StackOverflow2013

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plurals

PO
primarykey
Id
17249626
data
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0
AnswerCount
0
ClosedDate
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CommunityOwnedDate
CreationDate
2013-06-22T10:20:57.717
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2013-06-22T14:42:26.523
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2013-06-22T14:42:26.523
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849891
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17240248
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2
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text
Body
<pre><code>go m n (i,j) = [ (i+1,j) | i<m ] ++ [ (i-1,j) | i>1 ] ++ [ (i,j+1) | j<n ] ++ [ (i,j-1) | j>1 ] genPath p q acc m n input buf = g p q acc buf -- return all solutions where g p q acc buf | p==q = [(acc,buf)] -- return acc, buf g p q acc buf = [s | r <- go m n q, notElem r buf, notElem r acc, notElem r input, s <- g p r (r:acc) (r:buf)] ++ [s | r <- go m n q, notElem r acc, r==p, s <- g p r (r:acc) (r:buf)] </code></pre> your new code: <pre><code>genAllPaths([],A,A,_,_,_,_). genAllPaths([(I1,J1),(I2,J2)|T],Acc,S,M,N,Input,Bufor) :- genPath((I1,J1),(I2,J2),[(I2,J2)],X,M,N,Input,[(I2,J2)|Bufor],NewBufor), genAllPaths(T,[X|Acc],S,M,N,Input,NewBufor). </code></pre> The direct textual translation into Haskell is: <pre><code>genAllPaths points acc m n input buf = g points acc buf where g [] acc _ = [acc] g (p:q:t) acc buf = let sols = genPath p q [q] m n input (q:buf) -- => [(x,newbuf)] in concat [g t (x:acc) newbuf | (x,newbuf) <- sols] </code></pre> Another way to write it is <pre><code>genAllPaths points acc m n input buf = g points acc buf where g [] acc _ = [acc] g (p:q:t) acc buf = genPath p q [q] m n input (q:buf) >>= (\ (x,newbuf) -> g t (x:acc) newbuf ) </code></pre> This uses the bind operator <code>>>=</code> from the list monad. There's also the <code>do</code> notation, <pre><code>genAllPaths points acc m n input buf = g points acc buf where g [] acc _ = return acc -- same as writing `[acc]`, for list monad g (p:q:t) acc buf = do (x,newbuf) <- genPath p q [q] m n input (q:buf) g t (x:acc) newbuf </code></pre> which expresses the same computation without the explicit use of bind. List monad expresses non-deterministic computations by representing all possible choices as a list. True representation would use unordered lists with unpredictable order; normal Haskell lists induce order, but Prolog does the same with its left-to-right top-to-bottom strategy. Since Haskell is lazy, producing the resulting solutions one by one is equivalent to the backtracking search of Prolog, and <code>take 1</code> can be used to emulate <code>cut</code>.
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1. This table or related slice is empty.
PostParentId
1. POFind all paths between point on board
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PostTypePostTypeId
1. PTAnswer
UserLastEditorUserId
1. USWill Ness
UserOwnerUserId
1. USWill Ness
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