22C:111 — Fall 2005
page i
Homework V
(100 points)
This problem involves a common text processing computation — fully (left and
right) justifying a document. That is, replacing the “whitespace” in a text
document so that there are smooth left and right margins. In Haskell, text will
simply consist of a String. For this problem, the term “whitespace” will refer to
occurrences of spaces (' '), tabs ('\t'), and newlines or carriage returns ('\n').
The term “word” will denote a maximal length substring of non-whitespace
characters. The term “line” will denote substrings enclosed by newline
characters (or the beginning or end of string). Each string will be decomposed
into a sequence of “words” separated by whitespace.
Write a Haskell definition for a function 'justify' that takes two arguments, an
integer n>0 and a string 'text'. The resulting string should consist of the words
from text with all whitespace replaced so that each line (except the last):
• is exactly n characters long (not counting the newline character),
• includes as many words of the document as possible,
• has a first character that is the first character of some word and last
character that is the last character of some word, and
• has each pair of words in a line separated by a number of spaces differing
from other pairs in that line by at most one.
If for any reason this reformatting cannot be achieved, an error message
should be issued.
So for example
justify 15 "Now is the time for all good men to come to the aid of their
country."
should yield
"Now is the time
for all good
men to come to
the aid of
their country."
while
justify 10 "abracadabra"
should yield an error message like "ERROR - width too small".
Note that to insert the special whitespace characters into a string in Haskell,
the escape notations '\n' and '\t' are required. Also, to have the escape
notations replaced by actual whitespace, it is necessary to use the 'putStr'
function. For instance, "abc\ndef" returns "abc\ndef", but putStr "abc\ndef"
returns
abc
def
