CODEWITHSUNDEEP
parsinghaskellif-statement
Ray
Ray

Reputation: 31

Haskell 'else' parsing error

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"

Upvotes: 3

Views: 179

Answers (2)

rovaughn
rovaughn

Reputation: 1223

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:

Oh, and to make your life easier, Hoogle.

Upvotes: 1

sepp2k
sepp2k

Reputation: 370102

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.

Upvotes: 11

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