Difference between revisions of "99 questions/95 to 99"
m (shortened 96.parsec even further) 
APapageorge (talk  contribs) 

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__NOTOC__ 
__NOTOC__ 

−  This is part of [[H99:_NinetyNine_Haskell_ProblemsNinetyNine Haskell Problems]], based on [ 
+  This is part of [[H99:_NinetyNine_Haskell_ProblemsNinetyNine Haskell Problems]], based on [https://prof.ti.bfh.ch/hew1/informatik3/prolog/p99/ NinetyNine Prolog Problems]. 
−  
−  If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in Haskell>,<solution in haskell> and <description of implementation> fields. 

−  
−  == Miscellaneous problems == 

== Problem 95 == 
== Problem 95 == 

Line 14:  Line 10:  
Example in Haskell: 
Example in Haskell: 

−  <pre> 

−  > fullWords 175 

−  onesevenfive 

−  </pre> 

−  Solution: 

<haskell> 
<haskell> 

+  λ> fullWords 175 

−  import Data.List 

+  onesevenfive 

−  import Data.Maybe 

−  
−  fullWords :: Integer > String 

−  fullWords n = concat . intersperse "" . map (fromJust . (`lookup` table)) $ show n 

−  where table = [('0',"zero"), ('1',"one"), ('2',"two"), ('3',"three"), ('4',"four"), 

−  ('5',"five"), ('6',"six"), ('7',"seven"), ('8',"eight"), ('9',"nine")] 

</haskell> 
</haskell> 

+  [[99 questions/Solutions/95  Solutions]] 

−  This solution does a simple table lookup after converting the positive integer into a string. Thus dividing into digits is much simplified. 

−  A minor variant of the above solution: 

−  <haskell> 

−  import Data.Char 

−  import Data.List 

−  
−  fullWords :: Integer > String 

−  fullWords n = concat $ intersperse "" [digits!!digitToInt d  d < show n] 

−  where digits = ["zero", "one", "two", "three", "four", 

−  "five", "six", "seven", "eight", "nine"] 

−  </haskell> 

== Problem 96 == 
== Problem 96 == 

Line 49:  Line 25:  
In a certain programming language (Ada) identifiers are defined by the syntax diagram below. 
In a certain programming language (Ada) identifiers are defined by the syntax diagram below. 

−  http:// 
+  http://aperiodic.net/phil/scala/s99/p96.gif 
Transform the syntax diagram into a system of syntax diagrams which do not contain loops; i.e. which are purely recursive. Using these modified diagrams, write a predicate identifier/1 that can check whether or not a given string is a legal identifier. 
Transform the syntax diagram into a system of syntax diagrams which do not contain loops; i.e. which are purely recursive. Using these modified diagrams, write a predicate identifier/1 that can check whether or not a given string is a legal identifier. 

Example in Prolog: 
Example in Prolog: 

+  
<pre> 
<pre> 

% identifier(Str) : Str is a legal identifier 
% identifier(Str) : Str is a legal identifier 

Line 59:  Line 36:  
Example in Haskell: 
Example in Haskell: 

+  
−  <pre> 

+  <haskell> 

−  > identifier "thisisalongidentifier" 

+  λ> identifier "thisisalongidentifier" 

True 
True 

−  > identifier "thisendsin" 
+  λ> identifier "thisendsin" 
False 
False 

−  > identifier "twohyphens" 
+  λ> identifier "twohyphens" 
False 
False 

−  </pre> 

−  
−  Solution: 

−  <haskell> 

−  import Data.Char 

−  syntax_check :: String > Bool 

−  syntax_check [] = False 

−  syntax_check (x:xs) = isLetter x && loop xs 

−  where loop [] = True 

−  loop (y:ys)  y == '' = (not . null) ys && isAlphaNum (head ys) && loop (tail ys) 

−   isAlphaNum y = loop ys 

−   otherwise = False 

</haskell> 
</haskell> 

+  [[99 questions/Solutions/96  Solutions]] 

−  Simple functional transcription of the diagram. 

−  Another direct transcription of the diagram: 

−  <haskell> 

−  identifier :: String > Bool 

−  identifier (c:cs) = isLetter c && hyphen cs 

−  where hyphen [] = True 

−  hyphen ('':cs) = alphas cs 

−  hyphen cs = alphas cs 

−  alphas [] = False 

−  alphas (c:cs) = isAlphaNum c && hyphen cs 

−  </haskell> 

−  
−  The functions <tt>hyphen</tt> and <tt>alphas</tt> correspond to states in the automaton at the start of the loop and before a compulsory alphanumeric, respectively. 

−  
−  Here is a solution that parses the identifier using Parsec, a parser library that is commonly used in Haskell code: 

−  <haskell> 

−  identifier x = either (const False) (const True) $ parse parser "" x where 

−  parser = letter >> many (optional (char '') >> alphaNum) 

−  </haskell> 

== Problem 97 == 
== Problem 97 == 

Line 130:  Line 78:  
Every spot in the puzzle belongs to a (horizontal) row and a (vertical) column, as well as to one single 3x3 square (which we call "square" for short). At the beginning, some of the spots carry a singledigit number between 1 and 9. The problem is to fill the missing spots with digits in such a way that every number between 1 and 9 appears exactly once in each row, in each column, and in each square. 
Every spot in the puzzle belongs to a (horizontal) row and a (vertical) column, as well as to one single 3x3 square (which we call "square" for short). At the beginning, some of the spots carry a singledigit number between 1 and 9. The problem is to fill the missing spots with digits in such a way that every number between 1 and 9 appears exactly once in each row, in each column, and in each square. 

−  Solutions 
+  [[99 questions/Solutions/97  Solutions]] 
+  
== Problem 98 == 
== Problem 98 == 

Line 158:  Line 107:  
Example in Haskell: 
Example in Haskell: 

+  
−  <pre> 

+  <haskell> 

−  Nonogram> putStr $ nonogram [[3],[2,1],[3,2],[2,2],[6],[1,5],[6],[1],[2]] [[1,2],[3,1],[1,5],[7,1],[5],[3],[4],[3]] 

+  λ> putStr $ nonogram [[3],[2,1],[3,2],[2,2],[6],[1,5],[6],[1],[2]] [[1,2],[3,1],[1,5],[7,1],[5],[3],[4],[3]] 

_XXX____ 3 
_XXX____ 3 

XX_X____ 2 1 
XX_X____ 2 1 

Line 171:  Line 121:  
1 3 1 7 5 3 4 3 
1 3 1 7 5 3 4 3 

2 1 5 1 
2 1 5 1 

−  </pre> 

−  
−  Solutions: 

−  The first solution is a simple backtracking algorithm, but is quite slow for larger problems. 

−  <haskell> 

−  data Square = Blank  Cross deriving (Eq) 

−  instance Show Square where 

−  show Blank = " " 

−  show Cross = "X" 

−  
−   create all possibilities of arranging the given blocks in a line of "n" elements 

−  rows n [] = [replicate n Blank] 

−  rows n (k:ks)  n < k = [] 

−  rows n (k:ks) = 

−  [Blank : row  row < rows (n1) (k:ks)] ++ 

−  if null ks then [replicate k Cross ++ replicate (nk) Blank] 

−  else [replicate k Cross ++ Blank : row  row < rows (nk1) ks] 

−  
−   contract a given line into the block format 

−   i.e. contract [Cross,Blank,Cross] == [1,1] 

−  contract = map length . filter (\(x:_) > x==Cross) . group 

−  
−   create all solutions by combining all possible rows in all possible ways 

−   then pick a solution and check whether its block signature fits 

−  solver horz vert = filter fitsVert possSolution 

−  where possSolution = sequence $ map (rows (length vert)) horz 

−  fitsVert rs = map contract (transpose rs) == vert 

−  
−   output the (first) solution 

−  nonogram horz vert = printSolution $ head $ solver horz vert 

−  where printSolution = putStr . unlines . map (concatMap show) . transpose 

</haskell> 
</haskell> 

+  [[99 questions/Solutions/98  Solutions]] 

−  This is a solution done for simplicity rather than performance. It's SLOOOOW. 

−  It builds all combinations of blocks in a row (stolen from solution 2 :) and then builds all combinations of rows. The resulting columns are then contracted into the short block block form and the signature compared to the target. 

−  
−  We can make the search much faster (but more obscure) by deducing the values of as many squares as possible before guessing, as in this solution: 

−  <haskell> 

−  module Nonogram where 

−  
−  import Control.Monad 

−  import Data.List 

−  import Data.Maybe 

−  
−  data Square = Filled  Blank  Unknown 

−  deriving (Eq, Show) 

−  type Row = [Square] 

−  type Grid = [Row] 

−  
−   Print the first solution (if any) to the nonogram 

−  nonogram :: [[Int]] > [[Int]] > String 

−  nonogram rs cs = case solve rs cs of 

−  [] > "Inconsistent\n" 

−  (grid:_) > showGrid rs cs grid 

−  
−   All solutions to the nonogram 

−  solve :: [[Int]] > [[Int]] > [Grid] 

−  solve rs cs = [grid'  

−   deduce as many squares as we can 

−  grid < maybeToList (deduction rs cs), 

−   guess the rest, governed by rs 

−  grid' < zipWithM (rowsMatching nc) rs grid, 

−   check each guess against cs 

−  map contract (transpose grid') == cs] 

−  where nc = length cs 

−  contract = map length . filter (\(x:_) > x==Filled) . group 

−  
−   A nonogram with all the values we can deduce 

−  deduction :: [[Int]] > [[Int]] > Maybe Grid 

−  deduction rs cs = converge step init 

−  where nr = length rs 

−  nc = length cs 

−  init = replicate nr (replicate nc Unknown) 

−  step = (improve nc rs . transpose) <.> (improve nr cs . transpose) 

−  improve n = zipWithM (common n) 

−  (g <.> f) x = f x >>= g 

−  
−   repeatedly apply f until a fixed point is reached 

−  converge :: (Monad m, Eq a) => (a > m a) > a > m a 

−  converge f s = do 

−  s' < f s 

−  if s' == s then return s else converge f s' 

−  
−   common n ks partial = commonality between all possible ways of 

−   placing blocks of length ks in a row of length n that match partial. 

−  common :: Int > [Int] > Row > Maybe Row 

−  common n ks partial = case rowsMatching n ks partial of 

−  [] > Nothing 

−  rs > Just (foldr1 (zipWith unify) rs) 

−  
−   rowsMatching n ks partial = all possible ways of placing blocks of 

−   length ks in a row of length n that match partial. 

−  rowsMatching :: Int > [Int] > [Square] > [[Square]] 

−  rowsMatching n [] partial = [replicate n Blank  all (/= Filled) partial] 

−  rowsMatching n ks [] = [] 

−  rowsMatching n ks (Unknown:partial) = 

−  rowsMatching n ks (Filled:partial) ++ 

−  rowsMatching n ks (Blank:partial) 

−  rowsMatching n ks (Blank:partial) = 

−  [Blank : row  row < rowsMatching (n1) ks partial] 

−  rowsMatching n [k] (Filled:partial) = 

−  [replicate k Filled ++ replicate (nk) Blank  

−  n >= k && all (/= Blank) front && all (/= Filled) back] 

−  where (front, back) = splitAt (k1) partial 

−  rowsMatching n (k:ks) (Filled:partial) = 

−  [replicate k Filled ++ Blank : row  

−  n > k+1 && all (/= Blank) front && blank /= Filled, 

−  row < rowsMatching (nk1) ks partial'] 

−  where (front, blank:partial') = splitAt (k1) partial 

−  
−  unify :: Square > Square > Square 

−  unify Filled Filled = Filled 

−  unify Blank Blank = Blank 

−  unify _ _ = Unknown 

−  
−  showGrid :: [[Int]] > [[Int]] > Grid > String 

−  showGrid rs cs ss = unlines (zipWith showRow rs ss ++ showCols cs) 

−  where showRow rs ss = concat [['', name s]  s < ss] ++ " " ++ 

−  unwords (map show rs) 

−  showCols cs 

−   all null cs = [] 

−   otherwise = concatMap showCol cs : showCols (map advance cs) 

−  showCol (k:_) 

−   k < 10 = ' ':show k 

−   otherwise = show k 

−  showCol [] = " " 

−  advance [] = [] 

−  advance (x:xs) = xs 

−  
−  name :: Square > Char 

−  name Filled = 'X' 

−  name Blank = '_' 

−  name Unknown = '?' 

−  </haskell> 

−  We build up knowledge of which squares must be filled and which must be blank, until we can't make any more deductions. 

−  Some puzzles cannot be completely solved in this way, so then we guess values by the same method as the first solution for any remaining squares. 

== Problem 99 == 
== Problem 99 == 

Line 315:  Line 132:  
Given an empty (or almost empty) framework of a crossword puzzle and a set of words. The problem is to place the words into the framework. 
Given an empty (or almost empty) framework of a crossword puzzle and a set of words. The problem is to place the words into the framework. 

−  +  https://prof.ti.bfh.ch/hew1/informatik3/prolog/p99/p99.gif 

−  The particular crossword puzzle is specified in a text file which first lists the words (one word per line) in an arbitrary order. Then, after an empty line, the crossword framework is defined. In this framework specification, an empty character location is represented by a dot (.). In order to make the solution easier, character locations can also contain predefined character values. The puzzle above is defined in the file [ 
+  The particular crossword puzzle is specified in a text file which first lists the words (one word per line) in an arbitrary order. Then, after an empty line, the crossword framework is defined. In this framework specification, an empty character location is represented by a dot (.). In order to make the solution easier, character locations can also contain predefined character values. The puzzle above is defined in the file [https://sites.google.com/site/prologsite/prologproblems/7/solutions7/p7_09a.dat?attredirects=0&d=1 p7_09a.dat], other examples are [https://sites.google.com/site/prologsite/prologproblems/7/solutions7/p7_09b.dat?attredirects=0&d=1 p7_09b.dat] and [https://sites.google.com/site/prologsite/prologproblems/7/solutions7/p7_09d.dat?attredirects=0&d=1 p7_09d.dat]. There is also an example of a puzzle ([https://sites.google.com/site/prologsite/prologproblems/7/solutions7/p7_09c.dat?attredirects=0&d=1 p7_09c.dat]) which does not have a solution. 
Words are strings (character lists) of at least two characters. A horizontal or vertical sequence of character places in the crossword puzzle framework is called a site. Our problem is to find a compatible way of placing words onto sites. 
Words are strings (character lists) of at least two characters. A horizontal or vertical sequence of character places in the crossword puzzle framework is called a site. Our problem is to find a compatible way of placing words onto sites. 

Line 323:  Line 140:  
Hints: (1) The problem is not easy. You will need some time to thoroughly understand it. So, don't give up too early! And remember that the objective is a clean solution, not just a quickanddirty hack! 
Hints: (1) The problem is not easy. You will need some time to thoroughly understand it. So, don't give up too early! And remember that the objective is a clean solution, not just a quickanddirty hack! 

−  (2) Reading the data file is a tricky problem for which a solution is provided in the file [ 
+  (2) Reading the data file is a tricky problem for which a solution is provided in the file [https://sites.google.com/site/prologsite/prologproblems/7/solutions7/p7_09readfile.pl?attredirects=0&d=1 p7_09readfile.pl]. See the predicate read_lines/2. 
(3) For efficiency reasons it is important, at least for larger puzzles, to sort the words and the sites in a particular order. For this part of the problem, the solution of P28 may be very helpful. 
(3) For efficiency reasons it is important, at least for larger puzzles, to sort the words and the sites in a particular order. For this part of the problem, the solution of P28 may be very helpful. 

Example in Haskell: 
Example in Haskell: 

+  
<pre> 
<pre> 

ALPHA 
ALPHA 

Line 339:  Line 157:  
. . 
. . 

. 
. 

+  </pre> 

+  <haskell> 

−  > solve $ readCrossword "ALPHA\nARES\nPOPPY\n\n . \n . \n.....\n . .\n . .\n .\n" 

+  λ> solve $ readCrossword "ALPHA\nARES\nPOPPY\n\n . \n . \n.....\n . .\n . .\n .\n" 

[[((3,1),'A'),((3,2),'L'),((3,3),'P'),((3,4),'H'),((3,5),'A'),((1,3),'P'),((2,3) 
[[((3,1),'A'),((3,2),'L'),((3,3),'P'),((3,4),'H'),((3,5),'A'),((1,3),'P'),((2,3) 

,'O'),((3,3),'P'),((4,3),'P'),((5,3),'Y'),((3,5),'A'),((4,5),'R'),((5,5),'E'),(( 
,'O'),((3,3),'P'),((4,3),'P'),((5,3),'Y'),((3,5),'A'),((4,5),'R'),((5,5),'E'),(( 

6,5),'S')]] 
6,5),'S')]] 

−  </pre> 

−  
−  Solution: 

−  <haskell> 

−   import Control.Monad 

−   import Data.List 

−  
−  type Coord = (Int,Int) 

−  type Word = String 

−  data Site = Site {siteCoords :: [Coord], siteLen :: Int} deriving (Show,Eq) 

−  data Crossword = Crossword {cwWords :: [Word], cwSites :: [Site]} deriving (Show,Eq) 

−  
−  comparing f = \a b > f a `compare` f b 

−  equaling f = \a b > f a == f b 

−  
−   convert the text lines from the file to the "Site" datatype, 

−   which contain the adjacent coordinates of the site and its length 

−  toSites :: [String] > [Site] 

−  toSites lines = find (index_it lines) ++ find (transpose . index_it $ lines) 

−  where find = map makePos . concat . map extractor 

−  extractor = filter ((>1) . length) . map (filter (\(_,x) > x=='.')) . groupBy (equaling snd) 

−  index_it = map (\(row,e) > zip [(col,row)  col < [1..]] e) . zip [1..] 

−  makePos xs = Site {siteCoords = map fst xs, siteLen = length xs} 

−  
−   test whether there exist no two different letters at the same coordinate 

−  noCollision :: [(String, Site)] > Bool 

−  noCollision xs = all allEqual groupedByCoord 

−  where groupedByCoord = map (map snd) . groupBy (equaling fst) . sortBy (comparing fst) . concatMap together $ xs 

−  allEqual [] = True 

−  allEqual (x:xs) = all (x==) xs 

−  
−   merge a word and a site by assigning each letter to its respective coordinate 

−  together :: (Word, Site) > [(Coord, Char)] 

−  together (w,s) = zip (siteCoords s) w 

−  
−   returns all solutions for the crossword as lists of occupied coordinates and their respective letters 

−  solve :: Crossword > [[(Coord, Char)]] 

−  solve cw = map (concatMap together) solution 

−  where solution = solve' (cwWords cw) (cwSites cw) 

−  
−  solve' :: [Word] > [Site] > [[(Word, Site)]] 

−  solve' _ [] = [[]] 

−  solve' words (s:ss) = if null possWords 

−  then error ("too few words of length " ++ show (siteLen s)) 

−  else do try < possWords 

−  let restWords = Data.List.delete try words 

−  more < solve' restWords ss 

−  let attempt = (try,s):more 

−  Control.Monad.guard $ noCollision attempt 

−  return attempt 

−  where possWords = filter (\w > siteLen s == length w) words 

−  
−   read the content of a file into the "Crossword" datatype 

−  readCrossword :: String > Crossword 

−  readCrossword = (\(ws,ss) > Crossword ws (toSites (drop 1 ss))) . break (""==) . lines 

</haskell> 
</haskell> 

+  [[99 questions/Solutions/99  Solutions]] 

−  This is a simplistic solution with no consideration for speed. Especially sites and words aren't ordered as propesed in (3) of the problem. Words of the correct length are naively tried for all blanks (without heuristics) and the possible solutions are then backtracked. 

−  
−  To test for collisions, all (Word, Site) pairs are merged to result in a list of (Coord, Char) elements which represent all letters placed so far. If all (two) characters of the same coordinate are identical, there exist no collisions between words. 

[[Category:Tutorials]] 
[[Category:Tutorials]] 
Latest revision as of 02:36, 25 November 2022
This is part of NinetyNine Haskell Problems, based on NinetyNine Prolog Problems.
Problem 95
(**) English number words
On financial documents, like cheques, numbers must sometimes be written in full words. Example: 175 must be written as onesevenfive. Write a predicate fullwords/1 to print (nonnegative) integer numbers in full words.
Example in Haskell:
λ> fullWords 175
onesevenfive
Problem 96
(**) Syntax checker
In a certain programming language (Ada) identifiers are defined by the syntax diagram below.
Transform the syntax diagram into a system of syntax diagrams which do not contain loops; i.e. which are purely recursive. Using these modified diagrams, write a predicate identifier/1 that can check whether or not a given string is a legal identifier.
Example in Prolog:
% identifier(Str) : Str is a legal identifier
Example in Haskell:
λ> identifier "thisisalongidentifier"
True
λ> identifier "thisendsin"
False
λ> identifier "twohyphens"
False
Problem 97
(**) Sudoku
Sudoku puzzles go like this:
Problem statement Solution . . 4  8 . .  . 1 7 9 3 4  8 2 5  6 1 7     6 7 .  9 . .  . . . 6 7 2  9 1 4  8 5 3     5 . 8  . 3 .  . . 4 5 1 8  6 3 7  9 2 4 ++ ++ 3 . .  7 4 .  1 . . 3 2 5  7 4 8  1 6 9     . 6 9  . . .  7 8 . 4 6 9  1 5 3  7 8 2     . . 1  . 6 9  . . 5 7 8 1  2 6 9  4 3 5 ++ ++ 1 . .  . 8 .  3 . 6 1 9 7  5 8 2  3 4 6     . . .  . . 6  . 9 1 8 5 3  4 7 6  2 9 1     2 4 .  . . 1  5 . . 2 4 6  3 9 1  5 7 8
Every spot in the puzzle belongs to a (horizontal) row and a (vertical) column, as well as to one single 3x3 square (which we call "square" for short). At the beginning, some of the spots carry a singledigit number between 1 and 9. The problem is to fill the missing spots with digits in such a way that every number between 1 and 9 appears exactly once in each row, in each column, and in each square.
Problem 98
(***) Nonograms
Around 1994, a certain kind of puzzle was very popular in England. The "Sunday Telegraph" newspaper wrote: "Nonograms are puzzles from Japan and are currently published each week only in The Sunday Telegraph. Simply use your logic and skill to complete the grid and reveal a picture or diagram." As a Prolog programmer, you are in a better situation: you can have your computer do the work! Just write a little program ;).
The puzzle goes like this: Essentially, each row and column of a rectangular bitmap is annotated with the respective lengths of its distinct strings of occupied cells. The person who solves the puzzle must complete the bitmap given only these lengths.
Problem statement: Solution:
________ 3 _XXX____ 3 ________ 2 1 XX_X____ 2 1 ________ 3 2 _XXX__XX 3 2 ________ 2 2 __XX__XX 2 2 ________ 6 __XXXXXX 6 ________ 1 5 X_XXXXX_ 1 5 ________ 6 XXXXXX__ 6 ________ 1 ____X___ 1 ________ 2 ___XX___ 2 1 3 1 7 5 3 4 3 1 3 1 7 5 3 4 3 2 1 5 1 2 1 5 1
For the example above, the problem can be stated as the two lists [[3],[2,1],[3,2],[2,2],[6],[1,5],[6],[1],[2]] and [[1,2],[3,1],[1,5],[7,1],[5],[3],[4],[3]] which give the "solid" lengths of the rows and columns, toptobottom and lefttoright, respectively. Published puzzles are larger than this example, e.g. 25 x 20, and apparently always have unique solutions.
Example in Haskell:
λ> putStr $ nonogram [[3],[2,1],[3,2],[2,2],[6],[1,5],[6],[1],[2]] [[1,2],[3,1],[1,5],[7,1],[5],[3],[4],[3]]
_XXX____ 3
XX_X____ 2 1
_XXX__XX 3 2
__XX__XX 2 2
__XXXXXX 6
X_XXXXX_ 1 5
XXXXXX__ 6
____X___ 1
___XX___ 2
1 3 1 7 5 3 4 3
2 1 5 1
Problem 99
(***) Crossword puzzle
Given an empty (or almost empty) framework of a crossword puzzle and a set of words. The problem is to place the words into the framework.
The particular crossword puzzle is specified in a text file which first lists the words (one word per line) in an arbitrary order. Then, after an empty line, the crossword framework is defined. In this framework specification, an empty character location is represented by a dot (.). In order to make the solution easier, character locations can also contain predefined character values. The puzzle above is defined in the file p7_09a.dat, other examples are p7_09b.dat and p7_09d.dat. There is also an example of a puzzle (p7_09c.dat) which does not have a solution.
Words are strings (character lists) of at least two characters. A horizontal or vertical sequence of character places in the crossword puzzle framework is called a site. Our problem is to find a compatible way of placing words onto sites.
Hints: (1) The problem is not easy. You will need some time to thoroughly understand it. So, don't give up too early! And remember that the objective is a clean solution, not just a quickanddirty hack!
(2) Reading the data file is a tricky problem for which a solution is provided in the file p7_09readfile.pl. See the predicate read_lines/2.
(3) For efficiency reasons it is important, at least for larger puzzles, to sort the words and the sites in a particular order. For this part of the problem, the solution of P28 may be very helpful.
Example in Haskell:
ALPHA ARES POPPY . . ..... . . . . .
λ> solve $ readCrossword "ALPHA\nARES\nPOPPY\n\n . \n . \n.....\n . .\n . .\n .\n"
[[((3,1),'A'),((3,2),'L'),((3,3),'P'),((3,4),'H'),((3,5),'A'),((1,3),'P'),((2,3)
,'O'),((3,3),'P'),((4,3),'P'),((5,3),'Y'),((3,5),'A'),((4,5),'R'),((5,5),'E'),((
6,5),'S')]]