Haskell 'else' parsing error

Question

I'm having an parsing error on the input 'else' but I have no idea how to solve it. I'm still new to this haskell thing.

depot = 
do
putStr ("Enter deposit amount: ")
deAmount <- getLine
let redeAmount = read deAmount
if redeAmount > 0 then 
    let accBal = redeAmount + accBal
    else 
        putStrLn "Please enter amount greater than 0"

Answer 1

The first mistake is that the do (and everything after it) needs to be indented.

The next mistake is that syntactically let accBal = redeAmount + accBal can't stand on its own like that. Either it needs to be followed by an in or it must be directly inside a do block (being inside an if which is inside a do block does not count).

However even if you fix the syntax, it won't do what you want. You can't use let to reassign variables. As a matter of fact you can't reassign variables at all. If you fix the syntax of your let , it will simply create a new variable accBal which will shadow the old variable accBal . The scope of that new variable will be the then -block of the if . Once you leave the if -block, you'll accBal will again refer to the old value of accBal no matter what you did inside the if .

So to fix your issue, you need to restructure your logic, so it doesn't depend on accBal being reassigned.

Answer 2

I don't have much to add onto sepp2k's answer, but I thought I'd show you how I might write it:

type Balance          = Float               -- The type of the money balance
type TransactionT m a = StateT Balance m a  -- Something that modifies a balance
type Transaction a    = TransactionT IO a   -- Something that interacts with IO
                                            --  *and* modifies balance

-- Request the user to enter a deposit
getDeposit :: Transaction ()
getDeposit = do
 putStr "Enter deposit amount: "  -- You don't need () around the argument

 amountStr <- liftIO getLine  -- This is a consequence of using monad
                              -- transformers

 let amount = read amountStr

 if amount > 0
   then modify (+ amount) -- Update the balance
   else liftIO $ error "Please enter amount greater than 0." -- Raise an
                                                             --  exception

Here's a great little tutorial: http://learnyouahaskell.com/ .

Here are some sections relevant to the code I wrote:

• Let expressions
• Case expressions
• IO stuff
• Functors
• Monads
• More Monads
• Even More Monads

Oh, and to make your life easier, Hoogle .