PostgreSQL, Second Edition

Defining the Input and Output Functions in C

Now you will create the input and output functions for this data type. At this point, you have to decide what your external form will look like. You know that you need to deal with three components: the number of units, an optional exchange rate, and an optional currency name. You want the typical case (units only) to be easy to enter, so you will accept input in any of the following forms:

units
units(exchange-rate)
units(exchange-rate/currency-name)

If you see a number (and nothing else), assume that you have a number of units of the base currency. If you see a number followed by an open parenthesis, you will expect an exchange rate to follow. If the exchange rate is followed by a slash character, ...