How to use variable from do block assignment line in a where clause?

Question

I have the following sample of code:

{-# LANGUAGE ScopedTypeVariables #-}

main = do
  putStrLn \"Please input a number a: \"
  a :: Int  <- readLn
  prin         


        

Answer 1

Use let instead of where:

{-# LANGUAGE ScopedTypeVariables #-}

main = do
  putStrLn "Please input a number a: "
  a :: Int  <- readLn
  print a

  putStrLn "Please input a number b: "
  b :: Int  <- readLn
  print b

  let c = b^2
  putStrLn ("a+b+b^2:" ++ (show $ a+b+c))

The reason for the problem is that variables in the where clause are in scope for all of main, but b isn't in scope until after b :: Int <- readLn. In general, where clauses can't reference variables bound inside of a do block (or anywhere to the right of the =, for that matter: e.g., f x = y*2 where y = x+1 is fine but f = \x -> y*2 where y = x+1 is not).