Exported Identifiers

Go has no public or private keyword. Visibility is spelling: a name whose first letter is upper case is exported and visible to every package that imports yours, and a name that starts lower case is visible only inside the package that declares it.

func Title(s string) string   // callers can use this
func isWordChar(r rune) bool  // only this package can

The rule applies to everything with a name - functions, types, methods, struct fields, constants, variables - and it is checked at compile time, not by convention. bank.normalizeOwner is not a lint warning, it is a build error.

Why a field would ever be hidden

An exported field is a standing promise that anybody may set it to anything, at any time, without going through your code:

account.Balance = 1_000_000   // if Balance were exported, this is legal

Once the field is lower case, the only way in is the method set you wrote, so "a withdrawal larger than the balance is refused" is a rule the type can actually keep. Exporting a getter costs one line and gives up nothing:

func (a *Account) Balance() int { return a.balance }

The reverse is just as real. Anything you export is API: someone will depend on it, and changing it later breaks them. Export the smallest set that lets a caller do the job.

Sentinel errors are exported on purpose

ErrInsufficientFunds is a package-level var created with errors.New, and it is exported precisely so callers can recognise it:

if errors.Is(err, bank.ErrInsufficientFunds) { ... }

Comparing err.Error() against a string would work today and break the moment somebody rewords the message. The value is the contract; the text is not.

Two test files, two packages

bank/bank_test.go says package bank, so it is inside and can call normalizeOwner and read a.balance directly. main_test.go says package main: it goes through the exported surface like any other caller, and it uses reflect to assert that Account has no exported field - the visibility rule, checked mechanically rather than trusted.

Task

Finish the bank package - the hidden balance field, the constructor, the getters, Deposit, Withdraw, Transfer and normalizeOwner - then write FormatCents, Report and Describe in main against nothing but its exported names.

Hints

• Account needs a second field, balance int, and it stays lower case. Capitalising it fails the reflect test in main_test.go.
• A failed operation must not move money. Check every condition before you touch the balance, and return the error instead.
• Transfer is the same rule one level up: call Withdraw first and return early if it fails, so a rejected transfer never credits the destination.
• Zero is a valid opening balance but not a valid deposit or withdrawal. Read each doc comment for which comparison it wants.
• Describe orders its cases: nil first, then errors.Is for each sentinel, then err.Error() for anything else. A switch with no expression reads better than a chain of ifs.
• FormatCents is integer division and remainder, and the cents need two digits: fmt.Sprintf("$%d.%02d", ...).