Printing with mapM_ in Haskell

mapM_ runs an action per element and drops the results; forM_ is the same with flipped arguments.

import Control.Monad (forM_)

stations :: [(String, Int)]
stations = [("north", 4), ("central", 11), ("south", 7)]

-- Pad on the right so the columns line up.
pad :: Int -> String -> String
pad n s = s ++ replicate (n - length s) ' '

main :: IO ()
main = do
  -- mapM_ runs an action per element and discards the results.
  mapM_ print stations
  mapM_ (putStrLn . fst) stations
  -- forM_ is mapM_ with the arguments flipped, so the body comes last.
  forM_ stations $ \(name, bikes) ->
    putStrLn (pad 9 name ++ show bikes ++ " bikes free")

How it works

1. mapM_ print stations prints each pair, while mapM_ (putStrLn . fst) stations prints only the names.
2. pad lines the columns up by appending replicate (n - length s) ' '.
3. forM_ stations takes the list first, which lets the lambda body come last in the block.

Keywords and builtins used here

• IO
• Int
• String
• do
• forM_
• import
• main
• pad
• stations

The run, in numbers

Lines

17

Characters to type

532

Tokens

105

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 90 seconds.

Type this snippet

Step 3 of 3 in IO basics, step 24 of 27 in Language basics.

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