of the given document. This is  a
119              utility function; see empty/0 for details.
120
121       empty() -> document()
122
123              Yields  the  empty document, which has neither height nor width.
124              (empty is thus different from an empty text  string,  which  has
125              zero width but height 1.)
126
127              Empty  documents  are  occasionally  useful; in particular, they
128              have the property that above(X, empty()) will force a  new  line
129              after  X without leaving an empty line below it; since this is a
130              common idiom, the utility function break/1 will  place  a  given
131              document in such a context.
132
133              See also: text/1.
134
135       floating(D::document()) -> document()
136
137              Equivalent to floating(D, 0, 0).
138
139       floating(D::document(), Hp::integer(), Vp::integer()) -> document()
140
141              Creates  a  "floating"  document. The result represents the same
142              set of layouts as D; however, a floating document may  be  moved
143              relative to other floating documents immediately beside or above
144              it, according to their relative horizontal and vertical  priori‐
145              ties. These priorities are set with the Hp and Vp parameters; if
146              omitted, both default to zero.
147
148              Notes: Floating documents appear to work well, but are currently
149              less general than you might wish, losing effect when embedded in
150              certain contexts. It  is  possible  to  nest  floating-operators
151              (even  with different priorities), but the effects may be diffi‐
152              cult to predict. In any case, note that the  way  the  algorithm
153              reorders  floating documents amounts to a "bubblesort", so don't
154              expect it to be able to sort large sequences of  floating  docu‐
155              ments quickly.
156
157       follow(D1::document(), D2::document()) -> document()
158
159              Equivalent to follow(D1, D2, 0).
160
161       follow(D1::document(), D2::document(), Offset::integer()) -> document()
162
163              Separates  two  documents  by  either  a single space, or a line
164              break and intentation. In other words, one of the layouts
165
166                   abc def
167
168              or
169
170                   abc
171                    def
172
173              will be generated, using the optional offset in the latter case.
174              This  is  often useful for typesetting programming language con‐
175              structs.
176
177              This is a utility function; see par/2 for further details.
178
179              See also: follow/2.
180
181       format(D::document()) -> string()
182
183              Equivalent to format(D, 80).
184
185       format(D::document(), PaperWidth::integer()) -> string()
186
187              Equivalent to format(D, PaperWidth, 65).
188
189       format(D::document(), PaperWidth::integer(),  LineWidth::integer())  ->
190       string()
191
192              Computes  a  layout for a document and returns the corresponding
193              text. See document() for further information.  Throws  no_layout
194              if no layout could be selected.
195
196              PaperWidth specifies the total width (in character positions) of
197              the field for which the text is to be laid out. LineWidth speci‐
198              fies  the desired maximum width (in number of characters) of the
199              text printed on any single line, disregarding leading and trail‐
200              ing  white  space. These parameters need to be properly balanced
201              in order to produce good layouts. By default, PaperWidth  is  80
202              and LineWidth is 65.
203
204              See also: best/3.
205
206       nest(N::integer(), D::document()) -> document()
207
208              Indents a document a number of character positions to the right.
209              Note that N may be negative, shifting the text to the  left,  or
210              zero, in which case D is returned unchanged.
211
212       null_text(Characters::string()) -> document()
213
214              Similar  to  text/1,  but  the  result is treated as having zero
215              width. This is regardless of the actual length  of  the  string.
216              Null  text  is  typically  used for markup, which is supposed to
217              have no effect on the actual layout.
218
219              The standard example is when formatting source code as  HTML  to
220              be  placed  within <pre>...</pre> markup, and using e.g. <i> and
221              <b> to make parts of the source code stand out.  In  this  case,
222              the  markup does not add to the width of the text when viewed in
223              an HTML browser, so the layout engine should simply pretend that
224              the markup has zero width.
225
226              See also: empty/0, text/1.
227
228       par(Docs::[document()]) -> document()
229
230              Equivalent to par(Ds, 0).
231
232       par(Docs::[document()], Offset::integer()) -> document()
233
234              Arranges  documents  in a paragraph-like layout. Returns a docu‐
235              ment representing all possible left-aligned paragraph-like  lay‐
236              outs  of  the (nonempty) sequence Docs of documents. Elements in
237              Docs are separated horizontally by a single space character  and
238              vertically  with  a  single  line break. All lines following the
239              first (if any) are indented  to  the  same  left  column,  whose
240              indentation  is specified by the optional Offset parameter rela‐
241              tive to the position of the first element in Docs. For  example,
242              with  an offset of -4, the following layout can be produced, for
243              a list of documents representing the numbers 0 to 15:
244
245                       0 1 2 3
246                   4 5 6 7 8 9
247                   10 11 12 13
248                   14 15
249
250              or with an offset of +2:
251
252                   0 1 2 3 4 5 6
253                     7 8 9 10 11
254                     12 13 14 15
255
256              The utility function text_par/2 can be used to easily  transform
257              a  string of text into a par representation by splitting it into
258              words.
259
260              Note that whenever a document in Docs contains a line break,  it
261              will  be  placed on a separate line. Thus, neither a layout such
262              as
263
264                   ab cd
265                      ef
266
267              nor
268
269                   ab
270                   cd ef
271
272              will be generated. However, a useful idiom for making the former
273              variant possible (when wanted) is beside(par([D1, text("")], N),
274              D2) for two documents D1  and  D2.  This  will  break  the  line
275              between  D1  and D2 if D1 contains a line break (or if otherwise
276              necessary), and optionally further  indent  D2  by  N  character
277              positions.  The  utility  function follow/3 creates this context
278              for two documents D1 and D2, and an optional integer N.
279
280              See also: par/1, text_par/2.
281
282       sep(Docs::[document()]) -> document()
283
284              Arranges documents  horizontally  or  vertically,  separated  by
285              whitespace. Returns a document representing two alternative lay‐
286              outs of the (nonempty) sequence Docs  of  documents,  such  that
287              either  all  elements in Docs are concatenated horizontally, and
288              separated by a space character, or all elements are concatenated
289              vertically (without extra separation).
290
291              Note: If some document in Docs contains a line break, the verti‐
292              cal layout will always be selected.
293
294              Examples:
295
296                                                ab
297                   ab  cd  ef  =>  ab cd ef  |  cd
298                                                ef
299
300                   ab           ab
301                   cd  ef  =>   cd
302                                ef
303
304              See also: par/2.
305
306       text(Characters::string()) -> document()
307
308              Yields a document representing a fixed, unbreakable sequence  of
309              characters.  The string should contain only printable characters
310              (tabs allowed but not recommended), and not newline, line  feed,
311              vertical  tab,  etc. A tab character (\t) is interpreted as pad‐
312              ding of 1-8 space characters to the next column of 8  characters
313              within the string.
314
315              See also: empty/0, null_text/1, text_par/2.
316
317       text_par(Text::string()) -> document()
318
319              Equivalent to text_par(Text, 0).
320
321       text_par(Text::string(), Indentation::integer()) -> document()
322
323              Yields  a  document representing paragraph-formatted plain text.
324              The optional Indentation parameter specifies the extra  indenta‐
325              tion   of   the  first  line  of  the  paragraph.  For  example,
326              text_par("Lorem ipsum dolor sit amet", N) could represent
327
328                   Lorem ipsum dolor
329                   sit amet
330
331              if N = 0, or
332
333                     Lorem ipsum
334                   dolor sit amet
335
336              if N = 2, or
337
338                   Lorem ipsum dolor
339                     sit amet
340
341              if N = -2.
342
343              (The sign of the indentation is thus reversed  compared  to  the
344              par/2  function,  and the behaviour varies slightly depending on
345              the sign in order to match the expected layout of a paragraph of
346              text.)
347
348              Note  that  this  is just a utility function, which does all the
349              work of splitting the  given  string  into  words  separated  by
350              whitespace  and  setting  up  a par with the proper indentation,
351              containing a list of text elements.
352
353              See also: par/2, text/1, text_par/1.
354

AUTHORS

356       Richard Carlsson <carlsson.richard@gmail.com>
357
358
359
360                              syntax_tools 2.1.7                   prettypr(3)
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