'Cannot justify constraints in explicitly typed binding ', Hugs complained. - 10 views

• 
  #1 Xiaobin Huang on 14 Nov 08

   

  hi all,
  
  I'm new to Haskell.
  
  I wrote a permutation prog.
  
  permutation [] = [[]]
  permutation seq = foldr (++) [] (map appendEachTo seq)
  where appendEachTo i = (map (i:) (permutation (filter (i/=) seq)))
  It works.
  
  but I don't know how to give the type of 'permutation'.
  
  I wrote 'permutation::[a]->[[a]]', Hugs complain that
  
  *** Expression : permutation
  *** Type : [a] -> [[a]]
  *** Given context : ()
  *** Constraints : Eq a
  
  I think I must miss something when reading the tutorial.
  
  any help will be appreciated.

   

  <div class="cArrow"> </div><div class="cContentInner">hi all, <br /> <br /> I'm new to Haskell. <br /> <br /> I wrote a permutation prog. <br /> <br /> permutation [] = [[]] <br /> permutation seq = foldr (++) [] (map appendEachTo seq) <br /> where appendEachTo i = (map (i:) (permutation (filter (i/=) seq))) <br /> It works. <br /> <br /> but I don't know how to give the type of 'permutation'. <br /> <br /> I wrote 'permutation::[a]->[[a]]', Hugs complain that <br /> <br /> *** Expression : permutation <br /> *** Type : [a] -> [[a]] <br /> *** Given context : () <br /> *** Constraints : Eq a <br /> <br /> I think I must miss something when reading the tutorial. <br /> <br /> any help will be appreciated.</div>

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• 
  #2 Xiaobin Huang on 16 Nov 08

   

  Now I know, it should be 'permutation :: Eq a => [a]->[[a]]'.
  
  Xiaobin Huang wrote:
  > hi all,
  >
  > I'm new to Haskell.
  >
  > I wrote a permutation prog.
  >
  > permutation [] = [[]]
  > permutation seq = foldr (++) [] (map appendEachTo seq)
  > where appendEachTo i = (map (i:) (permutation (filter (i/=) seq)))
  > It works.
  >
  > but I don't know how to give the type of 'permutation'.
  >
  > I wrote 'permutation::[a]- >[[a]]', Hugs complain that
  >
  > *** Expression : permutation
  > *** Type : [a] - > [[a]]
  > *** Given context : ()
  > *** Constraints : Eq a
  >
  > I think I must miss something when reading the tutorial.
  >
  > any help will be appreciated.

   

  <div class="cArrow"> </div><div class="cContentInner">Now I know, it should be 'permutation :: Eq a => [a]->[[a]]'. <br /> <br /> Xiaobin Huang wrote: <br /> > hi all, <br /> > <br /> > I'm new to Haskell. <br /> > <br /> > I wrote a permutation prog. <br /> > <br /> > permutation [] = [[]] <br /> > permutation seq = foldr (++) [] (map appendEachTo seq) <br /> > where appendEachTo i = (map (i:) (permutation (filter (i/=) seq))) <br /> > It works. <br /> > <br /> > but I don't know how to give the type of 'permutation'. <br /> > <br /> > I wrote 'permutation::[a]- >[[a]]', Hugs complain that <br /> > <br /> > *** Expression : permutation <br /> > *** Type : [a] - > [[a]] <br /> > *** Given context : () <br /> > *** Constraints : Eq a <br /> > <br /> > I think I must miss something when reading the tutorial. <br /> > <br /> > any help will be appreciated.</div>

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