January 2018 1Liner 1HaskellADay problems and solutions

• January 8th, 2018: from Nicoλas‏ @BeRewt
  A small @1HaskellADay, old-school. Define foo:
  
  > foo 3 [1..5]
  [([1,2,3], 4), ([2,3,4], 5)]
  
  > foo 2 [1..4]
  [([1,2], 3), ([2,3], 4)]
  
  > foo 2 [1..20]
  [([1,2],3), ([2,3],4), ..., ([18,19],20)]
  
  > foo 20 [1..2]
  []
• Demiurge With a Teletype @mrkgrnao
  foo n
    = tails
    # filter (length # (> n))
    # map (splitAt n # second head)
  
  (#) = flip (.)
• Andreas Källberg @Anka213
  I haven't tested it, but this should work:
  foo n xs = [ (hd,x) | (hd , x:_) <- n="" splitat=""> tails xs ]
• <- n="" splitat="">Nicoλas @BeRewt foo n = zip <$> fmap (take n) . tails <*> drop n
• January 5th, 2018: You have the following DAG-paths:
  
  a -> b -> c -> e
  a -> b -> d -> e
  q -> r -> s
  w -> x
  y -> z
  
  and many more.
  
  From a path, provide a bi-directional encoding* given maximum graph depth is, say, 7, max number of roots is, say, 10, and max number of nodes is, say, 1000.
• *bi-directional encoding of a graph path:
  
  DAG path -> enc is unique for an unique DAG path
  enc -> DAG path yields the same DAG path that created the unique enc.
  
  *DAG: "Directed, acyclic graph."
• January 5th, 2018: given s :: Ord k => a -> (k,[v])
  
  define f using s
  
  f :: Ord k => [a] -> Map k [v]
  
  with no duplicate k in [a]
• Christian Bay @the_greenbourne f = foldr (\e acc -> uncurry M.insert (s e) acc) M.empty
• me: you can curry away the acc variable easily
• Christian Bay @the_greenbourne You're right :)
  f = foldr (uncurry M.insert . s) M.empty
• Bazzargh @bazzargh fromList.(map s) ?
• me: Yuppers