Haskell，打印出fmap结果

Question

I would like to print result of fmap like this

 putStrLn <- fmap (replicate 3) [1,2,3,4]

But I got putStrLn parse error. Please help.

Thank you.

Answer 1

The correct syntax here is:

putStrLn $ show $ fmap (replicate 3) [1,2,3,4]

or, even shorter:

print $ fmap (replicate 3) [1,2,3,4]

Answer 2

Totally not the answer the asker is looking for, but humorously his syntax misunderstanding is still valid Haskell code:

do putStrLn <- fmap (replicate 3) [1,2,3,4]
   return putStrLn

The above uses the list monad and the value putStrLn is just a variable (not the IO action one might expect). It might make more sense to people if expressed as a list comprehension instead of using the list monad:

> [ putStrLn | putStrLn <- map (replicate 3) [1,2,3,4]]
[[1,1,1],[2,2,2],[3,3,3],[4,4,4]]