在HLint的上下文中，eta意味着什么？

import System.Environment

main :: IO ()
main = getArgs >>= print . haqify . head

haqify s = \"Haq! \" ++ s

./Haq.hs:11:1: Warning: Eta reduce
Found:
  haqify s = \"Haq! \" ++ s
Why not:
  haqify = (\"Haq! \" ++ )

f

x

\\x -> f x

f

y

(\\x -> f x) y === f\' y -- (where f\' is obtained from f by substituting all x\'s by y)
              === f y  -- since f has no free occurrences of x

haqify

\\s -> \"Haq! \" ++ s

\\s -> (++) \"Haq! \"  s

(++) \"Haq! \"

f :: Int -> Int
g :: Int -> Int -> Int
f s = g 3 s 

f = g 3

(++)

[a] -> [a] -> [a]

 haqify ::  [Char] -> [Char]
 haqify = (++) \"Haq! \"

haqify = (\"Haq! \" ++ )

 \\x -> M x == M      -- if x is not free in M.

\\x -> abs x

abs