In the following code, line 4, I have an expression sandwiched between two IO actions in a do block:
1 doubleX :: (Show x, Num x) => x -> IO ()
2 doubleX x = do
3 putStrLn ("I will now double " ++ (show x))
4 let double = x * 2
5 putStrLn ("The result is " ++ (show double))
I understand do notation as chaining monadic operations together using >>= or >>. But how does that work when you have an expression in between? You couldn't just glue lines 3-5 together using >>.
I'm going to crib from my very similar answer here (though probably not a duplicate since that question doesn't explicitly deal with
let
).The Report gives a full translation from do syntax into kernel Haskell; the parts relevant to your question are:
So your code desugars like this: