lazynodet.hs

perm :: MonadPlus m => [a] -> m [a]
perm [] = return []
perm (x:xs) = do ys <- perm xs
                 zs <- insert x ys
                 return zs

insert :: MonadPlus m =>  a -> [a] -> m [a]
insert x xs = (return (x:xs))
              `mplus` case xs of 
                        []     -> mzero
                        (y:ys) -> liftM (y:) (insert  x ys)
 *Main> perm [1..10]

<interactive>:1:0:
    Ambiguous type variable `m' in the constraint:
      `MonadPlus m' arising from a use of `perm' at <interactive>:1:0-11
    Probable fix: add a type signature that fixes these type variable(s)