Question

I’m new to Haskell and need some help on this situation. I have the following list

-- create a type for bank account
type AcNo = String
type Name = String
type City = String
type Amnt = Int

type AcInfo = [(AcNo, Name, City, Amnt)]

-- function to get the data of bank accounts to a list of tuples
bankAccounts :: AcInfo
bankAccounts = [("oo1", "Sahan", "Colomb", 100),("002", "John", "Jafna", 200)]

My requirement is to get the amount corresponding to the account number, e.g., for 001 it should give 100.

The function I wrote was this

--Function to check the balance of a person
checkBalance :: bankAccounts -> AcNo -> Amnt
checkBalance dbase number = Amnt|(AcNo, Name, City, Amnt) <- dbase, AcNo==number}

The second line is where im stuck at which gives the error message

Syntax error in input (unexpected `|')

I’d like to have some help on this. Thanx.

Answer 1

Recall that names of Haskell types begin with capital letters, so the type of checkBalance should be

checkBalance :: AcInfo -> AcNo -> Amnt

In your question, you seem to aim at using a list comprehension, but you don’t have the syntax quite right.

checkBalance dbase number = head [amnt | (acNo, name, city, amnt) <- dbase,
                                         acNo == number]

This definition is fine if an account is in dbase

*Main> checkBalance bankAccounts "oo1"
100

but blows up when it isn’t.

*Main> checkBalance bankAccounts "001"
*** Exception: Prelude.head: empty list

A better type for checkBalance is

checkBalance :: AcInfo -> AcNo -> Maybe Amnt

to represent the general case, i.e., dbase may or may not contain number.