Let block gives indentation error - haskell

Question

I know what an indentation error is, but I have no idea why I'm getting this error here, while every is aligned, trying to solve it for 2 hours.
Account.hs:40:25: error:
parse error (possibly incorrect indentation or mismatched brackets)
|
40 | let amount = readLn :: IO Int
| ^
Failed, 0 modules loaded.
main = do
putStrLn $ "Press one to create a new account"
let g = getLine
enteredValue = read g :: Int
if g == 1
then do putStrLn $ "Enter your name "
let name = getLine
putStrLn $ "Enter the initial amount"
let amount = readLn :: IO Int
value = Account (name,1,amount) Saving
show value
else do putStrLn $ "Nothing"
I also tried this version but this also gives incorrect indentation or mismatched brackets:
main = do
putStrLn $ "Press one to create a new account"
let g = getLine
enteredValue = read g :: Int
if g == 1
then do putStrLn $ "Enter your name "
let name = getLine
putStrLn $ "Enter the initial amount"
amount = readLn :: IO Int
value = Account (name,1,amount) Saving
show value
else do putStrLn $ "Nothing"

Answer 1

The problem is here:
-- |<---- "column 0" of this 'do' block
then do putStrLn $ "Enter your name "
-- | still good; a 'let' statement:
let name = getLine
-- |<---- "column 0" of this 'let' block
putStrLn $ "Enter the initial amount"
-- | Huh, there's no '=' in ^this^ declaration?
let amount = readLn :: IO Int
-- ^^^ Why is there a 'let' within another let binding?
-- I still haven't seen a '='. Better throw a parse error.
Basically, putStrLn $ "Enter the initial amount" is aligned with name = ... in the preceding line, so the compiler reads it as a declaration (part of the same let block).
To fix your indentation errors, it should be:
main = do
putStrLn $ "Press one to create a new account"
let g = getLine
enteredValue = read g :: Int
if g == 1
then do putStrLn $ "Enter your name "
let name = getLine
putStrLn $ "Enter the initial amount"
let amount = readLn :: IO Int
value = Account (name,1,amount) Saving
show value
else do putStrLn $ "Nothing"
But then you'll run into type errors:
read g is wrong: read takes a String, but g :: IO String
g == 1 is wrong: 1 is an Int, but g :: IO String
show value is wrong: show returns a String, but you're using it as an IO action
You haven't shown the declaration of Account, but you're likely going to have issues with name and amount, too
You probably want something like:
main = do
putStrLn $ "Press one to create a new account"
g <- getLine
let enteredValue = read g :: Int
if enteredValue == 1
then do putStrLn $ "Enter your name "
name <- getLine
putStrLn $ "Enter the initial amount"
amount <- readLn :: IO Int
let value = Account (name,1,amount) Saving
putStrLn (show value)
else do putStrLn $ "Nothing"
Basically, use v <- expr to go from expr :: IO Something to v :: Something.
Other notes:
g <- getLine; let enteredValue = read g :: Int better written as enteredValue <- readLn :: IO Int
putStrLn (show value) can be shortened to print value
you don't need do for a single expression (nor $ for a single operand): ... else putStrLn "Nothing"

Answer 2

There is more wrong to your code than just the Indentation Errors - so my first suggestion would be reading a bit of learn you a haskell for great good.
Next there are two assignment operators in haskell - one binds the result of an action … <- … and the other one is a local definition/declaration of a pure computation let … = ….
Moreover you can improve your reading a value by taking account of the possible false input, that someone could give you (intentionally and unintentionally) by replacing read with readMaybe, where the latter returns a Maybe something, for example readMaybe "1" = Just 1 :: Maybe Int or readMaybe "foo" = Nothing :: Maybe Int.
Regarding your indentation it is best that you compare one solution to your program with yours own:
import Text.Read (readMaybe)
data Type = Saving | Checking
deriving (Show)
data Account = Account (String,Int,Int) Type
deriving (Show)
main :: IO ()
main = do
putStrLn "Press one to create a new account"
g <- getLine
let enteredValue = readMaybe g :: Maybe Int
here the result of getLine and entered value have the same scope so they have the same indentation - we only change the scope after the next if where the then-block - and the else-block do not share the 'declarations' of each branch, so you couldn't use name in the else-block, but enteredValue can be used in both.
if enteredValue == Just 1
then do putStrLn "Enter your name "
name <- getLine
putStrLn "Enter the initial amount"
amount' <- fmap readMaybe getLine
here again name and amount' share the same scope, and pattern matching on amount' creates a new scope where amount is visible and the match on Nothing where you cannot use this variable.
case amount' of
Just amount -> print $ Account (name,1,amount) Saving
Nothing -> putStrLn "Nothing"
else putStrLn "Nothing"

Answer 3

let is for binding values, which is done in the form let x = y+z, where x is the name (aka "identifier") being bound, and y+z is the expression to which it is being bound.
In your example, I see three bindings: name, amount, and value. The rest are not value bindings, but actions.
In the do notation, actions do not need a let. You just write them one after another. So:
let name = getLine
putStrLn $ "Enter the initial amount"
let amount = readLn :: IO Int
let value = Account (name,1,amount) Saving
show value
But wait! This is not all!
getLine is not actually an expression of type String, as you seem to be hoping here. Rather, getLine is an action. In order to get it to "run" and "produce" a String value, you need to use the <- construct instead of let:
name <- getLine
Similarly with readLn:
amount <- readLn :: IO Int
Finally, show value is not actually an action that would print the value to the screen. show is a function that takes a value and return a String. It doesn't "do" anything (i.e. doesn't produce any outside effects), so you can't use it in place of an action in the do notation. If you wanted an action that would print a value to the screen, that would be print:
print value
Gathering everything together:
name <- getLine
putStrLn $ "Enter the initial amount"
amount <- readLn :: IO Int
let value = Account (name,1,amount) Saving
print value
And after fixing all of that, you'll have similar difficulties with the first part of your program, where you have let g = getLine instead of g <- getLine.