let versus bind in IO in Haskell

Inside do, let names a pure value while <- runs an action and binds its result.

-- An IO action that produces a value rather than printing one.
readTank :: IO Double
readTank = pure 61.5

main :: IO ()
main = do
  -- let names a pure value; nothing is executed.
  let capacity = 80.0 :: Double
  -- <- runs the action and binds the result it yields.
  level <- readTank
  let percent = 100 * level / capacity
  -- show turns a number into the String putStrLn needs.
  putStrLn ("tank holds " ++ show level ++ " of " ++ show capacity)
  putStrLn ("that is " ++ show percent ++ " percent full")

How it works

1. let capacity = 80.0 :: Double binds a number without performing any IO at all.
2. level <- readTank executes the action and binds the Double it yields.
3. show turns each number into the String that putStrLn demands.

Keywords and builtins used here

• Double
• IO
• do
• let
• main
• readTank

The run, in numbers

Lines

14

Characters to type

496

Tokens

65

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 52 seconds.

Type this snippet

Step 2 of 3 in IO basics, step 23 of 27 in Language basics.

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